Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 103
Default Router and Gateways

On a TCP/IP network, there is a concept known as the default route. Except for proprietary implementations, this is not part of any other network protocol (XNS, AppleTalk, IPX, etc.). The default route can be maintained in two places: the router and the endstation.

For an endstation that does not support the active or passive functions of the RIP protocol, thereby allowing it to find a route dynamically, the default router (commonly called a default gateway) is assigned to it. This is the 32-bit address of the router the workstation should route to if remote routing is necessary. The IP layer in the endstation would determine that the destination network is not local and that the services of a router must be used. Instead of implementing the RIP protocol, the endstation may submit the packet to the default router as assigned by the default route number. The router will take care of ensuring the packet will reach its final destination. If that router does not have the best route, it will send a message (using the ICMP protocol) to the endstation to inform it of a better route. This will be explained later.

A router may also be assigned a default route. It is indicated as 0.0.0.0 in its routing table. There is no subnet mask associated with it. This is implemented for when a router receives a packet and does not have the network number in its table. The router will forward the packet to another router for which it has an assigned default route. This means that when a router has received a packet to route, and its table does not contain the network number indicated in the received packet, it will forward the packet to its default router, hoping that the default router will have the network number in its table and will be able to properly forward the packet. The default router will receive the packet and, if the network number is in its table, it will forward the packet. If the network number is not in its table with the best route, it, too, may have a default router, and it will forward the packet to that router. If there is no route and there is not another default route, the last router will send a control message (through ICMP) back to the originating station indicating it could not forward the packet.

The problem with default routes in workstations is that a workstation’s default router may go down and the workstation will not know if there is another router on the network. The network number may change or there may be a better path for the workstation to take. The default gateway allows for the elimination of routing tables in the network station and routers by allowing groups of networks to become available through the default route.

Default Router and Gateways