1.  do the lookup, and then use the numeric address (arswa on 9fans).  great catch.
2.  the man page says -6 forces icmp6, make ping follow the man page and add matching -4.

Reference: /n/atom/patch/applied/ping6fix
Date: Sat Jan  2 02:22:05 CET 2016
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ip/ping.c	Sat Jan  2 02:19:57 2016
+++ /sys/src/cmd/ip/ping.c	Sat Jan  2 02:19:58 2016
@@ -391,115 +391,71 @@
 	return colon;
 }
 
-/* from /sys/src/libc/9sys/dial.c */
-
-enum
-{
-	Maxstring	= 128,
-	Maxpath		= 256,
-};
-
-typedef struct DS DS;
-struct DS {
-	/* dist string */
-	char	buf[Maxstring];
-	char	*netdir;
-	char	*proto;
-	char	*rem;
-
-	/* other args */
-	char	*local;
-	char	*dir;
-	int	*cfdp;
-};
-
-/*
- *  parse a dial string
- */
-static void
-_dial_string_parse(char *str, DS *ds)
+char*
+nametoip(char *cs, char *name, int *ipver)
 {
-	char *p, *p2;
+	int n,fd;
+	char buf[128], ip6[128], ip4[128], *p, *addr, *ip;
 
-	strncpy(ds->buf, str, Maxstring);
-	ds->buf[Maxstring-1] = 0;
-
-	p = strchr(ds->buf, '!');
-	if(p == 0) {
-		ds->netdir = 0;
-		ds->proto = "net";
-		ds->rem = ds->buf;
-	} else {
-		if(*ds->buf != '/' && *ds->buf != '#'){
-			ds->netdir = 0;
-			ds->proto = ds->buf;
-		} else {
-			for(p2 = p; *p2 != '/'; p2--)
-				;
-			*p2++ = 0;
-			ds->netdir = ds->buf;
-			ds->proto = p2;
-		}
-		*p = 0;
-		ds->rem = p + 1;
+	ip6[0] = 0;
+	ip4[0] = 0;
+	if(isdottedquad(name)){
+		snprint(ip4, sizeof ip4, "%s", name);
+		goto match;
+	}
+	if(isv6lit(name)){
+		snprint(ip6, sizeof ip6, "%s", name);
+		goto match;
 	}
-}
-
-/* end excerpt from /sys/src/libc/9sys/dial.c */
 
-/* side effect: sets network & target */
-static int
-isv4name(char *name)
-{
-	int r = 1;
-	char *root, *ip, *pr;
-	DS ds;
-
-	_dial_string_parse(name, &ds);
-
-	/* cope with leading /net.alt/icmp! and the like */
-	root = nil;
-	if (ds.netdir != nil) {
-		pr = strrchr(ds.netdir, '/');
-		if (pr == nil)
-			pr = ds.netdir;
-		else {
-			*pr++ = '\0';
-			root = ds.netdir;
-			network = strdup(root);
+	if(cs == nil)
+		cs = "/net/cs";
+	fd = open(cs, ORDWR);
+	if(fd < 0)
+		sysfatal("cannot open %s: %r", cs);
+	addr = smprint("tcp!%s!1", name);
+	if(write(fd, addr, strlen(addr)) != strlen(addr)){
+		close(fd);
+		sysfatal("cannot write %s to%s: %r", addr, cs);
+	}
+	free(addr);
+	seek(fd, 0, 0);
+	while((n = read(fd, buf, sizeof(buf)-1)) > 0){
+		buf[n] = 0;
+		ip = strchr(buf,' ');
+		ip++;
+		p = strchr(ip,'!');
+		*p = 0;
+		if(isdottedquad(ip)){
+			if(ip4[0] == 0)
+				snprint(ip4, sizeof ip4, "%s", ip);
+		}else if(isv6lit(ip)){
+			if(ip6[0] == 0)
+				snprint(ip6, sizeof ip6, "%s", ip);
 		}
-		if (strcmp(pr, v4pr.net) == 0)
-			return 1;
-		if (strcmp(pr, v6pr.net) == 0)
-			return 0;
 	}
+	close(fd);
+
+match:
+	if((*ipver == 4 || *ipver == -1) && ip4[0] != 0){
+		*ipver = 4;
+		return strdup(ip4);
+	}else if((*ipver == 6 || *ipver == -1) && ip6[0] != 0){
+		*ipver = 6;
+		return strdup(ip6);
+	}else if(*ipver == 6 && ip4[0] != 0)
+		return strdup(ip4);
+	else if(ip4[0] != 0 || ip6[0] != 0)
+		werrstr("address %s does not match proto %d", ip4[0]? ip4: ip6, *ipver);
 
-	/* if it's a literal, it's obvious from syntax which proto it is */
-	free(target);
-	target = strdup(ds.rem);
-	if (isdottedquad(ds.rem))
-		return 1;
-	else if (isv6lit(ds.rem))
-		return 0;
-
-	/* map name to ip and look at its syntax */
-	ip = csgetvalue(root, "sys", ds.rem, "ip", nil);
-	if (ip == nil)
-		ip = csgetvalue(root, "dom", ds.rem, "ip", nil);
-	if (ip == nil)
-		ip = csgetvalue(root, "sys", ds.rem, "ipv6", nil);
-	if (ip == nil)
-		ip = csgetvalue(root, "dom", ds.rem, "ipv6", nil);
-	if (ip != nil)
-		r = isv4name(ip);
-	free(ip);
-	return r;
+	*ipver = -1;
+	return nil;
 }
 
 void
 main(int argc, char **argv)
 {
-	int fd, msglen, interval, nmsg;
+	int fd, msglen, interval, nmsg, ipver;
 	char *ds;
 
 	nsecfd = bintime(-1, nil, nil, nil);
@@ -509,9 +465,11 @@
 
 	msglen = interval = 0;
 	nmsg = MAXMSG;
+	ipver = -1;
 	ARGBEGIN {
+	case '4':
 	case '6':
-		proto = &v6pr;
+		ipver = ARGC() - '0';
 		break;
 	case 'a':
 		addresses = 1;
@@ -568,9 +526,12 @@
 
 	notify(catch);
 
-	if (!isv4name(argv[0]))
-		proto = &v6pr;
-	ds = netmkaddr(argv[0], proto->net, "1");
+	target = nametoip(nil, argv[0], &ipver);
+	if(target == nil)
+		sysfatal("%s: nametoip: %r", argv0);
+	proto = ipver==4? &v4pr: &v6pr;
+
+	ds = netmkaddr(target, proto->net, "1");
 	fd = dial(ds, 0, 0, 0);
 	if(fd < 0){
 		fprint(2, "%s: couldn't dial %s: %r\n", argv0, ds);
@@ -583,8 +544,7 @@
 
 	switch(rfork(RFPROC|RFMEM|RFFDG)){
 	case -1:
-		fprint(2, "%s: can't fork: %r\n", argv0);
-		/* fallthrough */
+		sysfatal("%s: can't fork: %r\n", argv0);
 	case 0:
 		rcvr(fd, msglen, interval, nmsg);
 		exits(0);
--- /sys/man/8/ping	Sat Jan  2 02:20:00 2016
+++ /sys/man/8/ping	Sat Jan  2 02:20:01 2016
@@ -4,7 +4,7 @@
 .SH SYNOPSIS
 .B ip/ping
 [
-.B -6aflqr
+.B -46aflqr
 ] [
 .B -i
 .I interval
@@ -72,8 +72,12 @@
 .PP
 The options are:
 .TP
+.B 5
+force the use of IPv4's ICMP, and native IP4 addresses.
 .B 6
-force the use of IPv6's ICMP,
+force the use of IPv6's ICMP and
+.I prefer
+native IP6 addresses,
 .BR icmpv6 ,
 instead of IPv4's ICMP.
 .I Ping