A default route is a catch-all route for routes not in the routing table.  It is often confused with default gateways, which provide end devices a router to send all non-local traffic for forwarding.

Maps versus Routing

Routing tables list the direction to reach a network.  When there is no path in the table, a default route can be used.  A default route is represented by all 0’s.

0.0.0.0/0

There are other methods to introduce a default route into router but a common method is to add a static route

Cisco IOS

ip route 0.0.0.0 0.0.0.0 ${Gateway-IP}

Comware

ip route-static 0.0.0.0 0.0.0.0 ${Gateway-IP}

Provision

ip route 0.0.0.0 0.0.0.0 ${Gateway-IP}

Brocade

ip route 0.0.0.0 0.0.0.0 ${Gateway-IP}

For Layer 2 switches, routing is not available so a default gateway is used to define where packets should be routed.