#!/bin/bash

gateway=`ip route | head -n 1 | grep -Eow '([0-9]{1,3}.){3}[0-9]{1,3}'`
[[ $gateway == '' ]] && exit 9

num1=`echo $gateway | awk -F '.' '{print $1}'`
num2=`echo $gateway | awk -F '.' '{print $2}'`
num3=`echo $gateway | awk -F '.' '{print $3}'`

for num4 in `seq 200 254`; do
    IP=$num1'.'$num2'.'$num3'.'$num4
    ping -c 2 $IP
    [[ $? -ne 0 ]] && break
done

 ip link add main2wrt link enp1s0 type macvlan mode bridge
ip addr add $IP dev main2wrt
ip link set main2wrt up
ip route add $IP dev main2wrt

ip link set main2wrt promisc on