Default-free zone

In Internet routing, the default-free zone (DFZ) is the collection of all Internet autonomous systems (AS) that do not require a default route to route a packet to any destination. Conceptually, DFZ routers have a "complete" Border Gateway Protocol table, sometimes referred to as the Internet routing table, global routing table or global BGP table. However, internet routing changes rapidly and the widespread use of route filtering ensures that no router has a complete view of all routes. Any routing table created would look different from the perspective of different routers, even if a stable view could be achieved. The Weekly Routing Reports used by the ISP community come from the Asia-Pacific Network Information Centre (APNIC) router in Tokyo, which is a well-connected router that has as good a view of the Internet as any other single router. For serious routing research, however, routing information will be captured at multiple well-connected sites, including high-traffic ISPs (see the "skitter core") below. As of May 12, 2014, there were 494,105 routes seen by the APNIC router. These came from 46,795 autonomous systems, of which only 172 were transit-only and 35787 were stub/origin-only. 6087 autonomous systems provided some level of transit. The term "default-free zone" is sometimes confused with an "Internet core" or Internet backbone, but there has been no true "core" since before the Border Gateway Protocol (BGP) was introduced. In pre-BGP days, when the Exterior Gateway Protocol (EGP) was the exterior routing protocol, it indeed could be assumed there was a single Internet core. That concept, however, has been obsolete for a long time. At best, today's definition of the Internet core is statistical, with the "skitter core" being some number of AS with the greatest traffic according to the CAIDA measurements, previously made with its measuring tool called "skitter". The CAIDA measurements are constantly updated.