Index: sys/netinet/in.c
===================================================================
--- sys/netinet/in.c
+++ sys/netinet/in.c
@@ -35,6 +35,8 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#define IN_HISTORICAL_NETS		/* include class masks */
+
 #include <sys/param.h>
 #include <sys/eventhandler.h>
 #include <sys/systm.h>
@@ -449,17 +451,24 @@
 		ia->ia_sockmask = *mask;
 		ia->ia_subnetmask = ntohl(ia->ia_sockmask.sin_addr.s_addr);
 	} else {
+		in_addr_t i = ntohl(addr->sin_addr.s_addr);
+
 		/*
-	 	 * If netmask isn't supplied, use default for now.
+	 	 * If netmask isn't supplied, use historical default.
 		 * This is deprecated for interfaces other than loopback
 		 * or point-to-point; warn in other cases.  In the future
 		 * we should return an error rather than warning.
 	 	 */
 		if ((ifp->if_flags & (IFF_POINTOPOINT | IFF_LOOPBACK)) == 0)
-			printf("%s: set address: WARNING: network mask"
-			     " should be specified; using default mask\n",
+			printf("%s: set address: WARNING: network mask "
+			     "should be specified; using historical default\n",
 			     ifp->if_xname);
-		ia->ia_subnetmask = IN_NETMASK_DEFAULT;
+		if (IN_CLASSA(i))
+			ia->ia_subnetmask = IN_CLASSA_NET;
+		else if (IN_CLASSB(i))
+			ia->ia_subnetmask = IN_CLASSB_NET;
+		else
+			ia->ia_subnetmask = IN_CLASSC_NET;
 		ia->ia_sockmask.sin_addr.s_addr = htonl(ia->ia_subnetmask);
 	}
 	ia->ia_subnet = ntohl(addr->sin_addr.s_addr) & ia->ia_subnetmask;
Index: sys/nfs/bootp_subr.c
===================================================================
--- sys/nfs/bootp_subr.c
+++ sys/nfs/bootp_subr.c
@@ -42,6 +42,8 @@
  *	$NetBSD: krpc_subr.c,v 1.10 1995/08/08 20:43:43 gwr Exp $
  */
 
+#define IN_HISTORICAL_NETS		/* include class masks */
+
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
@@ -1486,11 +1488,16 @@
 
 	if (ifctx->gotnetmask == 0) {
 		/*
-		 * If there is no netmask, use a default, but we really
-		 * need the right mask from the server.
+		 * If there is no netmask, use historical default,
+		 * but we really need the right mask from the server.
 		 */
 		printf("%s: no netmask received!\n", ifctx->ireq.ifr_name);
-		ifctx->netmask.sin_addr.s_addr = htonl(IN_NETMASK_DEFAULT);
+		if (IN_CLASSA(ntohl(ifctx->myaddr.sin_addr.s_addr)))
+			ifctx->netmask.sin_addr.s_addr = htonl(IN_CLASSA_NET);
+		else if (IN_CLASSB(ntohl(ifctx->myaddr.sin_addr.s_addr)))
+			ifctx->netmask.sin_addr.s_addr = htonl(IN_CLASSB_NET);
+		else
+			ifctx->netmask.sin_addr.s_addr = htonl(IN_CLASSC_NET);
 	}
 }