diff --git a/ipam/allocate.go b/ipam/allocate.go
index c11f75ed..72e3af69 100644
--- a/ipam/allocate.go
+++ b/ipam/allocate.go
@@ -25,15 +25,15 @@ func (g *allocate) Try(alloc *Allocator) bool {
 		return true
 	}
 
-	if addr, found := alloc.ownedInRange(g.ident, g.r); found {
+	if addrs := alloc.ownedInRange(g.ident, g.r); len(addrs) > 0 {
 		// If we had heard that this container died, resurrect it
 		delete(alloc.dead, g.ident) // delete is no-op if key not in map
-		g.resultChan <- allocateResult{addr, nil}
+		g.resultChan <- allocateResult{addrs[0], nil}
 		return true
 	}
 
 	if !alloc.universe.Overlaps(g.r) {
-		g.resultChan <- allocateResult{0, fmt.Errorf("range %s out of bounds: %s", g.r, alloc.universe)}
+		g.resultChan <- allocateResult{err: fmt.Errorf("range %s out of bounds: %s", g.r, alloc.universe)}
 		return true
 	}
 
@@ -66,7 +66,7 @@ func (g *allocate) Try(alloc *Allocator) bool {
 }
 
 func (g *allocate) Cancel() {
-	g.resultChan <- allocateResult{0, &errorCancelled{"Allocate", g.ident}}
+	g.resultChan <- allocateResult{err: &errorCancelled{"Allocate", g.ident}}
 }
 
 func (g *allocate) ForContainer(ident string) bool {
diff --git a/ipam/allocator.go b/ipam/allocator.go
index 4ea0eb81..235ab65d 100644
--- a/ipam/allocator.go
+++ b/ipam/allocator.go
@@ -203,18 +203,13 @@ func (alloc *Allocator) Allocate(ident string, r address.Range, hasBeenCancelled
 	return result.addr, result.err
 }
 
-// Lookup (Sync) - get existing IP address for container with given name in range
-func (alloc *Allocator) Lookup(ident string, r address.Range) (address.Address, error) {
-	resultChan := make(chan allocateResult)
+// Lookup (Sync) - get existing IP addresses for container with given name in range
+func (alloc *Allocator) Lookup(ident string, r address.Range) ([]address.Address, error) {
+	resultChan := make(chan []address.Address)
 	alloc.actionChan <- func() {
-		if addr, found := alloc.ownedInRange(ident, r); found {
-			resultChan <- allocateResult{addr: addr}
-			return
-		}
-		resultChan <- allocateResult{err: fmt.Errorf("lookup: no address found for %s in range %s", ident, r)}
+		resultChan <- alloc.ownedInRange(ident, r)
 	}
-	result := <-resultChan
-	return result.addr, result.err
+	return <-resultChan, nil
 }
 
 // Claim an address that we think we should own (Sync)
@@ -861,13 +856,14 @@ func (alloc *Allocator) removeOwned(ident string, addrToFree address.Address) bo
 	return false
 }
 
-func (alloc *Allocator) ownedInRange(ident string, r address.Range) (address.Address, bool) {
+func (alloc *Allocator) ownedInRange(ident string, r address.Range) []address.Address {
+	var a []address.Address
 	for _, addr := range alloc.owned[ident] {
 		if r.Contains(addr) {
-			return addr, true
+			a = append(a, addr)
 		}
 	}
-	return 0, false
+	return a
 }
 
 func (alloc *Allocator) findOwner(addr address.Address) string {
diff --git a/ipam/allocator_test.go b/ipam/allocator_test.go
index c9eef59c..55a631e7 100644
--- a/ipam/allocator_test.go
+++ b/ipam/allocator_test.go
@@ -49,6 +49,10 @@ func TestAllocFree(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, testAddr2, addr2.String(), "address")
 
+	addrs, err := alloc.Lookup(container1, subnet)
+	require.NoError(t, err)
+	require.Equal(t, []address.Address{addr1, addr2}, addrs)
+
 	// Ask for another address for a different container and check it's different
 	addr1b, _ := alloc.Allocate(container2, cidr1.HostRange(), returnFalse)
 	if addr1b.String() == testAddr1 {
diff --git a/ipam/http.go b/ipam/http.go
index eb51b87a..2cfc26fa 100644
--- a/ipam/http.go
+++ b/ipam/http.go
@@ -29,6 +29,15 @@ func parseCIDR(w http.ResponseWriter, cidrStr string) (address.CIDR, bool) {
 	return cidr, true
 }
 
+func writeAddresses(w http.ResponseWriter, addrs []address.Address, subnet address.CIDR) {
+	for i, addr := range addrs {
+		fmt.Fprintf(w, "%s/%d", addr, subnet.PrefixLen)
+		if i < len(addrs)-1 {
+			w.Write([]byte{' '})
+		}
+	}
+}
+
 func (alloc *Allocator) handleHTTPAllocate(dockerCli *docker.Client, w http.ResponseWriter, ident string, checkAlive bool, subnet address.CIDR) {
 	closedChan := w.(http.CloseNotifier).CloseNotify()
 	addr, err := alloc.Allocate(ident, subnet.HostRange(),
@@ -80,22 +89,22 @@ func (alloc *Allocator) HandleHTTP(router *mux.Router, defaultSubnet address.CID
 	router.Methods("GET").Path("/ip/{id}/{ip}/{prefixlen}").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		vars := mux.Vars(r)
 		if subnet, ok := parseCIDR(w, vars["ip"]+"/"+vars["prefixlen"]); ok {
-			addr, err := alloc.Lookup(vars["id"], subnet.HostRange())
+			addrs, err := alloc.Lookup(vars["id"], subnet.HostRange())
 			if err != nil {
 				http.NotFound(w, r)
 				return
 			}
-			fmt.Fprintf(w, "%s/%d", addr, subnet.PrefixLen)
+			writeAddresses(w, addrs, subnet)
 		}
 	})
 
 	router.Methods("GET").Path("/ip/{id}").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		addr, err := alloc.Lookup(mux.Vars(r)["id"], defaultSubnet.HostRange())
+		addrs, err := alloc.Lookup(mux.Vars(r)["id"], defaultSubnet.HostRange())
 		if err != nil {
 			http.NotFound(w, r)
 			return
 		}
-		fmt.Fprintf(w, "%s/%d", addr, defaultSubnet.PrefixLen)
+		writeAddresses(w, addrs, defaultSubnet)
 	})
 
 	router.Methods("POST").Path("/ip/{id}/{ip}/{prefixlen}").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
diff --git a/weave b/weave
index 6364bcfd..2d6d4135 100755
--- a/weave
+++ b/weave
@@ -1343,7 +1343,7 @@ ipam_cidrs() {
                     echo "IP address allocation must be enabled to use 'net:'" >&2
                     return 1
                 fi
-            elif ! is_cidr "$CIDR" ; then
+            elif [ -n "$CIDR" ] && ! is_cidr "$CIDR" ; then
                 echo "$CIDR" >&2
                 return 1
             else