Default Route
A default route of a computer that is participating in
computer networking is the packet forwarding rule (route) taking effect when no
other route can be determined for a given Internet Protocol (IP) destination address.
All packets for destinations not established in the routing table are sent via
the default route. This route generally points to 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 process repeats until a packet is delivered to the destination.
Each router traversal counts as one hop in the distance calculation for the
transmission path.
The route evaluation process in each router uses the longest
prefix match method to obtain the most specific route. The network with the
longest subnet mask that matches the destination IP address is the next-hop
network gateway.
The default route in Internet Protocol Version 4 (IPv4) is
designated as the zero-address 0.0.0.0/0 in CIDR notation, often called the
quad-zero route. The subnet mask is given as /0, which effectively specifies
all networks, and is the shortest match possible. A route lookup that does not
match any other route, falls back to this route. Similarly, in IPv6, the
default route is specified by ::/0.
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 a network service provider. Therefore, packets with
destinations outside the organization's local area network, typically
destinations on the Internet or a wide area network, are forwarded to the
router with the connection to that provider.
The device to which the default route points is often called
the default gateway, and it often carries out other functions such as packet
filtering, firewalling, or proxy server operations.
Command : R1(config)#ip route 0.0.0.0 0.0.0.0 <exit
interface type> <exit interface number >
Or
R1(config)#ip route 0.0.0.0 0.0.0.0 <valid
next hop ip address>
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