Pense bête routing :

route -n
route add -net 10.0.101.0 netmask 255.255.255.0 gw 12.34.56.1
route add -net 10.0.100.0 netmask 255.255.255.0 gw 12.34.56.1
route delete -net default gw 12.34.56.1
route del -net 192.168.100.0 gw 12.34.56.1 netmask 0.0.0.0 dev eth0
route add -host 98.76.54.32 gw 12.34.56.1

IP route

ip route show
ip route del default via 12.34.56.1 dev ppp0 scope link
ip route add 10.0.0.0/1 via 12.34.56.1 dev ppp0 scope link
ip route add 10/1 via 12.34.56.1 dev ppp0 scope link
ip route add 10.0.0.0/16 via 12.34.56.1 dev ppp0 scope link