diff --git a/sbin/ipfw/ipfw.8 b/sbin/ipfw/ipfw.8
--- a/sbin/ipfw/ipfw.8
+++ b/sbin/ipfw/ipfw.8
@@ -1,5 +1,5 @@
 .\"
-.Dd April 18, 2025
+.Dd May 29, 2025
 .Dt IPFW 8
 .Os
 .Sh NAME
@@ -3834,11 +3834,11 @@
 .Pp
 The following parameters can be configured:
 .Bl -tag -width indent
-.It Cm int_prefix Ar ipv6_prefix
-IPv6 prefix used in internal network.
+.It Cm int_prefix Ar ipv6_netaddr
+IPv6 network address of prefix used in internal network.
 NPTv6 module translates source address when it matches this prefix.
-.It Cm ext_prefix Ar ipv6_prefix
-IPv6 prefix used in external network.
+.It Cm ext_prefix Ar ipv6_netaddr
+IPv6 network address of prefix used in external network.
 NPTv6 module translates destination address when it matches this prefix.
 .It Cm ext_if Ar nic
 The NPTv6 module will use first global IPv6 address from interface
diff --git a/sbin/ipfw/nptv6.c b/sbin/ipfw/nptv6.c
--- a/sbin/ipfw/nptv6.c
+++ b/sbin/ipfw/nptv6.c
@@ -155,28 +155,22 @@
 
 
 static void
-nptv6_parse_prefix(const char *arg, struct in6_addr *prefix, int *len)
+nptv6_parse_prefix(const char *arg, struct in6_addr *prefix)
 {
-	char *p, *l;
+	char *p;
 
 	p = strdup(arg);
 	if (p == NULL)
 		err(EX_OSERR, NULL);
-	if ((l = strchr(p, '/')) != NULL)
-		*l++ = '\0';
+	if (strchr(p, '/') != NULL)
+		errx(EX_USAGE,
+			"Bad IPv6 network address: %s\nUse prefixlen option instead", arg);
 	if (inet_pton(AF_INET6, p, prefix) != 1)
 		errx(EX_USAGE, "Bad prefix: %s", p);
-	if (l != NULL) {
-		*len = (int)strtol(l, &l, 10);
-		if (*l != '\0' || *len <= 0 || *len > 64)
-			errx(EX_USAGE, "Bad prefix length: %s", arg);
-	} else
-		*len = 0;
 	free(p);
 }
 /*
  * Creates new nptv6 instance
- * ipfw nptv6 <NAME> create int_prefix <prefix> ext_prefix <prefix>
  * Request: [ ipfw_obj_lheader ipfw_nptv6_cfg ]
  */
 #define	NPTV6_HAS_INTPREFIX	0x01
@@ -204,22 +198,18 @@
 
 		switch (tcmd) {
 		case TOK_INTPREFIX:
-			NEED1("IPv6 prefix required");
-			nptv6_parse_prefix(*av, &cfg->internal, &plen);
+			NEED1("IPv6 network address required");
+			nptv6_parse_prefix(*av, &cfg->internal);
 			flags |= NPTV6_HAS_INTPREFIX;
-			if (plen > 0)
-				goto check_prefix;
 			ac--; av++;
 			break;
 		case TOK_EXTPREFIX:
 			if (flags & NPTV6_HAS_EXTPREFIX)
 				errx(EX_USAGE,
 				    "Only one ext_prefix or ext_if allowed");
-			NEED1("IPv6 prefix required");
-			nptv6_parse_prefix(*av, &cfg->external, &plen);
+			NEED1("IPv6 network address required");
+			nptv6_parse_prefix(*av, &cfg->external);
 			flags |= NPTV6_HAS_EXTPREFIX;
-			if (plen > 0)
-				goto check_prefix;
 			ac--; av++;
 			break;
 		case TOK_EXTIF:
@@ -237,16 +227,8 @@
 		case TOK_PREFIXLEN:
 			NEED1("IPv6 prefix length required");
 			plen = strtol(*av, &p, 10);
-check_prefix:
 			if (*p != '\0' || plen < 8 || plen > 64)
 				errx(EX_USAGE, "wrong prefix length: %s", *av);
-			/* RFC 6296 Sec. 3.1 */
-			if (cfg->plen > 0 && cfg->plen != plen) {
-				warnx("Prefix length mismatch (%d vs %d).  "
-				    "It was extended up to %d",
-				    cfg->plen, plen, MAX(plen, cfg->plen));
-				plen = MAX(plen, cfg->plen);
-			}
 			cfg->plen = plen;
 			flags |= NPTV6_HAS_PREFIXLEN;
 			ac--; av++;