#!/bin/sh

gen_filter()
{
  IFS=. read a b c d
  [ "$a" -a "$b" -a "$c" -a "$d" ] || return
  cprime=$((c+1))
  echo s/$a\\.$b\\.$c\\.\\\([0-9]*\\\)/$a.$b.$cprime.\\\\1/g
}

get_default()
{
  ip route | grep '^default via 192\.168\.[^.]*\.1' |
    (read default via what ip; [ "$what" ] && echo $what)
}

get_default | gen_filter