diff --git a/src/pkg/net/udpsock_plan9.go b/src/pkg/net/udpsock_plan9.go
index 66dcbab..4cf0ca3 100644
--- a/src/pkg/net/udpsock_plan9.go
+++ b/src/pkg/net/udpsock_plan9.go
@@ -164,7 +164,10 @@
 }
 
 // ListenUDP listens for incoming UDP packets addressed to the local
-// address laddr.  The returned connection c's ReadFrom and WriteTo
+// address laddr.  Net must be "udp", "udp4", or "udp6".  If laddr has
+// a port of 0, ListenUDP will choose an available port.
+// The LocalAddr method of the returned UDPConn can be used to
+// discover the port.  The returned connection's ReadFrom and WriteTo
 // methods can be used to receive and send UDP packets with per-packet
 // addressing.
 func ListenUDP(net string, laddr *UDPAddr) (*UDPConn, error) {