default route
termThe route of last resort - 0.0.0.0/0 - matching everything no more specific route claims; the packet's way out of the building.
On hosts it is the 'default gateway'; on the internet's core routers there famously is none - the default-free zone.
RFC 1812 defines it as the route whose prefix length is zero, last in the order of preference, so it matches every destination and loses to every longer prefix. RFC 4632's table lists 0.0.0.0/0, 4,294,967,296 addresses, as the default route and requires every implementation to accept it, while advising that it be advertised to another routing domain only when a router is explicitly configured to do so. IPv6 writes it ::/0, and Linux simply says default.
Hosts learn it by hand, from DHCP option 3 or option 121, which makes the client ignore option 3, from ICMP Router Discovery, or on IPv6 from a Router Advertisement whose Router Lifetime is not zero. IS-IS installs one too: a Level 1 router points a default at the nearest Level 1-2 router that sets the attached bit.
Its failures are quiet. A wrong or dead default leaves the local prefix working, since on-link destinations never touch the gateway, while everything else vanishes into a black hole, and a rogue DHCP server or Router Advertisement can hand hosts a bogus default. A router that advertises a summary must not follow its default for parts of that summary, which is why it installs a discard route.
Sources
- RFC 1812, Requirements for IP Version 4 Routers, June 1995: section 5.2.4.3 and Appendix B (read 2026-10-19)
- RFC 4632, Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan, August 2006: sections 3.1, 5.1 and 5.2 (read 2026-10-19)
- RFC 3442, The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4, December 2002: DHCP Client Behavior (read 2026-10-19)