post-up route add -net <network identifier> netmask <netmask> gw <links gateway>
pre-down route del -net <network identifier> netmask <netmask> gw <links gateway>

iptables -t nat -A POSTROUTING -s 172.16.0.2/24 -j SNAT --to-source <ip address you want it to go out of>

iptables -t nat -A PREROUTING -d <ip/mask bit> -j DNAT --to-destination 172.16.0.2