Fix install script using IPv6 CIDR notation (#1570)

automated install/basic-install.sh

@@ -368,8 +368,8 @@ useIPv6dialog() {
   # Determine type of found IPv6 addresses
   for i in "${IPV6_ADDRESSES[@]}"; do
     result=$(testIPv6 "$i")
-    [[ "${result}" == "ULA" ]] && ULA_ADDRESS="$i"
-    [[ "${result}" == "GUA" ]] && GUA_ADDRESS="$i"
+    [[ "${result}" == "ULA" ]] && ULA_ADDRESS="${i%/*}"
+    [[ "${result}" == "GUA" ]] && GUA_ADDRESS="${i%/*}"
   done
 
   # Determine which address to be used: Prefer ULA over GUA or don't use any if none found