IP Unnumbered (Different major network)
IP Secondary
1.     Not enough host address for a particular network segment.
2.     Old networks (L2 Bridge), not subneted.
3.     Two subnets of a single network , otherwise be separated by another work.(likes In ip routing)
Routing Assistance when IP routing is disable.
1.     Proxy ARP
2.     Default Gateway
3.     ICMP Router Discovery Protocol