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Incomputer networking, thedefault route is a configuration of theInternet Protocol (IP) that establishes a forwarding rule forpackets when no specific address of a next-hop host is available from therouting table or other routing mechanisms.
The default route is generally the address of another router, which treats the packet the same way: if a route matches, the packet is forwarded accordingly, otherwise the packet is forwarded to the default route of that router. The route evaluation process in each router uses thelongest prefix match method to obtain the most specific route. The network with the longestsubnet mask or network prefix that matches the destinationIP address is the next-hop network gateway. The process repeats until a packet is delivered to the destination host, or earlier along the route, when a router has no default route available and cannot route the packet otherwise. In the latter case, the packet is dropped and anICMPDestination Unreachable message may be returned.[1] Each router traversal counts as one hop in the distance calculation for the transmission path.
The device to which the default route points is often called thedefault gateway, and it often carries out other functions such as packet filtering,firewalling, orproxy server operations.
The default route inInternet Protocol Version 4 (IPv4) is designated as the zero address,0.0.0.0/0 inCIDR notation.[2] Similarly, inIPv6, the default route is specified by::/0. The subnet mask is specified as/0, which effectively specifies all networks and is the shortest match possible. A route lookup that does not match any other rule falls back to this route.
In the highest-level segment of a network, administrators generally point the default route for a given host towards the router that has a connection to anetwork service provider. Therefore, packets with destinations outside the organization'sLAN, typically destinations on theInternet or awide area network, are forwarded to the router with the connection to that provider.
Note that the degenerate route to prefix 0.0.0.0/0 is used as a default route and MUST be accepted by all implementations.