diff -Nru /n/sources/plan9/sys/src/cmd/ip/ipconfig/main.c /sys/src/cmd/ip/ipconfig/main.c
--- /n/sources/plan9/sys/src/cmd/ip/ipconfig/main.c	Mon Jun 24 22:35:14 2013
+++ /sys/src/cmd/ip/ipconfig/main.c	Sat Dec  6 00:00:00 2014
@@ -1227,6 +1227,8 @@
 		if(!validip(conf.mask) || !Oflag){
 			if(!optgetaddr(bp->optdata, OBmask, conf.mask))
 				ipmove(conf.mask, IPnoaddr);
+			if(ipcmp(conf.mask, IPv4bcast) == 0)
+				ipmove(conf.mask, IPnoaddr);
 		}
 		DEBUG("ipaddr=%I ipmask=%M ", conf.laddr, conf.mask);