--- ip6rd.sh.orig	2021-05-01 10:02:07.979716992 +0200
+++ ip6rd.sh	2021-05-01 11:27:15.194443097 +0200
@@ -7,6 +7,7 @@
 ip6rd_depend()
 {
 	program ip
+	program ipv6calc
 	after interface
 }
 
@@ -58,17 +59,17 @@
 	fi
 	for addr in ${addrs}; do
 		# Strip the subnet
-		local ip="${addr%/*}" subnet="${addr#*/}"
+		local ip="${addr%/*}" subnet="$(_ip6rd_inet_get_network ${addr%/*} ${ipv4mask})"
 		# We don't work on private IPv4 addresses
 		if _ip6rd_inet_is_private_network "${ip}"
 		then
 			continue
 		fi
 
-		local ip6= ip6_prefix="${prefix%::/*}" ip6_subnet="${prefix#*/}"
-		ip6_subnet=$((ip6_subnet + (32-ipv4mask)))
-		eval ip6="$(printf "${ip6_prefix}:%s::%s" \
-		$(_ip6rd_prefix_shave_bits  ${ip} ${ipv4mask}) ${suffix})"
+		local ip6 ip6_subnet
+		ip6=$(ipv6calc --in ipv4addr --out ipv6addr --action 6rd_local_prefix --6rd_prefix "${prefix}" --6rd_relay_prefix "${subnet}/${ipv4mask}" "${ip}")
+		ip6_subnet=${ip6#*/}
+		ip6=${ip6%/*}
 		veinfo "Derived IPv6 address: ${ip6}"
 
 		# Now apply our IPv6 address to our config
@@ -90,7 +91,7 @@
 	if [ "${IFACE}" != "sit0" ]; then
 		_tunnel add "${IFACE}" mode sit ttl 255 remote any local "${localip}"
 	fi
-	_tunnel 6rd dev "${IFACE}" 6rd-prefix "${prefix}"
+	_tunnel 6rd dev "${IFACE}" 6rd-prefix "${prefix}" 6rd-relay_prefix "${subnet}/${ipv4mask}"
 	eend $? || return 1
 	_up
 
@@ -143,9 +144,8 @@
 _ip6rd_inet_get_network()
 {
 	local a=$(_ip6rd_inet_atoi $1)
-	local net=$(( a & ( (1<<$2)-1 ) ))
-	local cidr=$(( 32 - $2 ))
-	echo $(_ip6rd_inet_itoa $(( (net << cidr ) )) )
+	local net=$(( a & ( 0xffffffff << (32 - $2)) ))
+	echo $(_ip6rd_inet_itoa $net)
 }
 
 _ip6rd_inet_is_private_network()