Add support for a subnet prefix length on interface network addresses

This allows the user code to add a network address with a subnet prefix length.
The prefix length value is stored in the network endpoint and provided back to
the user in the ProtocolAddress type.

PiperOrigin-RevId: 259807693

12 files changed, 392 insertions, 187 deletions

diff --git a/pkg/sentry/socket/epsocket/stack.go b/pkg/sentry/socket/epsocket/stack.go
index 7eef19f74..8fe489c0e 100644
--- a/pkg/sentry/socket/epsocket/stack.go
+++ b/pkg/sentry/socket/epsocket/stack.go
@@ -75,8 +75,8 @@ func (s *Stack) InterfaceAddrs() map[int32][]inet.InterfaceAddr {
 
 			addrs = append(addrs, inet.InterfaceAddr{
 				Family:    family,
-				PrefixLen: uint8(len(a.Address) * 8),
-				Addr:      []byte(a.Address),
+				PrefixLen: uint8(a.AddressWithPrefix.PrefixLen),
+				Addr:      []byte(a.AddressWithPrefix.Address),
 				// TODO(b/68878065): Other fields.
 			})
 		}
diff --git a/pkg/tcpip/network/arp/arp.go b/pkg/tcpip/network/arp/arp.go
index cb35635fc..60070874d 100644
--- a/pkg/tcpip/network/arp/arp.go
+++ b/pkg/tcpip/network/arp/arp.go
@@ -46,7 +46,6 @@ const (
 // endpoint implements stack.NetworkEndpoint.
 type endpoint struct {
 	nicid         tcpip.NICID
-	addr          tcpip.Address
 	linkEP        stack.LinkEndpoint
 	linkAddrCache stack.LinkAddressCache
 }
@@ -73,6 +72,10 @@ func (e *endpoint) ID() *stack.NetworkEndpointID {
 	return &stack.NetworkEndpointID{ProtocolAddress}
 }
 
+func (e *endpoint) PrefixLen() int {
+	return 0
+}
+
 func (e *endpoint) MaxHeaderLength() uint16 {
 	return e.linkEP.MaxHeaderLength() + header.ARPSize
 }
@@ -122,19 +125,19 @@ type protocol struct {
 
 func (p *protocol) Number() tcpip.NetworkProtocolNumber { return ProtocolNumber }
 func (p *protocol) MinimumPacketSize() int              { return header.ARPSize }
+func (p *protocol) DefaultPrefixLen() int               { return 0 }
 
 func (*protocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
 	h := header.ARP(v)
 	return tcpip.Address(h.ProtocolAddressSender()), ProtocolAddress
 }
 
-func (p *protocol) NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, sender stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
-	if addr != ProtocolAddress {
+func (p *protocol) NewEndpoint(nicid tcpip.NICID, addrWithPrefix tcpip.AddressWithPrefix, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, sender stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
+	if addrWithPrefix.Address != ProtocolAddress {
 		return nil, tcpip.ErrBadLocalAddress
 	}
 	return &endpoint{
 		nicid:         nicid,
-		addr:          addr,
 		linkEP:        sender,
 		linkAddrCache: linkAddrCache,
 	}, nil
diff --git a/pkg/tcpip/network/ip_test.go b/pkg/tcpip/network/ip_test.go
index 8ff428445..55e9eec99 100644
--- a/pkg/tcpip/network/ip_test.go
+++ b/pkg/tcpip/network/ip_test.go
@@ -29,16 +29,18 @@ import (
 )
 
 const (
-	localIpv4Addr  = "\x0a\x00\x00\x01"
-	remoteIpv4Addr = "\x0a\x00\x00\x02"
-	ipv4SubnetAddr = "\x0a\x00\x00\x00"
-	ipv4SubnetMask = "\xff\xff\xff\x00"
-	ipv4Gateway    = "\x0a\x00\x00\x03"
-	localIpv6Addr  = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01"
-	remoteIpv6Addr = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02"
-	ipv6SubnetAddr = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
-	ipv6SubnetMask = "\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00"
-	ipv6Gateway    = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x03"
+	localIpv4Addr      = "\x0a\x00\x00\x01"
+	localIpv4PrefixLen = 24
+	remoteIpv4Addr     = "\x0a\x00\x00\x02"
+	ipv4SubnetAddr     = "\x0a\x00\x00\x00"
+	ipv4SubnetMask     = "\xff\xff\xff\x00"
+	ipv4Gateway        = "\x0a\x00\x00\x03"
+	localIpv6Addr      = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01"
+	localIpv6PrefixLen = 120
+	remoteIpv6Addr     = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02"
+	ipv6SubnetAddr     = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
+	ipv6SubnetMask     = "\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00"
+	ipv6Gateway        = "\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x03"
 )
 
 // testObject implements two interfaces: LinkEndpoint and TransportDispatcher.
@@ -197,7 +199,7 @@ func buildIPv6Route(local, remote tcpip.Address) (stack.Route, *tcpip.Error) {
 func TestIPv4Send(t *testing.T) {
 	o := testObject{t: t, v4: true}
 	proto := ipv4.NewProtocol()
-	ep, err := proto.NewEndpoint(1, localIpv4Addr, nil, nil, &o)
+	ep, err := proto.NewEndpoint(1, tcpip.AddressWithPrefix{localIpv4Addr, localIpv4PrefixLen}, nil, nil, &o)
 	if err != nil {
 		t.Fatalf("NewEndpoint failed: %v", err)
 	}
@@ -229,7 +231,7 @@ func TestIPv4Send(t *testing.T) {
 func TestIPv4Receive(t *testing.T) {
 	o := testObject{t: t, v4: true}
 	proto := ipv4.NewProtocol()
-	ep, err := proto.NewEndpoint(1, localIpv4Addr, nil, &o, nil)
+	ep, err := proto.NewEndpoint(1, tcpip.AddressWithPrefix{localIpv4Addr, localIpv4PrefixLen}, nil, &o, nil)
 	if err != nil {
 		t.Fatalf("NewEndpoint failed: %v", err)
 	}
@@ -295,7 +297,7 @@ func TestIPv4ReceiveControl(t *testing.T) {
 		t.Run(c.name, func(t *testing.T) {
 			o := testObject{t: t}
 			proto := ipv4.NewProtocol()
-			ep, err := proto.NewEndpoint(1, localIpv4Addr, nil, &o, nil)
+			ep, err := proto.NewEndpoint(1, tcpip.AddressWithPrefix{localIpv4Addr, localIpv4PrefixLen}, nil, &o, nil)
 			if err != nil {
 				t.Fatalf("NewEndpoint failed: %v", err)
 			}
@@ -359,7 +361,7 @@ func TestIPv4ReceiveControl(t *testing.T) {
 func TestIPv4FragmentationReceive(t *testing.T) {
 	o := testObject{t: t, v4: true}
 	proto := ipv4.NewProtocol()
-	ep, err := proto.NewEndpoint(1, localIpv4Addr, nil, &o, nil)
+	ep, err := proto.NewEndpoint(1, tcpip.AddressWithPrefix{localIpv4Addr, localIpv4PrefixLen}, nil, &o, nil)
 	if err != nil {
 		t.Fatalf("NewEndpoint failed: %v", err)
 	}
@@ -426,7 +428,7 @@ func TestIPv4FragmentationReceive(t *testing.T) {
 func TestIPv6Send(t *testing.T) {
 	o := testObject{t: t}
 	proto := ipv6.NewProtocol()
-	ep, err := proto.NewEndpoint(1, localIpv6Addr, nil, nil, &o)
+	ep, err := proto.NewEndpoint(1, tcpip.AddressWithPrefix{localIpv6Addr, localIpv6PrefixLen}, nil, nil, &o)
 	if err != nil {
 		t.Fatalf("NewEndpoint failed: %v", err)
 	}
@@ -458,7 +460,7 @@ func TestIPv6Send(t *testing.T) {
 func TestIPv6Receive(t *testing.T) {
 	o := testObject{t: t}
 	proto := ipv6.NewProtocol()
-	ep, err := proto.NewEndpoint(1, localIpv6Addr, nil, &o, nil)
+	ep, err := proto.NewEndpoint(1, tcpip.AddressWithPrefix{localIpv6Addr, localIpv6PrefixLen}, nil, &o, nil)
 	if err != nil {
 		t.Fatalf("NewEndpoint failed: %v", err)
 	}
@@ -532,7 +534,7 @@ func TestIPv6ReceiveControl(t *testing.T) {
 		t.Run(c.name, func(t *testing.T) {
 			o := testObject{t: t}
 			proto := ipv6.NewProtocol()
-			ep, err := proto.NewEndpoint(1, localIpv6Addr, nil, &o, nil)
+			ep, err := proto.NewEndpoint(1, tcpip.AddressWithPrefix{localIpv6Addr, localIpv6PrefixLen}, nil, &o, nil)
 			if err != nil {
 				t.Fatalf("NewEndpoint failed: %v", err)
 			}
diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index e44a73d96..b7a06f525 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -49,16 +49,18 @@ const (
 type endpoint struct {
 	nicid         tcpip.NICID
 	id            stack.NetworkEndpointID
+	prefixLen     int
 	linkEP        stack.LinkEndpoint
 	dispatcher    stack.TransportDispatcher
 	fragmentation *fragmentation.Fragmentation
 }
 
 // NewEndpoint creates a new ipv4 endpoint.
-func (p *protocol) NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
+func (p *protocol) NewEndpoint(nicid tcpip.NICID, addrWithPrefix tcpip.AddressWithPrefix, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
 	e := &endpoint{
 		nicid:         nicid,
-		id:            stack.NetworkEndpointID{LocalAddress: addr},
+		id:            stack.NetworkEndpointID{LocalAddress: addrWithPrefix.Address},
+		prefixLen:     addrWithPrefix.PrefixLen,
 		linkEP:        linkEP,
 		dispatcher:    dispatcher,
 		fragmentation: fragmentation.NewFragmentation(fragmentation.HighFragThreshold, fragmentation.LowFragThreshold, fragmentation.DefaultReassembleTimeout),
@@ -93,6 +95,11 @@ func (e *endpoint) ID() *stack.NetworkEndpointID {
 	return &e.id
 }
 
+// PrefixLen returns the ipv4 endpoint subnet prefix length in bits.
+func (e *endpoint) PrefixLen() int {
+	return e.prefixLen
+}
+
 // MaxHeaderLength returns the maximum length needed by ipv4 headers (and
 // underlying protocols).
 func (e *endpoint) MaxHeaderLength() uint16 {
@@ -338,6 +345,11 @@ func (p *protocol) MinimumPacketSize() int {
 	return header.IPv4MinimumSize
 }
 
+// DefaultPrefixLen returns the IPv4 default prefix length.
+func (p *protocol) DefaultPrefixLen() int {
+	return header.IPv4AddressSize * 8
+}
+
 // ParseAddresses implements NetworkProtocol.ParseAddresses.
 func (*protocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
 	h := header.IPv4(v)
diff --git a/pkg/tcpip/network/ipv6/icmp_test.go b/pkg/tcpip/network/ipv6/icmp_test.go
index d46d68e73..726362c87 100644
--- a/pkg/tcpip/network/ipv6/icmp_test.go
+++ b/pkg/tcpip/network/ipv6/icmp_test.go
@@ -99,7 +99,11 @@ func TestICMPCounts(t *testing.T) {
 		}},
 	)
 
-	ep, err := s.NetworkProtocolInstance(ProtocolNumber).NewEndpoint(0, lladdr1, &stubLinkAddressCache{}, &stubDispatcher{}, nil)
+	netProto := s.NetworkProtocolInstance(ProtocolNumber)
+	if netProto == nil {
+		t.Fatalf("cannot find protocol instance for network protocol %d", ProtocolNumber)
+	}
+	ep, err := netProto.NewEndpoint(0, tcpip.AddressWithPrefix{lladdr1, netProto.DefaultPrefixLen()}, &stubLinkAddressCache{}, &stubDispatcher{}, nil)
 	if err != nil {
 		t.Fatalf("NewEndpoint(_) = _, %s, want = _, nil", err)
 	}
diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index e3e8739fd..331a8bdaa 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -46,6 +46,7 @@ const (
 type endpoint struct {
 	nicid         tcpip.NICID
 	id            stack.NetworkEndpointID
+	prefixLen     int
 	linkEP        stack.LinkEndpoint
 	linkAddrCache stack.LinkAddressCache
 	dispatcher    stack.TransportDispatcher
@@ -72,6 +73,11 @@ func (e *endpoint) ID() *stack.NetworkEndpointID {
 	return &e.id
 }
 
+// PrefixLen returns the ipv6 endpoint subnet prefix length in bits.
+func (e *endpoint) PrefixLen() int {
+	return e.prefixLen
+}
+
 // Capabilities implements stack.NetworkEndpoint.Capabilities.
 func (e *endpoint) Capabilities() stack.LinkEndpointCapabilities {
 	return e.linkEP.Capabilities()
@@ -172,6 +178,11 @@ func (p *protocol) MinimumPacketSize() int {
 	return header.IPv6MinimumSize
 }
 
+// DefaultPrefixLen returns the IPv6 default prefix length.
+func (p *protocol) DefaultPrefixLen() int {
+	return header.IPv6AddressSize * 8
+}
+
 // ParseAddresses implements NetworkProtocol.ParseAddresses.
 func (*protocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
 	h := header.IPv6(v)
@@ -179,10 +190,11 @@ func (*protocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
 }
 
 // NewEndpoint creates a new ipv6 endpoint.
-func (p *protocol) NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
+func (p *protocol) NewEndpoint(nicid tcpip.NICID, addrWithPrefix tcpip.AddressWithPrefix, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
 	return &endpoint{
 		nicid:         nicid,
-		id:            stack.NetworkEndpointID{LocalAddress: addr},
+		id:            stack.NetworkEndpointID{LocalAddress: addrWithPrefix.Address},
+		prefixLen:     addrWithPrefix.PrefixLen,
 		linkEP:        linkEP,
 		linkAddrCache: linkAddrCache,
 		dispatcher:    dispatcher,
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 30c0dee42..3e6ff4afb 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -129,7 +129,7 @@ func (n *NIC) setSpoofing(enable bool) {
 	n.mu.Unlock()
 }
 
-func (n *NIC) getMainNICAddress(protocol tcpip.NetworkProtocolNumber) (tcpip.Address, tcpip.Subnet, *tcpip.Error) {
+func (n *NIC) getMainNICAddress(protocol tcpip.NetworkProtocolNumber) (tcpip.AddressWithPrefix, *tcpip.Error) {
 	n.mu.RLock()
 	defer n.mu.RUnlock()
 
@@ -148,21 +148,16 @@ func (n *NIC) getMainNICAddress(protocol tcpip.NetworkProtocolNumber) (tcpip.Add
 	}
 
 	if r == nil {
-		return "", tcpip.Subnet{}, tcpip.ErrNoLinkAddress
+		return tcpip.AddressWithPrefix{}, tcpip.ErrNoLinkAddress
 	}
 
-	address := r.ep.ID().LocalAddress
+	addressWithPrefix := tcpip.AddressWithPrefix{
+		Address:   r.ep.ID().LocalAddress,
+		PrefixLen: r.ep.PrefixLen(),
+	}
 	r.decRef()
 
-	// Find the least-constrained matching subnet for the address, if one
-	// exists, and return it.
-	var subnet tcpip.Subnet
-	for _, s := range n.subnets {
-		if s.Contains(address) && !subnet.Contains(s.ID()) {
-			subnet = s
-		}
-	}
-	return address, subnet, nil
+	return addressWithPrefix, nil
 }
 
 // primaryEndpoint returns the primary endpoint of n for the given network
@@ -213,23 +208,26 @@ func (n *NIC) findEndpoint(protocol tcpip.NetworkProtocolNumber, address tcpip.A
 	n.mu.Lock()
 	ref = n.endpoints[id]
 	if ref == nil || !ref.tryIncRef() {
-		ref, _ = n.addAddressLocked(protocol, address, peb, true)
-		if ref != nil {
-			ref.holdsInsertRef = false
+		if netProto, ok := n.stack.networkProtocols[protocol]; ok {
+			addrWithPrefix := tcpip.AddressWithPrefix{address, netProto.DefaultPrefixLen()}
+			ref, _ = n.addAddressLocked(protocol, addrWithPrefix, peb, true)
+			if ref != nil {
+				ref.holdsInsertRef = false
+			}
 		}
 	}
 	n.mu.Unlock()
 	return ref
 }
 
-func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior, replace bool) (*referencedNetworkEndpoint, *tcpip.Error) {
+func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addrWithPrefix tcpip.AddressWithPrefix, peb PrimaryEndpointBehavior, replace bool) (*referencedNetworkEndpoint, *tcpip.Error) {
 	netProto, ok := n.stack.networkProtocols[protocol]
 	if !ok {
 		return nil, tcpip.ErrUnknownProtocol
 	}
 
 	// Create the new network endpoint.
-	ep, err := netProto.NewEndpoint(n.id, addr, n.stack, n, n.linkEP)
+	ep, err := netProto.NewEndpoint(n.id, addrWithPrefix, n.stack, n, n.linkEP)
 	if err != nil {
 		return nil, err
 	}
@@ -278,16 +276,10 @@ func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.
 
 // AddAddress adds a new address to n, so that it starts accepting packets
 // targeted at the given address (and network protocol).
-func (n *NIC) AddAddress(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) *tcpip.Error {
-	return n.AddAddressWithOptions(protocol, addr, CanBePrimaryEndpoint)
-}
-
-// AddAddressWithOptions is the same as AddAddress, but allows you to specify
-// whether the new endpoint can be primary or not.
-func (n *NIC) AddAddressWithOptions(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior) *tcpip.Error {
+func (n *NIC) AddAddress(protocol tcpip.NetworkProtocolNumber, addrWithPrefix tcpip.AddressWithPrefix, peb PrimaryEndpointBehavior) *tcpip.Error {
 	// Add the endpoint.
 	n.mu.Lock()
-	_, err := n.addAddressLocked(protocol, addr, peb, false)
+	_, err := n.addAddressLocked(protocol, addrWithPrefix, peb, false)
 	n.mu.Unlock()
 
 	return err
@@ -298,10 +290,13 @@ func (n *NIC) Addresses() []tcpip.ProtocolAddress {
 	n.mu.RLock()
 	defer n.mu.RUnlock()
 	addrs := make([]tcpip.ProtocolAddress, 0, len(n.endpoints))
-	for nid, ep := range n.endpoints {
+	for nid, ref := range n.endpoints {
 		addrs = append(addrs, tcpip.ProtocolAddress{
-			Protocol: ep.protocol,
-			Address:  nid.LocalAddress,
+			Protocol: ref.protocol,
+			AddressWithPrefix: tcpip.AddressWithPrefix{
+				Address:   nid.LocalAddress,
+				PrefixLen: ref.ep.PrefixLen(),
+			},
 		})
 	}
 	return addrs
@@ -415,7 +410,12 @@ func (n *NIC) joinGroup(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address
 	id := NetworkEndpointID{addr}
 	joins := n.mcastJoins[id]
 	if joins == 0 {
-		if _, err := n.addAddressLocked(protocol, addr, NeverPrimaryEndpoint, false); err != nil {
+		netProto, ok := n.stack.networkProtocols[protocol]
+		if !ok {
+			return tcpip.ErrUnknownProtocol
+		}
+		addrWithPrefix := tcpip.AddressWithPrefix{addr, netProto.DefaultPrefixLen()}
+		if _, err := n.addAddressLocked(protocol, addrWithPrefix, NeverPrimaryEndpoint, false); err != nil {
 			return err
 		}
 	}
@@ -572,7 +572,13 @@ func (n *NIC) getRef(protocol tcpip.NetworkProtocolNumber, dst tcpip.Address) *r
 			n.mu.Unlock()
 			return ref
 		}
-		ref, err := n.addAddressLocked(protocol, dst, CanBePrimaryEndpoint, true)
+		netProto, ok := n.stack.networkProtocols[protocol]
+		if !ok {
+			n.mu.Unlock()
+			return nil
+		}
+		addrWithPrefix := tcpip.AddressWithPrefix{dst, netProto.DefaultPrefixLen()}
+		ref, err := n.addAddressLocked(protocol, addrWithPrefix, CanBePrimaryEndpoint, true)
 		n.mu.Unlock()
 		if err == nil {
 			ref.holdsInsertRef = false
diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index 462265281..2037eef9f 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -181,6 +181,9 @@ type NetworkEndpoint interface {
 	// ID returns the network protocol endpoint ID.
 	ID() *NetworkEndpointID
 
+	// PrefixLen returns the network endpoint's subnet prefix length in bits.
+	PrefixLen() int
+
 	// NICID returns the id of the NIC this endpoint belongs to.
 	NICID() tcpip.NICID
 
@@ -203,12 +206,15 @@ type NetworkProtocol interface {
 	// than this targeted at this protocol.
 	MinimumPacketSize() int
 
+	// DefaultPrefixLen returns the protocol's default prefix length.
+	DefaultPrefixLen() int
+
 	// ParsePorts returns the source and destination addresses stored in a
 	// packet of this protocol.
 	ParseAddresses(v buffer.View) (src, dst tcpip.Address)
 
 	// NewEndpoint creates a new endpoint of this protocol.
-	NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache LinkAddressCache, dispatcher TransportDispatcher, sender LinkEndpoint) (NetworkEndpoint, *tcpip.Error)
+	NewEndpoint(nicid tcpip.NICID, addrWithPrefix tcpip.AddressWithPrefix, linkAddrCache LinkAddressCache, dispatcher TransportDispatcher, sender LinkEndpoint) (NetworkEndpoint, *tcpip.Error)
 
 	// SetOption allows enabling/disabling protocol specific features.
 	// SetOption returns an error if the option is not supported or the
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 3e8fb2a6c..57b8a9994 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -751,9 +751,26 @@ func (s *Stack) AddAddress(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber,
 	return s.AddAddressWithOptions(id, protocol, addr, CanBePrimaryEndpoint)
 }
 
+// AddAddressWithPrefix adds a new network-layer address/prefixLen to the
+// specified NIC.
+func (s *Stack) AddAddressWithPrefix(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addrWithPrefix tcpip.AddressWithPrefix) *tcpip.Error {
+	return s.AddAddressWithPrefixAndOptions(id, protocol, addrWithPrefix, CanBePrimaryEndpoint)
+}
+
 // AddAddressWithOptions is the same as AddAddress, but allows you to specify
 // whether the new endpoint can be primary or not.
 func (s *Stack) AddAddressWithOptions(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior) *tcpip.Error {
+	netProto, ok := s.networkProtocols[protocol]
+	if !ok {
+		return tcpip.ErrUnknownProtocol
+	}
+	addrWithPrefix := tcpip.AddressWithPrefix{addr, netProto.DefaultPrefixLen()}
+	return s.AddAddressWithPrefixAndOptions(id, protocol, addrWithPrefix, peb)
+}
+
+// AddAddressWithPrefixAndOptions is the same as AddAddressWithPrefixLen,
+// but allows you to specify whether the new endpoint can be primary or not.
+func (s *Stack) AddAddressWithPrefixAndOptions(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addrWithPrefix tcpip.AddressWithPrefix, peb PrimaryEndpointBehavior) *tcpip.Error {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
@@ -762,7 +779,7 @@ func (s *Stack) AddAddressWithOptions(id tcpip.NICID, protocol tcpip.NetworkProt
 		return tcpip.ErrUnknownNICID
 	}
 
-	return nic.AddAddressWithOptions(protocol, addr, peb)
+	return nic.AddAddress(protocol, addrWithPrefix, peb)
 }
 
 // AddSubnet adds a subnet range to the specified NIC.
@@ -821,7 +838,7 @@ func (s *Stack) RemoveAddress(id tcpip.NICID, addr tcpip.Address) *tcpip.Error {
 // contains it) for the given NIC and protocol. Returns an arbitrary endpoint's
 // address if no primary addresses exist. Returns an error if the NIC doesn't
 // exist or has no endpoints.
-func (s *Stack) GetMainNICAddress(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber) (tcpip.Address, tcpip.Subnet, *tcpip.Error) {
+func (s *Stack) GetMainNICAddress(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber) (tcpip.AddressWithPrefix, *tcpip.Error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
@@ -829,7 +846,7 @@ func (s *Stack) GetMainNICAddress(id tcpip.NICID, protocol tcpip.NetworkProtocol
 		return nic.getMainNICAddress(protocol)
 	}
 
-	return "", tcpip.Subnet{}, tcpip.ErrUnknownNICID
+	return tcpip.AddressWithPrefix{}, tcpip.ErrUnknownNICID
 }
 
 func (s *Stack) getRefEP(nic *NIC, localAddr tcpip.Address, netProto tcpip.NetworkProtocolNumber) (ref *referencedNetworkEndpoint) {
diff --git a/pkg/tcpip/stack/stack_test.go b/pkg/tcpip/stack/stack_test.go
index 959071dbe..9d082bba4 100644
--- a/pkg/tcpip/stack/stack_test.go
+++ b/pkg/tcpip/stack/stack_test.go
@@ -21,6 +21,7 @@ import (
 	"bytes"
 	"fmt"
 	"math"
+	"sort"
 	"strings"
 	"testing"
 
@@ -32,8 +33,9 @@ import (
 )
 
 const (
-	fakeNetNumber    tcpip.NetworkProtocolNumber = math.MaxUint32
-	fakeNetHeaderLen                             = 12
+	fakeNetNumber        tcpip.NetworkProtocolNumber = math.MaxUint32
+	fakeNetHeaderLen                                 = 12
+	fakeDefaultPrefixLen                             = 8
 
 	// fakeControlProtocol is used for control packets that represent
 	// destination port unreachable.
@@ -55,6 +57,7 @@ const (
 type fakeNetworkEndpoint struct {
 	nicid      tcpip.NICID
 	id         stack.NetworkEndpointID
+	prefixLen  int
 	proto      *fakeNetworkProtocol
 	dispatcher stack.TransportDispatcher
 	linkEP     stack.LinkEndpoint
@@ -68,6 +71,10 @@ func (f *fakeNetworkEndpoint) NICID() tcpip.NICID {
 	return f.nicid
 }
 
+func (f *fakeNetworkEndpoint) PrefixLen() int {
+	return f.prefixLen
+}
+
 func (*fakeNetworkEndpoint) DefaultTTL() uint8 {
 	return 123
 }
@@ -170,14 +177,19 @@ func (f *fakeNetworkProtocol) MinimumPacketSize() int {
 	return fakeNetHeaderLen
 }
 
+func (f *fakeNetworkProtocol) DefaultPrefixLen() int {
+	return fakeDefaultPrefixLen
+}
+
 func (*fakeNetworkProtocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
 	return tcpip.Address(v[1:2]), tcpip.Address(v[0:1])
 }
 
-func (f *fakeNetworkProtocol) NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
+func (f *fakeNetworkProtocol) NewEndpoint(nicid tcpip.NICID, addrWithPrefix tcpip.AddressWithPrefix, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
 	return &fakeNetworkEndpoint{
 		nicid:      nicid,
-		id:         stack.NetworkEndpointID{addr},
+		id:         stack.NetworkEndpointID{addrWithPrefix.Address},
+		prefixLen:  addrWithPrefix.PrefixLen,
 		proto:      f,
 		dispatcher: dispatcher,
 		linkEP:     linkEP,
@@ -212,15 +224,15 @@ func TestNetworkReceive(t *testing.T) {
 	id, linkEP := channel.New(10, defaultMTU, "")
 	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x02"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	fakeNet := s.NetworkProtocolInstance(fakeNetNumber).(*fakeNetworkProtocol)
@@ -280,13 +292,13 @@ func TestNetworkReceive(t *testing.T) {
 func sendTo(t *testing.T, s *stack.Stack, addr tcpip.Address, payload buffer.View) {
 	r, err := s.FindRoute(0, "", addr, fakeNetNumber, false /* multicastLoop */)
 	if err != nil {
-		t.Fatalf("FindRoute failed: %v", err)
+		t.Fatal("FindRoute failed:", err)
 	}
 	defer r.Release()
 
 	hdr := buffer.NewPrependable(int(r.MaxHeaderLength()))
 	if err := r.WritePacket(nil /* gso */, hdr, payload.ToVectorisedView(), fakeTransNumber, 123); err != nil {
-		t.Errorf("WritePacket failed: %v", err)
+		t.Error("WritePacket failed:", err)
 	}
 }
 
@@ -297,13 +309,13 @@ func TestNetworkSend(t *testing.T) {
 	id, linkEP := channel.New(10, defaultMTU, "")
 	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("NewNIC failed: %v", err)
+		t.Fatal("NewNIC failed:", err)
 	}
 
 	s.SetRouteTable([]tcpip.Route{{"\x00", "\x00", "\x00", 1}})
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	// Make sure that the link-layer endpoint received the outbound packet.
@@ -321,28 +333,28 @@ func TestNetworkSendMultiRoute(t *testing.T) {
 
 	id1, linkEP1 := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id1); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x03"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	id2, linkEP2 := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(2, id2); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(2, fakeNetNumber, "\x02"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	if err := s.AddAddress(2, fakeNetNumber, "\x04"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	// Set a route table that sends all packets with odd destination
@@ -371,7 +383,7 @@ func TestNetworkSendMultiRoute(t *testing.T) {
 func testRoute(t *testing.T, s *stack.Stack, nic tcpip.NICID, srcAddr, dstAddr, expectedSrcAddr tcpip.Address) {
 	r, err := s.FindRoute(nic, srcAddr, dstAddr, fakeNetNumber, false /* multicastLoop */)
 	if err != nil {
-		t.Fatalf("FindRoute failed: %v", err)
+		t.Fatal("FindRoute failed:", err)
 	}
 
 	defer r.Release()
@@ -388,7 +400,7 @@ func testRoute(t *testing.T, s *stack.Stack, nic tcpip.NICID, srcAddr, dstAddr,
 func testNoRoute(t *testing.T, s *stack.Stack, nic tcpip.NICID, srcAddr, dstAddr tcpip.Address) {
 	_, err := s.FindRoute(nic, srcAddr, dstAddr, fakeNetNumber, false /* multicastLoop */)
 	if err != tcpip.ErrNoRoute {
-		t.Fatalf("FindRoute returned unexpected error, expected tcpip.ErrNoRoute, got %v", err)
+		t.Fatalf("FindRoute returned unexpected error, got = %v, want = %s", err, tcpip.ErrNoRoute)
 	}
 }
 
@@ -400,28 +412,28 @@ func TestRoutes(t *testing.T) {
 
 	id1, _ := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id1); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x03"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	id2, _ := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(2, id2); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(2, fakeNetNumber, "\x02"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	if err := s.AddAddress(2, fakeNetNumber, "\x04"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	// Set a route table that sends all packets with odd destination
@@ -464,11 +476,11 @@ func TestAddressRemoval(t *testing.T) {
 
 	id, linkEP := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	fakeNet := s.NetworkProtocolInstance(fakeNetNumber).(*fakeNetworkProtocol)
@@ -486,7 +498,7 @@ func TestAddressRemoval(t *testing.T) {
 	// Remove the address, then check that packet doesn't get delivered
 	// anymore.
 	if err := s.RemoveAddress(1, "\x01"); err != nil {
-		t.Fatalf("RemoveAddress failed: %v", err)
+		t.Fatal("RemoveAddress failed:", err)
 	}
 
 	linkEP.Inject(fakeNetNumber, buf.ToVectorisedView())
@@ -496,7 +508,7 @@ func TestAddressRemoval(t *testing.T) {
 
 	// Check that removing the same address fails.
 	if err := s.RemoveAddress(1, "\x01"); err != tcpip.ErrBadLocalAddress {
-		t.Fatalf("RemoveAddress failed: %v", err)
+		t.Fatalf("RemoveAddress returned unexpected error, got = %v, want = %s", err, tcpip.ErrBadLocalAddress)
 	}
 }
 
@@ -505,11 +517,11 @@ func TestDelayedRemovalDueToRoute(t *testing.T) {
 
 	id, linkEP := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	s.SetRouteTable([]tcpip.Route{
@@ -531,7 +543,7 @@ func TestDelayedRemovalDueToRoute(t *testing.T) {
 	// Get a route, check that packet is still deliverable.
 	r, err := s.FindRoute(0, "", "\x02", fakeNetNumber, false /* multicastLoop */)
 	if err != nil {
-		t.Fatalf("FindRoute failed: %v", err)
+		t.Fatal("FindRoute failed:", err)
 	}
 
 	linkEP.Inject(fakeNetNumber, buf.ToVectorisedView())
@@ -542,7 +554,7 @@ func TestDelayedRemovalDueToRoute(t *testing.T) {
 	// Remove the address, then check that packet is still deliverable
 	// because the route is keeping the address alive.
 	if err := s.RemoveAddress(1, "\x01"); err != nil {
-		t.Fatalf("RemoveAddress failed: %v", err)
+		t.Fatal("RemoveAddress failed:", err)
 	}
 
 	linkEP.Inject(fakeNetNumber, buf.ToVectorisedView())
@@ -552,7 +564,7 @@ func TestDelayedRemovalDueToRoute(t *testing.T) {
 
 	// Check that removing the same address fails.
 	if err := s.RemoveAddress(1, "\x01"); err != tcpip.ErrBadLocalAddress {
-		t.Fatalf("RemoveAddress failed: %v", err)
+		t.Fatalf("RemoveAddress returned unexpected error, got = %v, want = %s", err, tcpip.ErrBadLocalAddress)
 	}
 
 	// Release the route, then check that packet is not deliverable anymore.
@@ -568,7 +580,7 @@ func TestPromiscuousMode(t *testing.T) {
 
 	id, linkEP := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	s.SetRouteTable([]tcpip.Route{
@@ -590,7 +602,7 @@ func TestPromiscuousMode(t *testing.T) {
 
 	// Set promiscuous mode, then check that packet is delivered.
 	if err := s.SetPromiscuousMode(1, true); err != nil {
-		t.Fatalf("SetPromiscuousMode failed: %v", err)
+		t.Fatal("SetPromiscuousMode failed:", err)
 	}
 
 	linkEP.Inject(fakeNetNumber, buf.ToVectorisedView())
@@ -601,13 +613,13 @@ func TestPromiscuousMode(t *testing.T) {
 	// Check that we can't get a route as there is no local address.
 	_, err := s.FindRoute(0, "", "\x02", fakeNetNumber, false /* multicastLoop */)
 	if err != tcpip.ErrNoRoute {
-		t.Fatalf("FindRoute returned unexpected status: expected %v, got %v", tcpip.ErrNoRoute, err)
+		t.Fatalf("FindRoute returned unexpected error: got = %v, want = %s", err, tcpip.ErrNoRoute)
 	}
 
 	// Set promiscuous mode to false, then check that packet can't be
 	// delivered anymore.
 	if err := s.SetPromiscuousMode(1, false); err != nil {
-		t.Fatalf("SetPromiscuousMode failed: %v", err)
+		t.Fatal("SetPromiscuousMode failed:", err)
 	}
 
 	linkEP.Inject(fakeNetNumber, buf.ToVectorisedView())
@@ -624,11 +636,11 @@ func TestAddressSpoofing(t *testing.T) {
 
 	id, _ := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, dstAddr); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 
 	s.SetRouteTable([]tcpip.Route{
@@ -645,11 +657,11 @@ func TestAddressSpoofing(t *testing.T) {
 	// With address spoofing enabled, FindRoute permits any address to be used
 	// as the source.
 	if err := s.SetSpoofing(1, true); err != nil {
-		t.Fatalf("SetSpoofing failed: %v", err)
+		t.Fatal("SetSpoofing failed:", err)
 	}
 	r, err = s.FindRoute(0, srcAddr, dstAddr, fakeNetNumber, false /* multicastLoop */)
 	if err != nil {
-		t.Fatalf("FindRoute failed: %v", err)
+		t.Fatal("FindRoute failed:", err)
 	}
 	if r.LocalAddress != srcAddr {
 		t.Errorf("Route has wrong local address: got %v, wanted %v", r.LocalAddress, srcAddr)
@@ -664,17 +676,17 @@ func TestBroadcastNeedsNoRoute(t *testing.T) {
 
 	id, _ := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 	s.SetRouteTable([]tcpip.Route{})
 
 	// If there is no endpoint, it won't work.
 	if _, err := s.FindRoute(1, header.IPv4Any, header.IPv4Broadcast, fakeNetNumber, false /* multicastLoop */); err != tcpip.ErrNetworkUnreachable {
-		t.Fatalf("got FindRoute(1, %v, %v, %v) = %v, want = %v", header.IPv4Any, header.IPv4Broadcast, fakeNetNumber, err, tcpip.ErrNetworkUnreachable)
+		t.Fatalf("got FindRoute(1, %v, %v, %v) = %v, want = %s", header.IPv4Any, header.IPv4Broadcast, fakeNetNumber, err, tcpip.ErrNetworkUnreachable)
 	}
 
 	if err := s.AddAddress(1, fakeNetNumber, header.IPv4Any); err != nil {
-		t.Fatalf("AddAddress(%v, %v) failed: %v", fakeNetNumber, header.IPv4Any, err)
+		t.Fatalf("AddAddress(%v, %v) failed: %s", fakeNetNumber, header.IPv4Any, err)
 	}
 	r, err := s.FindRoute(1, header.IPv4Any, header.IPv4Broadcast, fakeNetNumber, false /* multicastLoop */)
 	if err != nil {
@@ -735,7 +747,7 @@ func TestMulticastOrIPv6LinkLocalNeedsNoRoute(t *testing.T) {
 
 			id, _ := channel.New(10, defaultMTU, "")
 			if err := s.CreateNIC(1, id); err != nil {
-				t.Fatalf("CreateNIC failed: %v", err)
+				t.Fatal("CreateNIC failed:", err)
 			}
 
 			s.SetRouteTable([]tcpip.Route{})
@@ -791,7 +803,7 @@ func TestSubnetAcceptsMatchingPacket(t *testing.T) {
 
 	id, linkEP := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	s.SetRouteTable([]tcpip.Route{
@@ -806,10 +818,10 @@ func TestSubnetAcceptsMatchingPacket(t *testing.T) {
 	fakeNet.packetCount[1] = 0
 	subnet, err := tcpip.NewSubnet(tcpip.Address("\x00"), tcpip.AddressMask("\xF0"))
 	if err != nil {
-		t.Fatalf("NewSubnet failed: %v", err)
+		t.Fatal("NewSubnet failed:", err)
 	}
 	if err := s.AddSubnet(1, fakeNetNumber, subnet); err != nil {
-		t.Fatalf("AddSubnet failed: %v", err)
+		t.Fatal("AddSubnet failed:", err)
 	}
 
 	linkEP.Inject(fakeNetNumber, buf.ToVectorisedView())
@@ -824,7 +836,7 @@ func TestSubnetRejectsNonmatchingPacket(t *testing.T) {
 
 	id, linkEP := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	s.SetRouteTable([]tcpip.Route{
@@ -839,10 +851,10 @@ func TestSubnetRejectsNonmatchingPacket(t *testing.T) {
 	fakeNet.packetCount[1] = 0
 	subnet, err := tcpip.NewSubnet(tcpip.Address("\x10"), tcpip.AddressMask("\xF0"))
 	if err != nil {
-		t.Fatalf("NewSubnet failed: %v", err)
+		t.Fatal("NewSubnet failed:", err)
 	}
 	if err := s.AddSubnet(1, fakeNetNumber, subnet); err != nil {
-		t.Fatalf("AddSubnet failed: %v", err)
+		t.Fatal("AddSubnet failed:", err)
 	}
 	linkEP.Inject(fakeNetNumber, buf.ToVectorisedView())
 	if fakeNet.packetCount[1] != 0 {
@@ -894,38 +906,38 @@ func TestSubnetAddRemove(t *testing.T) {
 	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
 	id, _ := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	addr := tcpip.Address("\x01\x01\x01\x01")
 	mask := tcpip.AddressMask(strings.Repeat("\xff", len(addr)))
 	subnet, err := tcpip.NewSubnet(addr, mask)
 	if err != nil {
-		t.Fatalf("NewSubnet failed: %v", err)
+		t.Fatal("NewSubnet failed:", err)
 	}
 
 	if contained, err := s.ContainsSubnet(1, subnet); err != nil {
-		t.Fatalf("ContainsSubnet failed: %v", err)
+		t.Fatal("ContainsSubnet failed:", err)
 	} else if contained {
 		t.Fatal("got s.ContainsSubnet(...) = true, want = false")
 	}
 
 	if err := s.AddSubnet(1, fakeNetNumber, subnet); err != nil {
-		t.Fatalf("AddSubnet failed: %v", err)
+		t.Fatal("AddSubnet failed:", err)
 	}
 
 	if contained, err := s.ContainsSubnet(1, subnet); err != nil {
-		t.Fatalf("ContainsSubnet failed: %v", err)
+		t.Fatal("ContainsSubnet failed:", err)
 	} else if !contained {
 		t.Fatal("got s.ContainsSubnet(...) = false, want = true")
 	}
 
 	if err := s.RemoveSubnet(1, subnet); err != nil {
-		t.Fatalf("RemoveSubnet failed: %v", err)
+		t.Fatal("RemoveSubnet failed:", err)
 	}
 
 	if contained, err := s.ContainsSubnet(1, subnet); err != nil {
-		t.Fatalf("ContainsSubnet failed: %v", err)
+		t.Fatal("ContainsSubnet failed:", err)
 	} else if contained {
 		t.Fatal("got s.ContainsSubnet(...) = true, want = false")
 	}
@@ -941,11 +953,11 @@ func TestGetMainNICAddressAddPrimaryNonPrimary(t *testing.T) {
 							s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
 							id, _ := channel.New(10, defaultMTU, "")
 							if err := s.CreateNIC(1, id); err != nil {
-								t.Fatalf("CreateNIC failed: %v", err)
+								t.Fatal("CreateNIC failed:", err)
 							}
 							// Insert <canBe> primary and <never> never-primary addresses.
 							// Each one will add a network endpoint to the NIC.
-							primaryAddrAdded := make(map[tcpip.Address]tcpip.Subnet)
+							primaryAddrAdded := make(map[tcpip.AddressWithPrefix]struct{})
 							for i := 0; i < canBe+never; i++ {
 								var behavior stack.PrimaryEndpointBehavior
 								if i < canBe {
@@ -953,46 +965,39 @@ func TestGetMainNICAddressAddPrimaryNonPrimary(t *testing.T) {
 								} else {
 									behavior = stack.NeverPrimaryEndpoint
 								}
-								// Add an address and in case of a primary one also add a
-								// subnet.
+								// Add an address and in case of a primary one include a
+								// prefixLen.
 								address := tcpip.Address(bytes.Repeat([]byte{byte(i)}, addrLen))
-								if err := s.AddAddressWithOptions(1, fakeNetNumber, address, behavior); err != nil {
-									t.Fatalf("AddAddressWithOptions failed: %v", err)
-								}
 								if behavior == stack.CanBePrimaryEndpoint {
-									mask := tcpip.AddressMask(strings.Repeat("\xff", len(address)))
-									subnet, err := tcpip.NewSubnet(address, mask)
-									if err != nil {
-										t.Fatalf("NewSubnet failed: %v", err)
+									addressWithPrefix := tcpip.AddressWithPrefix{address, addrLen * 8}
+									if err := s.AddAddressWithPrefixAndOptions(1, fakeNetNumber, addressWithPrefix, behavior); err != nil {
+										t.Fatal("AddAddressWithPrefixAndOptions failed:", err)
 									}
-									if err := s.AddSubnet(1, fakeNetNumber, subnet); err != nil {
-										t.Fatalf("AddSubnet failed: %v", err)
+									// Remember the address/prefix.
+									primaryAddrAdded[addressWithPrefix] = struct{}{}
+								} else {
+									if err := s.AddAddressWithOptions(1, fakeNetNumber, address, behavior); err != nil {
+										t.Fatal("AddAddressWithOptions failed:", err)
 									}
-									// Remember the address/subnet.
-									primaryAddrAdded[address] = subnet
 								}
 							}
 							// Check that GetMainNICAddress returns an address if at least
 							// one primary address was added. In that case make sure the
-							// address/subnet matches what we added.
+							// address/prefixLen matches what we added.
 							if len(primaryAddrAdded) == 0 {
 								// No primary addresses present, expect an error.
-								if _, _, err := s.GetMainNICAddress(1, fakeNetNumber); err != tcpip.ErrNoLinkAddress {
-									t.Fatalf("got s.GetMainNICAddress(...) = %v, wanted = %v", err, tcpip.ErrNoLinkAddress)
+								if _, err := s.GetMainNICAddress(1, fakeNetNumber); err != tcpip.ErrNoLinkAddress {
+									t.Fatalf("got s.GetMainNICAddress(...) = %v, wanted = %s", err, tcpip.ErrNoLinkAddress)
 								}
 							} else {
 								// At least one primary address was added, expect a valid
-								// address and subnet.
-								gotAddress, gotSubnet, err := s.GetMainNICAddress(1, fakeNetNumber)
+								// address and prefixLen.
+								gotAddressWithPefix, err := s.GetMainNICAddress(1, fakeNetNumber)
 								if err != nil {
-									t.Fatalf("GetMainNICAddress failed: %v", err)
-								}
-								expectedSubnet, ok := primaryAddrAdded[gotAddress]
-								if !ok {
-									t.Fatalf("GetMainNICAddress: got address = %v, wanted any in {%v}", gotAddress, primaryAddrAdded)
+									t.Fatal("GetMainNICAddress failed:", err)
 								}
-								if gotSubnet != expectedSubnet {
-									t.Fatalf("GetMainNICAddress: got subnet = %v, wanted %v", gotSubnet, expectedSubnet)
+								if _, ok := primaryAddrAdded[gotAddressWithPefix]; !ok {
+									t.Fatalf("GetMainNICAddress: got addressWithPrefix = %v, wanted any in {%v}", gotAddressWithPefix, primaryAddrAdded)
 								}
 							}
 						})
@@ -1007,65 +1012,194 @@ func TestGetMainNICAddressAddRemove(t *testing.T) {
 	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
 	id, _ := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 
 	for _, tc := range []struct {
-		name    string
-		address tcpip.Address
+		name      string
+		address   tcpip.Address
+		prefixLen int
 	}{
-		{"IPv4", "\x01\x01\x01\x01"},
-		{"IPv6", "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01"},
+		{"IPv4", "\x01\x01\x01\x01", 24},
+		{"IPv6", "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01", 116},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
-			address := tc.address
-			mask := tcpip.AddressMask(strings.Repeat("\xff", len(address)))
-			subnet, err := tcpip.NewSubnet(address, mask)
-			if err != nil {
-				t.Fatalf("NewSubnet failed: %v", err)
+			addressWithPrefix := tcpip.AddressWithPrefix{tc.address, tc.prefixLen}
+
+			if err := s.AddAddressWithPrefix(1, fakeNetNumber, addressWithPrefix); err != nil {
+				t.Fatal("AddAddressWithPrefix failed:", err)
 			}
 
-			if err := s.AddAddress(1, fakeNetNumber, address); err != nil {
-				t.Fatalf("AddAddress failed: %v", err)
+			// Check that we get the right initial address and prefix length.
+			if gotAddressWithPrefix, err := s.GetMainNICAddress(1, fakeNetNumber); err != nil {
+				t.Fatal("GetMainNICAddress failed:", err)
+			} else if gotAddressWithPrefix != addressWithPrefix {
+				t.Fatalf("got GetMainNICAddress = %+v, want = %+v", gotAddressWithPrefix, addressWithPrefix)
 			}
 
-			if err := s.AddSubnet(1, fakeNetNumber, subnet); err != nil {
-				t.Fatalf("AddSubnet failed: %v", err)
+			if err := s.RemoveAddress(1, addressWithPrefix.Address); err != nil {
+				t.Fatal("RemoveAddress failed:", err)
 			}
 
-			// Check that we get the right initial address and subnet.
-			if gotAddress, gotSubnet, err := s.GetMainNICAddress(1, fakeNetNumber); err != nil {
-				t.Fatalf("GetMainNICAddress failed: %v", err)
-			} else if gotAddress != address {
-				t.Fatalf("got GetMainNICAddress = (%v, ...), want = (%v, ...)", gotAddress, address)
-			} else if gotSubnet != subnet {
-				t.Fatalf("got GetMainNICAddress = (..., %v), want = (..., %v)", gotSubnet, subnet)
+			// Check that we get an error after removal.
+			if _, err := s.GetMainNICAddress(1, fakeNetNumber); err != tcpip.ErrNoLinkAddress {
+				t.Fatalf("got s.GetMainNICAddress(...) = %v, want = %s", err, tcpip.ErrNoLinkAddress)
 			}
+		})
+	}
+}
+
+// Simple network address generator. Good for 255 addresses.
+type addressGenerator struct{ cnt byte }
+
+func (g *addressGenerator) next(addrLen int) tcpip.Address {
+	g.cnt++
+	return tcpip.Address(bytes.Repeat([]byte{g.cnt}, addrLen))
+}
+
+func verifyAddresses(t *testing.T, expectedAddresses, gotAddresses []tcpip.ProtocolAddress) {
+	if len(gotAddresses) != len(expectedAddresses) {
+		t.Fatalf("got len(addresses) = %d, wanted = %d", len(gotAddresses), len(expectedAddresses))
+	}
+
+	sort.Slice(gotAddresses, func(i, j int) bool {
+		return gotAddresses[i].AddressWithPrefix.Address < gotAddresses[j].AddressWithPrefix.Address
+	})
+	sort.Slice(expectedAddresses, func(i, j int) bool {
+		return expectedAddresses[i].AddressWithPrefix.Address < expectedAddresses[j].AddressWithPrefix.Address
+	})
+
+	for i, gotAddr := range gotAddresses {
+		expectedAddr := expectedAddresses[i]
+		if gotAddr != expectedAddr {
+			t.Errorf("got address = %+v, wanted = %+v", gotAddr, expectedAddr)
+		}
+	}
+}
+
+func TestAddAddress(t *testing.T) {
+	const nicid = 1
+	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
+	id, _ := channel.New(10, defaultMTU, "")
+	if err := s.CreateNIC(nicid, id); err != nil {
+		t.Fatal("CreateNIC failed:", err)
+	}
 
-			if err := s.RemoveSubnet(1, subnet); err != nil {
-				t.Fatalf("RemoveSubnet failed: %v", err)
+	var addrGen addressGenerator
+	expectedAddresses := make([]tcpip.ProtocolAddress, 0, 2)
+	for _, addrLen := range []int{4, 16} {
+		address := addrGen.next(addrLen)
+		if err := s.AddAddress(nicid, fakeNetNumber, address); err != nil {
+			t.Fatalf("AddAddress(address=%s) failed: %s", address, err)
+		}
+		expectedAddresses = append(expectedAddresses, tcpip.ProtocolAddress{
+			Protocol:          fakeNetNumber,
+			AddressWithPrefix: tcpip.AddressWithPrefix{address, fakeDefaultPrefixLen},
+		})
+	}
+
+	gotAddresses := s.NICInfo()[nicid].ProtocolAddresses
+	verifyAddresses(t, expectedAddresses, gotAddresses)
+}
+
+func TestAddAddressWithPrefix(t *testing.T) {
+	const nicid = 1
+	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
+	id, _ := channel.New(10, defaultMTU, "")
+	if err := s.CreateNIC(nicid, id); err != nil {
+		t.Fatal("CreateNIC failed:", err)
+	}
+
+	var addrGen addressGenerator
+	addrLenRange := []int{4, 16}
+	prefixLenRange := []int{8, 13, 20, 32}
+	expectedAddresses := make([]tcpip.ProtocolAddress, 0, len(addrLenRange)*len(prefixLenRange))
+	for _, addrLen := range addrLenRange {
+		for _, prefixLen := range prefixLenRange {
+			address := addrGen.next(addrLen)
+			if err := s.AddAddressWithPrefix(nicid, fakeNetNumber, tcpip.AddressWithPrefix{address, prefixLen}); err != nil {
+				t.Errorf("AddAddressWithPrefix(address=%s, prefixLen=%d) failed: %s", address, prefixLen, err)
 			}
+			expectedAddresses = append(expectedAddresses, tcpip.ProtocolAddress{
+				Protocol:          fakeNetNumber,
+				AddressWithPrefix: tcpip.AddressWithPrefix{address, prefixLen},
+			})
+		}
+	}
 
-			if err := s.RemoveAddress(1, address); err != nil {
-				t.Fatalf("RemoveAddress failed: %v", err)
+	gotAddresses := s.NICInfo()[nicid].ProtocolAddresses
+	verifyAddresses(t, expectedAddresses, gotAddresses)
+}
+
+func TestAddAddressWithOptions(t *testing.T) {
+	const nicid = 1
+	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
+	id, _ := channel.New(10, defaultMTU, "")
+	if err := s.CreateNIC(nicid, id); err != nil {
+		t.Fatal("CreateNIC failed:", err)
+	}
+
+	addrLenRange := []int{4, 16}
+	behaviorRange := []stack.PrimaryEndpointBehavior{stack.CanBePrimaryEndpoint, stack.FirstPrimaryEndpoint, stack.NeverPrimaryEndpoint}
+	expectedAddresses := make([]tcpip.ProtocolAddress, 0, len(addrLenRange)*len(behaviorRange))
+	var addrGen addressGenerator
+	for _, addrLen := range addrLenRange {
+		for _, behavior := range behaviorRange {
+			address := addrGen.next(addrLen)
+			if err := s.AddAddressWithOptions(nicid, fakeNetNumber, address, behavior); err != nil {
+				t.Fatalf("AddAddressWithOptions(address=%s, behavior=%d) failed: %s", address, behavior, err)
 			}
+			expectedAddresses = append(expectedAddresses, tcpip.ProtocolAddress{
+				Protocol:          fakeNetNumber,
+				AddressWithPrefix: tcpip.AddressWithPrefix{address, fakeDefaultPrefixLen},
+			})
+		}
+	}
 
-			// Check that we get an error after removal.
-			if _, _, err := s.GetMainNICAddress(1, fakeNetNumber); err != tcpip.ErrNoLinkAddress {
-				t.Fatalf("got s.GetMainNICAddress(...) = %v, want = %v", err, tcpip.ErrNoLinkAddress)
+	gotAddresses := s.NICInfo()[nicid].ProtocolAddresses
+	verifyAddresses(t, expectedAddresses, gotAddresses)
+}
+
+func TestAddAddressWithPrefixAndOptions(t *testing.T) {
+	const nicid = 1
+	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
+	id, _ := channel.New(10, defaultMTU, "")
+	if err := s.CreateNIC(nicid, id); err != nil {
+		t.Fatal("CreateNIC failed:", err)
+	}
+
+	addrLenRange := []int{4, 16}
+	prefixLenRange := []int{8, 13, 20, 32}
+	behaviorRange := []stack.PrimaryEndpointBehavior{stack.CanBePrimaryEndpoint, stack.FirstPrimaryEndpoint, stack.NeverPrimaryEndpoint}
+	expectedAddresses := make([]tcpip.ProtocolAddress, 0, len(addrLenRange)*len(prefixLenRange)*len(behaviorRange))
+	var addrGen addressGenerator
+	for _, addrLen := range addrLenRange {
+		for _, prefixLen := range prefixLenRange {
+			for _, behavior := range behaviorRange {
+				address := addrGen.next(addrLen)
+				if err := s.AddAddressWithPrefixAndOptions(nicid, fakeNetNumber, tcpip.AddressWithPrefix{address, prefixLen}, behavior); err != nil {
+					t.Fatalf("AddAddressWithPrefixAndOptions(address=%s, prefixLen=%d, behavior=%d) failed: %s", address, prefixLen, behavior, err)
+				}
+				expectedAddresses = append(expectedAddresses, tcpip.ProtocolAddress{
+					Protocol:          fakeNetNumber,
+					AddressWithPrefix: tcpip.AddressWithPrefix{address, prefixLen},
+				})
 			}
-		})
+		}
 	}
+
+	gotAddresses := s.NICInfo()[nicid].ProtocolAddresses
+	verifyAddresses(t, expectedAddresses, gotAddresses)
 }
 
 func TestNICStats(t *testing.T) {
 	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
 	id1, linkEP1 := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id1); err != nil {
-		t.Fatalf("CreateNIC failed: %v", err)
+		t.Fatal("CreateNIC failed:", err)
 	}
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress failed: %v", err)
+		t.Fatal("AddAddress failed:", err)
 	}
 	// Route all packets for address \x01 to NIC 1.
 	s.SetRouteTable([]tcpip.Route{
@@ -1104,18 +1238,18 @@ func TestNICForwarding(t *testing.T) {
 
 	id1, linkEP1 := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(1, id1); err != nil {
-		t.Fatalf("CreateNIC #1 failed: %v", err)
+		t.Fatal("CreateNIC #1 failed:", err)
 	}
 	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatalf("AddAddress #1 failed: %v", err)
+		t.Fatal("AddAddress #1 failed:", err)
 	}
 
 	id2, linkEP2 := channel.New(10, defaultMTU, "")
 	if err := s.CreateNIC(2, id2); err != nil {
-		t.Fatalf("CreateNIC #2 failed: %v", err)
+		t.Fatal("CreateNIC #2 failed:", err)
 	}
 	if err := s.AddAddress(2, fakeNetNumber, "\x02"); err != nil {
-		t.Fatalf("AddAddress #2 failed: %v", err)
+		t.Fatal("AddAddress #2 failed:", err)
 	}
 
 	// Route all packets to address 3 to NIC 2.
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index c5d79da5e..4208c0303 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -1059,14 +1059,23 @@ func ParseMACAddress(s string) (LinkAddress, error) {
 	return LinkAddress(addr), nil
 }
 
+// AddressWithPrefix is an address with its subnet prefix length.
+type AddressWithPrefix struct {
+	// Address is a network address.
+	Address Address
+
+	// PrefixLen is the subnet prefix length.
+	PrefixLen int
+}
+
 // ProtocolAddress is an address and the network protocol it is associated
 // with.
 type ProtocolAddress struct {
 	// Protocol is the protocol of the address.
 	Protocol NetworkProtocolNumber
 
-	// Address is a network address.
-	Address Address
+	// AddressWithPrefix is a network address with its subnet prefix length.
+	AddressWithPrefix AddressWithPrefix
 }
 
 // danglingEndpointsMu protects access to danglingEndpoints.
diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index 958d5712e..56c285f88 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -967,14 +967,14 @@ func TestTTL(t *testing.T) {
 					multicast = false
 					switch variant {
 					case "v4", "mapped":
-						ep, err := ipv4.NewProtocol().NewEndpoint(0, "", nil, nil, nil)
+						ep, err := ipv4.NewProtocol().NewEndpoint(0, tcpip.AddressWithPrefix{}, nil, nil, nil)
 						if err != nil {
 							t.Fatal(err)
 						}
 						wantTTL = ep.DefaultTTL()
 						ep.Close()
 					case "v6":
-						ep, err := ipv6.NewProtocol().NewEndpoint(0, "", nil, nil, nil)
+						ep, err := ipv6.NewProtocol().NewEndpoint(0, tcpip.AddressWithPrefix{}, nil, nil, nil)
 						if err != nil {
 							t.Fatal(err)
 						}