diff --git a/src/net/lookup.go b/src/net/lookup.go
index a7ceee823f1..9008322dc5a 100644
--- a/src/net/lookup.go
+++ b/src/net/lookup.go
@@ -123,6 +123,9 @@ func lookupIPDeadline(host string, deadline time.Time) (addrs []IPAddr, err erro
 
 // LookupPort looks up the port for the given network and service.
 func LookupPort(network, service string) (port int, err error) {
+	if n, i, ok := dtoi(service, 0); ok && i == len(service) {
+		return n, nil
+	}
 	return lookupPort(network, service)
 }
 
diff --git a/src/net/port_test.go b/src/net/port_test.go
index 2dacd975e7a..258a5bda48f 100644
--- a/src/net/port_test.go
+++ b/src/net/port_test.go
@@ -27,6 +27,7 @@ var portTests = []struct {
 	{"tcp", "time", 37, true},
 	{"tcp", "domain", 53, true},
 	{"tcp", "finger", 79, true},
+	{"tcp", "42", 42, true},
 
 	{"udp", "echo", 7, true},
 	{"udp", "tftp", 69, true},
@@ -36,6 +37,7 @@ var portTests = []struct {
 	{"udp", "ntp", 123, true},
 	{"udp", "snmp", 161, true},
 	{"udp", "syslog", 514, true},
+	{"udp", "42", 42, true},
 
 	{"--badnet--", "zzz", 0, false},
 	{"tcp", "--badport--", 0, false},