diff --git a/network.go b/network.go
index 1397de0557..fc50a5a907 100644
--- a/network.go
+++ b/network.go
@@ -225,16 +225,22 @@ func getIfaceAddr(name string) (net.Addr, error) {
 // up iptables rules.
 // It keeps track of all mappings and is able to unmap at will
 type PortMapper struct {
-	tcpMapping map[int]*net.TCPAddr
-	tcpProxies map[int]proxy.Proxy
-	udpMapping map[int]*net.UDPAddr
-	udpProxies map[int]proxy.Proxy
+	tcpMapping map[string]*net.TCPAddr
+	tcpProxies map[string]proxy.Proxy
+	udpMapping map[string]*net.UDPAddr
+	udpProxies map[string]proxy.Proxy
 
-	iptables  *iptables.Chain
-	defaultIp net.IP
+	iptables         *iptables.Chain
+	defaultIp        net.IP
+	proxyFactoryFunc func(net.Addr, net.Addr) (proxy.Proxy, error)
 }
 
 func (mapper *PortMapper) Map(ip net.IP, port int, backendAddr net.Addr) error {
+	mapKey := (&net.TCPAddr{Port: port, IP: ip}).String()
+	if _, exists := mapper.tcpProxies[mapKey]; exists {
+		return fmt.Errorf("Port %s is already in use", mapKey)
+	}
+
 	if _, isTCP := backendAddr.(*net.TCPAddr); isTCP {
 		backendPort := backendAddr.(*net.TCPAddr).Port
 		backendIP := backendAddr.(*net.TCPAddr).IP
@@ -243,13 +249,13 @@ func (mapper *PortMapper) Map(ip net.IP, port int, backendAddr net.Addr) error {
 				return err
 			}
 		}
-		mapper.tcpMapping[port] = backendAddr.(*net.TCPAddr)
-		proxy, err := proxy.NewProxy(&net.TCPAddr{IP: ip, Port: port}, backendAddr)
+		mapper.tcpMapping[mapKey] = backendAddr.(*net.TCPAddr)
+		proxy, err := mapper.proxyFactoryFunc(&net.TCPAddr{IP: ip, Port: port}, backendAddr)
 		if err != nil {
 			mapper.Unmap(ip, port, "tcp")
 			return err
 		}
-		mapper.tcpProxies[port] = proxy
+		mapper.tcpProxies[mapKey] = proxy
 		go proxy.Run()
 	} else {
 		backendPort := backendAddr.(*net.UDPAddr).Port
@@ -259,49 +265,50 @@ func (mapper *PortMapper) Map(ip net.IP, port int, backendAddr net.Addr) error {
 				return err
 			}
 		}
-		mapper.udpMapping[port] = backendAddr.(*net.UDPAddr)
-		proxy, err := proxy.NewProxy(&net.UDPAddr{IP: ip, Port: port}, backendAddr)
+		mapper.udpMapping[mapKey] = backendAddr.(*net.UDPAddr)
+		proxy, err := mapper.proxyFactoryFunc(&net.UDPAddr{IP: ip, Port: port}, backendAddr)
 		if err != nil {
 			mapper.Unmap(ip, port, "udp")
 			return err
 		}
-		mapper.udpProxies[port] = proxy
+		mapper.udpProxies[mapKey] = proxy
 		go proxy.Run()
 	}
 	return nil
 }
 
 func (mapper *PortMapper) Unmap(ip net.IP, port int, proto string) error {
+	mapKey := (&net.TCPAddr{Port: port, IP: ip}).String()
 	if proto == "tcp" {
-		backendAddr, ok := mapper.tcpMapping[port]
+		backendAddr, ok := mapper.tcpMapping[mapKey]
 		if !ok {
-			return fmt.Errorf("Port tcp/%v is not mapped", port)
+			return fmt.Errorf("Port tcp/%s is not mapped", mapKey)
 		}
-		if proxy, exists := mapper.tcpProxies[port]; exists {
+		if proxy, exists := mapper.tcpProxies[mapKey]; exists {
 			proxy.Close()
-			delete(mapper.tcpProxies, port)
+			delete(mapper.tcpProxies, mapKey)
 		}
 		if mapper.iptables != nil {
 			if err := mapper.iptables.Forward(iptables.Delete, ip, port, proto, backendAddr.IP.String(), backendAddr.Port); err != nil {
 				return err
 			}
 		}
-		delete(mapper.tcpMapping, port)
+		delete(mapper.tcpMapping, mapKey)
 	} else {
-		backendAddr, ok := mapper.udpMapping[port]
+		backendAddr, ok := mapper.udpMapping[mapKey]
 		if !ok {
-			return fmt.Errorf("Port udp/%v is not mapped", port)
+			return fmt.Errorf("Port udp/%s is not mapped", mapKey)
 		}
-		if proxy, exists := mapper.udpProxies[port]; exists {
+		if proxy, exists := mapper.udpProxies[mapKey]; exists {
 			proxy.Close()
-			delete(mapper.udpProxies, port)
+			delete(mapper.udpProxies, mapKey)
 		}
 		if mapper.iptables != nil {
 			if err := mapper.iptables.Forward(iptables.Delete, ip, port, proto, backendAddr.IP.String(), backendAddr.Port); err != nil {
 				return err
 			}
 		}
-		delete(mapper.udpMapping, port)
+		delete(mapper.udpMapping, mapKey)
 	}
 	return nil
 }
@@ -321,12 +328,13 @@ func newPortMapper(config *DaemonConfig) (*PortMapper, error) {
 	}
 
 	mapper := &PortMapper{
-		tcpMapping: make(map[int]*net.TCPAddr),
-		tcpProxies: make(map[int]proxy.Proxy),
-		udpMapping: make(map[int]*net.UDPAddr),
-		udpProxies: make(map[int]proxy.Proxy),
-		iptables:   chain,
-		defaultIp:  config.DefaultIp,
+		tcpMapping:       make(map[string]*net.TCPAddr),
+		tcpProxies:       make(map[string]proxy.Proxy),
+		udpMapping:       make(map[string]*net.UDPAddr),
+		udpProxies:       make(map[string]proxy.Proxy),
+		iptables:         chain,
+		defaultIp:        config.DefaultIp,
+		proxyFactoryFunc: proxy.NewProxy,
 	}
 	return mapper, nil
 }
@@ -334,7 +342,7 @@ func newPortMapper(config *DaemonConfig) (*PortMapper, error) {
 // Port allocator: Automatically allocate and release networking ports
 type PortAllocator struct {
 	sync.Mutex
-	inUse    map[int]struct{}
+	inUse    map[string]struct{}
 	fountain chan int
 	quit     chan bool
 }
@@ -354,20 +362,22 @@ func (alloc *PortAllocator) runFountain() {
 }
 
 // FIXME: Release can no longer fail, change its prototype to reflect that.
-func (alloc *PortAllocator) Release(port int) error {
+func (alloc *PortAllocator) Release(addr net.IP, port int) error {
+	mapKey := (&net.TCPAddr{Port: port, IP: addr}).String()
 	utils.Debugf("Releasing %d", port)
 	alloc.Lock()
-	delete(alloc.inUse, port)
+	delete(alloc.inUse, mapKey)
 	alloc.Unlock()
 	return nil
 }
 
-func (alloc *PortAllocator) Acquire(port int) (int, error) {
-	utils.Debugf("Acquiring %d", port)
+func (alloc *PortAllocator) Acquire(addr net.IP, port int) (int, error) {
+	mapKey := (&net.TCPAddr{Port: port, IP: addr}).String()
+	utils.Debugf("Acquiring %s", mapKey)
 	if port == 0 {
 		// Allocate a port from the fountain
 		for port := range alloc.fountain {
-			if _, err := alloc.Acquire(port); err == nil {
+			if _, err := alloc.Acquire(addr, port); err == nil {
 				return port, nil
 			}
 		}
@@ -375,10 +385,10 @@ func (alloc *PortAllocator) Acquire(port int) (int, error) {
 	}
 	alloc.Lock()
 	defer alloc.Unlock()
-	if _, inUse := alloc.inUse[port]; inUse {
+	if _, inUse := alloc.inUse[mapKey]; inUse {
 		return -1, fmt.Errorf("Port already in use: %d", port)
 	}
-	alloc.inUse[port] = struct{}{}
+	alloc.inUse[mapKey] = struct{}{}
 	return port, nil
 }
 
@@ -391,7 +401,7 @@ func (alloc *PortAllocator) Close() error {
 
 func newPortAllocator() (*PortAllocator, error) {
 	allocator := &PortAllocator{
-		inUse:    make(map[int]struct{}),
+		inUse:    make(map[string]struct{}),
 		fountain: make(chan int),
 		quit:     make(chan bool),
 	}
@@ -546,25 +556,25 @@ func (iface *NetworkInterface) AllocatePort(port Port, binding PortBinding) (*Na
 	hostPort, _ := parsePort(nat.Binding.HostPort)
 
 	if nat.Port.Proto() == "tcp" {
-		extPort, err := iface.manager.tcpPortAllocator.Acquire(hostPort)
+		extPort, err := iface.manager.tcpPortAllocator.Acquire(ip, hostPort)
 		if err != nil {
 			return nil, err
 		}
 
 		backend := &net.TCPAddr{IP: iface.IPNet.IP, Port: containerPort}
 		if err := iface.manager.portMapper.Map(ip, extPort, backend); err != nil {
-			iface.manager.tcpPortAllocator.Release(extPort)
+			iface.manager.tcpPortAllocator.Release(ip, extPort)
 			return nil, err
 		}
 		nat.Binding.HostPort = strconv.Itoa(extPort)
 	} else {
-		extPort, err := iface.manager.udpPortAllocator.Acquire(hostPort)
+		extPort, err := iface.manager.udpPortAllocator.Acquire(ip, hostPort)
 		if err != nil {
 			return nil, err
 		}
 		backend := &net.UDPAddr{IP: iface.IPNet.IP, Port: containerPort}
 		if err := iface.manager.portMapper.Map(ip, extPort, backend); err != nil {
-			iface.manager.udpPortAllocator.Release(extPort)
+			iface.manager.udpPortAllocator.Release(ip, extPort)
 			return nil, err
 		}
 		nat.Binding.HostPort = strconv.Itoa(extPort)
@@ -596,16 +606,19 @@ func (iface *NetworkInterface) Release() {
 			continue
 		}
 		ip := net.ParseIP(nat.Binding.HostIp)
-		utils.Debugf("Unmaping %s/%s", nat.Port.Proto, nat.Binding.HostPort)
+		utils.Debugf("Unmaping %s/%s:%s", nat.Port.Proto, ip.String(), nat.Binding.HostPort)
 		if err := iface.manager.portMapper.Unmap(ip, hostPort, nat.Port.Proto()); err != nil {
 			log.Printf("Unable to unmap port %s: %s", nat, err)
 		}
+
 		if nat.Port.Proto() == "tcp" {
-			if err := iface.manager.tcpPortAllocator.Release(hostPort); err != nil {
+			if err := iface.manager.tcpPortAllocator.Release(ip, hostPort); err != nil {
 				log.Printf("Unable to release port %s", nat)
 			}
-		} else if err := iface.manager.udpPortAllocator.Release(hostPort); err != nil {
-			log.Printf("Unable to release port %s: %s", nat, err)
+		} else if nat.Port.Proto() == "udp" {
+			if err := iface.manager.tcpPortAllocator.Release(ip, hostPort); err != nil {
+				log.Printf("Unable to release port %s: %s", nat, err)
+			}
 		}
 	}
 
@@ -732,6 +745,7 @@ func newNetworkManager(config *DaemonConfig) (*NetworkManager, error) {
 	if err != nil {
 		return nil, err
 	}
+
 	udpPortAllocator, err := newPortAllocator()
 	if err != nil {
 		return nil, err
diff --git a/network_test.go b/network_test.go
index e2631ddcb7..184b497938 100644
--- a/network_test.go
+++ b/network_test.go
@@ -1,42 +1,52 @@
 package docker
 
 import (
+	"github.com/dotcloud/docker/iptables"
+	"github.com/dotcloud/docker/proxy"
 	"net"
 	"testing"
 )
 
 func TestPortAllocation(t *testing.T) {
+	ip := net.ParseIP("192.168.0.1")
+	ip2 := net.ParseIP("192.168.0.2")
 	allocator, err := newPortAllocator()
 	if err != nil {
 		t.Fatal(err)
 	}
-	if port, err := allocator.Acquire(80); err != nil {
+	if port, err := allocator.Acquire(ip, 80); err != nil {
 		t.Fatal(err)
 	} else if port != 80 {
 		t.Fatalf("Acquire(80) should return 80, not %d", port)
 	}
-	port, err := allocator.Acquire(0)
+	port, err := allocator.Acquire(ip, 0)
 	if err != nil {
 		t.Fatal(err)
 	}
 	if port <= 0 {
 		t.Fatalf("Acquire(0) should return a non-zero port")
 	}
-	if _, err := allocator.Acquire(port); err == nil {
+	if _, err := allocator.Acquire(ip, port); err == nil {
 		t.Fatalf("Acquiring a port already in use should return an error")
 	}
-	if newPort, err := allocator.Acquire(0); err != nil {
+	if newPort, err := allocator.Acquire(ip, 0); err != nil {
 		t.Fatal(err)
 	} else if newPort == port {
 		t.Fatalf("Acquire(0) allocated the same port twice: %d", port)
 	}
-	if _, err := allocator.Acquire(80); err == nil {
+	if _, err := allocator.Acquire(ip, 80); err == nil {
 		t.Fatalf("Acquiring a port already in use should return an error")
 	}
-	if err := allocator.Release(80); err != nil {
+	if _, err := allocator.Acquire(ip2, 80); err != nil {
+		t.Fatalf("It should be possible to allocate the same port on a different interface")
+	}
+	if _, err := allocator.Acquire(ip2, 80); err == nil {
+		t.Fatalf("Acquiring a port already in use should return an error")
+	}
+	if err := allocator.Release(ip, 80); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := allocator.Acquire(80); err != nil {
+	if _, err := allocator.Acquire(ip, 80); err != nil {
 		t.Fatal(err)
 	}
 }
@@ -311,3 +321,66 @@ func TestCheckNameserverOverlaps(t *testing.T) {
 		t.Fatalf("%s should not overlap %v but it does", netX, nameservers)
 	}
 }
+
+type StubProxy struct {
+	frontendAddr *net.Addr
+	backendAddr  *net.Addr
+}
+
+func (proxy *StubProxy) Run()                   {}
+func (proxy *StubProxy) Close()                 {}
+func (proxy *StubProxy) FrontendAddr() net.Addr { return *proxy.frontendAddr }
+func (proxy *StubProxy) BackendAddr() net.Addr  { return *proxy.backendAddr }
+
+func NewStubProxy(frontendAddr, backendAddr net.Addr) (proxy.Proxy, error) {
+	return &StubProxy{
+		frontendAddr: &frontendAddr,
+		backendAddr:  &backendAddr,
+	}, nil
+}
+
+func TestPortMapper(t *testing.T) {
+	var chain *iptables.Chain
+	mapper := &PortMapper{
+		tcpMapping:       make(map[string]*net.TCPAddr),
+		tcpProxies:       make(map[string]proxy.Proxy),
+		udpMapping:       make(map[string]*net.UDPAddr),
+		udpProxies:       make(map[string]proxy.Proxy),
+		iptables:         chain,
+		defaultIp:        net.IP("0.0.0.0"),
+		proxyFactoryFunc: NewStubProxy,
+	}
+
+	dstIp1 := net.ParseIP("192.168.0.1")
+	dstIp2 := net.ParseIP("192.168.0.2")
+	srcAddr1 := &net.TCPAddr{Port: 1080, IP: net.ParseIP("172.16.0.1")}
+	srcAddr2 := &net.TCPAddr{Port: 1080, IP: net.ParseIP("172.16.0.2")}
+
+	if err := mapper.Map(dstIp1, 80, srcAddr1); err != nil {
+		t.Fatalf("Failed to allocate port: %s", err)
+	}
+
+	if mapper.Map(dstIp1, 80, srcAddr1) == nil {
+		t.Fatalf("Port is in use - mapping should have failed")
+	}
+
+	if mapper.Map(dstIp1, 80, srcAddr2) == nil {
+		t.Fatalf("Port is in use - mapping should have failed")
+	}
+
+	if err := mapper.Map(dstIp2, 80, srcAddr2); err != nil {
+		t.Fatalf("Failed to allocate port: %s", err)
+	}
+
+	if mapper.Unmap(dstIp1, 80, "tcp") != nil {
+		t.Fatalf("Failed to release port")
+	}
+
+	if mapper.Unmap(dstIp2, 80, "tcp") != nil {
+		t.Fatalf("Failed to release port")
+	}
+
+	if mapper.Unmap(dstIp2, 80, "tcp") == nil {
+		t.Fatalf("Port already released, but no error reported")
+	}
+}