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A default route, also known as the gateway of last resort, is the network route used by a router when no other known route exists for a given IP packet's destination address. All the packets for destinations not known by the router's routing table are sent to the default route. This route generally leads to another router, which treats the packet the same way: If the route is known, the packet will get forwarded to the known route. If not, the packet is forwarded to the default-route of that router which generally leads to another router. And so on. Each router traversal adds a one-hop distance to the route.
Once the router with a known route to a host destination is reached, the router determines which route is valid by finding the "most specific match". The network with the longest subnet mask that matches the destination IP address wins.
The default route in IPv4 (in CIDR notation) is 0.0.0.0/0, often called the quad-zero route. Since the subnet mask given is /0, it effectively specifies no network, and is the "shortest" match possible. A route lookup that doesn't match anything will naturally fall back onto this route. Similarly, in IPv6 the default address is given by ::/0.
Routers in an organization generally point the default route towards the router that has a connection to a network service provider. This way, packets with destinations outside the organization's local area network (LAN)—typically to the Internet, WAN, or VPN—will be forwarded by the router with the connection to that provider.
Default router should be on the same network
Default router should be on the same network
Once it is routed outside the network, if that router does not know the route of the destination, it will forward it to its own Default Route, which is usually a router connected to larger number of networks. Similarly, the packet will progress to internet backbone if still no route is known about the destination IP. It is then considered that the network does not exist, and the packet is discarded.
Host devices in an organization generally refer to the default route as a default gateway which can be, and usually is, a filtration device such as a firewall or Proxy server.
There is no uniformity between Unix flavors in storing default route information:
/etc/sysconfig/networkor other file in sysconfig tree. Here is the content of
/etc/systconfig/networkfile for RHEL:
NETWORKING=yes NETWORKING_IPV6=no HOSTNAME=unlab12 GATEWAY=10.194.144.1
created: 2003-07-11 00:00:00 last updated: 2003-09-29 13:31:07
Step 1: edit /etc/defaultrouter
Using the editor of your choice, edit the file /etc/defaultrouter -- the only line in the file should be the default route of the system, for example: 192.168.1.1. This change will not take effect until the system is rebooted.
Step 2: Delete the current default route
To make the route change take effect immediately, you must first delete the default route. If the current default route is 192.168.254.1, then the command would be:
route delete default 192.168.254.1
Step 3: Assign the current default route
To implement the new default route without rebooting the system, use the following command substituting your default route for 192.168.1.1:
route add default 192.168.1.1
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