diff --git a/p2p/nat/natpmp.go b/p2p/nat/natpmp.go
index 94ef1f4b68..5598b6da73 100644
--- a/p2p/nat/natpmp.go
+++ b/p2p/nat/natpmp.go
@@ -49,6 +49,9 @@ func (n *pmp) AddMapping(protocol string, extport, intport int, name string, lif
 	if lifetime <= 0 {
 		return 0, errors.New("lifetime must not be <= 0")
 	}
+	if extport == 0 {
+		extport = intport
+	}
 	// Note order of port arguments is switched between our
 	// AddMapping and the client's AddPortMapping.
 	res, err := n.c.AddPortMapping(strings.ToLower(protocol), intport, extport, int(lifetime/time.Second))
diff --git a/p2p/nat/natupnp.go b/p2p/nat/natupnp.go
index 673a0b36d6..b07f67f0b9 100644
--- a/p2p/nat/natupnp.go
+++ b/p2p/nat/natupnp.go
@@ -87,6 +87,13 @@ func (n *upnp) AddMapping(protocol string, extport, intport int, desc string, li
 	protocol = strings.ToUpper(protocol)
 	lifetimeS := uint32(lifetime / time.Second)
 
+	if extport == 0 {
+		extport = intport
+	} else {
+		// Only delete port mapping if the external port was already used by geth.
+		n.DeleteMapping(protocol, extport, intport)
+	}
+
 	err = n.withRateLimit(func() error {
 		return n.client.AddPortMapping("", uint16(extport), protocol, uint16(intport), ip.String(), true, desc, lifetimeS)
 	})
diff --git a/p2p/server_nat.go b/p2p/server_nat.go
index 933993bc1f..599fda8fa1 100644
--- a/p2p/server_nat.go
+++ b/p2p/server_nat.go
@@ -150,10 +150,7 @@ func (srv *Server) portMappingLoop() {
 					continue
 				}
 
-				external := m.port
-				if m.extPort != 0 {
-					external = m.extPort
-				}
+				external := m.extPort
 				log := newLogger(m.protocol, external, m.port)
 
 				log.Trace("Attempting port mapping")