diff --git a/bind8/lang/en b/bind8/lang/en
index a52e39543..a266de3d7 100644
--- a/bind8/lang/en
+++ b/bind8/lang/en
@@ -557,6 +557,7 @@ net_topol=Nameserver choice topology
 net_recur=Allow recursive queries from
 net_err=Failed to save address and topology options
 net_eport='$1' is not a valid port number
+net_eusedport=Only one line of addresses is allowed for port $1
 net_etopology=No topology addresses entered
 net_erecur=No addresses to allow recursion from entered
 
diff --git a/bind8/save_net.cgi b/bind8/save_net.cgi
index 38f14c5f9..fe37d6e82 100755
--- a/bind8/save_net.cgi
+++ b/bind8/save_net.cgi
@@ -20,6 +20,8 @@ if (!$in{'listen_def'}) {
 				&error(&text('net_eport', $in{"port_$i"}));
 			$l->{'values'} = [ 'port', $in{"port_$i"} ];
 			}
+		$port = $in{"pdef_$i"} ? 53 : $in{"port_$i"};
+		$used{$port}++ && &error(&text('net_eusedport', $port));
 		$l->{'members'} =
 			[ map { { 'name' => $_ } } split(/\s+/, $addr) ];
 		push(@listen, $l);