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In Internet routing, a default-free zone (DFZ) is a set of all autonomous systems that do not require a default route to send a destination packet. Routers are used to route data packets across computer networks according to a packet’s destination address or protocol format details. A packet with a unique destination address that does not match an available route is dropped by routers.
In a DFZ, Internet routers only support routes that are defined in their routing information bases, which are usually obtained from local configuration and dynamic routing protocols when there is not a default route in the routing information chart.
The DFZ is defined as a set of all Internet networks that are operated without a default route. DFZ routers have complete Border Gateway Protocol (BGP) tables. It is impossible for a single router to have an absolute view of all existing routes, although such routing tables may be observed from the viewpoint of different routers - even under stable conditions.
The geographical scope of any particular network, transit status or peering of such large networks does not change the absence or presence of a default route.
End-user networks obtain global connectivity through Internet Service Providers (ISP) that use a default route to connect to larger ISPs. Smaller networks usually have limited options to handle Internet traffic. On the other hand, large Internet exchange points that include complete routes of multiple ISPs use BGP for their default-free zone.
In common practice, when more than one router is connected to a large ISP - like Sprint or Q West - it receives more routes that are available in the DFZ. Usually, ISPs are connected to a number of customer multihomed anonymous systems, which guide a customer toward a specific route to connect with another customer.