In routing, one route may be defined as the one to be followed by all packets whose destination address does not match any prefix in the routing table. A common application is to have explicit routes for all destinations within a routing domain, but to have the default take all packets either to an internal "backbone of backbones" connecting several routing domains, or to the public Internet. Another common use is in hierarchical networks, where most destinations will be servers within the same section of the network, such as an OSPF nonzero area.
By convention, a default route is shown with a prefix length of zero, as in the Internet Protocol version 4 route
0.0.0.0/0. Some routing protocols will generate a default route as part of hierarchical routing, as does OSPF for a stub area. Most often, default routes are hard-coded static routes. Different static routes can be specified, either in order of preference for backup, or to have several closest path exits.
- Baker, F., ed. (June 1995), Requirements for IP Version 4 Routers, Intenet Engineering Task Force, RFC1812