diff --git a/sbin/ipfw/nptv6.c b/sbin/ipfw/nptv6.c
--- a/sbin/ipfw/nptv6.c
+++ b/sbin/ipfw/nptv6.c
@@ -175,20 +175,6 @@
 	free(p);
 }
 
-/* RFC 6296 Sec. 3.1 */
-static void
-nptv6_prefixlen_check(int len, int *plen)
-{
-	if (len == 0)
-		return;
-
-	if (*plen == 0)
-		*plen = len;
-	else if (len != *plen)
-		errx(EX_USAGE, "Prefix length mismatch (%d vs %d).",
-			len, *plen);
-}
-
 /*
  * Creates new nptv6 instance
  * ipfw nptv6 <NAME> create int_prefix <prefix> ext_prefix <prefix>
@@ -204,10 +190,9 @@
 	struct in6_addr mask;
 	ipfw_nptv6_cfg *cfg;
 	ipfw_obj_lheader *olh;
-	int tcmd, flags, plen, iplen, eplen, pplen;
+	int tcmd, flags, iplen, eplen, pplen;
 	char *p;
 
-	plen = 0;
 	iplen = 0;
 	eplen = 0;
 	pplen = 0;
@@ -224,7 +209,6 @@
 		case TOK_INTPREFIX:
 			NEED1("IPv6 prefix required");
 			nptv6_parse_prefix(*av, &cfg->internal, &iplen);
-			nptv6_prefixlen_check(iplen, &pplen);
 			flags |= NPTV6_HAS_INTPREFIX;
 			ac--; av++;
 			break;
@@ -234,7 +218,6 @@
 				    "Only one ext_prefix or ext_if allowed");
 			NEED1("IPv6 prefix required");
 			nptv6_parse_prefix(*av, &cfg->external, &eplen);
-			nptv6_prefixlen_check(eplen, &pplen);
 			flags |= NPTV6_HAS_EXTPREFIX;
 			ac--; av++;
 			break;
@@ -252,18 +235,25 @@
 			break;
 		case TOK_PREFIXLEN:
 			NEED1("IPv6 prefix length required");
-			plen = strtol(*av, &p, 10);
-			if (*p != '\0' || plen < 8 || plen > 64)
+			pplen = strtol(*av, &p, 10);
+			if (*p != '\0' || pplen < 8 || pplen > 64)
 				errx(EX_USAGE, "wrong prefix length: %s", *av);
-			nptv6_prefixlen_check(plen, &pplen);
 			flags |= NPTV6_HAS_PREFIXLEN;
 			ac--; av++;
 			break;
 		}
 	}
 
-	if ((flags & NPTV6_HAS_PREFIXLEN) != NPTV6_HAS_PREFIXLEN && pplen != 0) {
-		warnx("Use prefixlen instead");
+	/* RFC 6296 Sec. 3.1 */
+	if (pplen != 0) {
+		if ((eplen != 0 && eplen != pplen) || (iplen != 0 && iplen != pplen))
+			errx(EX_USAGE, "prefix length mismatch");
+		cfg->plen = pplen;
+	} else if (eplen != 0 || iplen != 0) {
+		if (eplen != 0 && iplen != 0 && eplen != iplen)
+			errx(EX_USAGE, "prefix length mismatch");
+		warnx("use prefixlen instead");
+		cfg->plen = eplen ? eplen : iplen;
 		flags |= NPTV6_HAS_PREFIXLEN;
 	}
 
@@ -275,8 +265,6 @@
 	if ((flags & NPTV6_HAS_PREFIXLEN) != NPTV6_HAS_PREFIXLEN)
 		errx(EX_USAGE, "prefixlen required");
 
-	cfg->plen = pplen;
-
 	n2mask(&mask, cfg->plen);
 	APPLY_MASK(&cfg->internal, &mask);
 	if ((cfg->flags & NPTV6_DYNAMIC_PREFIX) == 0)