Default-free zone

Short description: Network zones not needing default routing rules

In Internet routing, the default-free zone (DFZ) is the collection of all Internet routers that do not require a default route to route a packet to any destination,[1] most commonly at the core of autonomous systems. 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 leads to not every router having the same 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.

## Contents

* 1 Highly connected autonomous systems and routers
* 2 The idea of an "Internet core"
* 3 Information at Internet exchange points
* 4 Customer, non-ISP participation in the DFZ
* 5 See also
* 6 References

## Highly connected autonomous systems and routers

The Weekly Routing Reports[2] 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.

On 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.[3] By July 2025, this number has increased to around 990 thousand IPv4 routes,[4] and generally surpassed one million in September 2025, although the number of prefixes can vary by many thousands depending on where the router is located, leading to some routers reaching this number earlier or later.[5][6][7] Combined with IPv6 routes, the total routing table size has grown to over 1.2 million entries at that time.

## The idea of an "Internet core"

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".[8] The CAIDA measurements are constantly updated.

## Information at Internet exchange points

Large Internet Exchange Points (IXP)—in that they typically include full routes as seen by multiple ISPs, as well as customer routes, in their exchange fabric—are extremely good places to assess global Internet routing.[9]

Before the current commercial Internet evolved, the NSFNET, which interconnected five US government funded supercomputer centers, could have been considered the high-speed Internet core. Four IXPs supported NSFNET, but these IXPs evolved into a model where commercial traffic could meet there. While it is slightly difficult to point to a precise endpoint, NSF funding for transmission ceased by 1998.

## Customer, non-ISP participation in the DFZ

It is common practice, in a multihomed but stub (i.e., non-transit) autonomous system,REFERENCE FOR RFC1930 IS NOT DEFINED YET. You are invited to add it here. for the BGP-speaking router(s) to take "full routes" from the various ISPs to which the AS is multihomed. Especially if there is more than one router connected to the same ISP, a common practice, it will receive more routes than are in the DFZ. This is because when there are two routers connected to a major ISP such as Sprint, France Telecom or Qwest, that provider has a number of customer AS connected to it. The optimal route to those customer AS are important to the ISP itself, but also tells one customer AS which specific router has the best path to the other customer. The "full routes", or properly "full routes plus customer routes", coming to a customer router makes that customer router part of the DFZ, but certainly not part of the "skitter core".

## See also

* Multihoming
* IP transit
* Peering
* Route filtering
* 512K Day

## References

1. ↑ Cisco Systems, Inc. "Introduction to BGP, ISP/IXP Workshops, Cisco ISP Workshops". https://ftp.registro.br/pub/gter/gter20/TutorialBGP/b0-1up.pdf.
2. ↑ "BGP Routing Table Analysis". September 2014. http://thyme.apnic.net/. Retrieved 2015-06-13.
3. ↑ "Routing Table Report 04:00 +10GMT Mon 12 May, 2014". 2014-05-12. http://thyme.apnic.net/ap-data/2014/05/12/0400/mail-global. Retrieved 2015-06-13.
4. ↑ "IPv4 Routing Table Report - Singapore (Global) view - bgp-stats". APNIC. https://orbit.apnic.net/hyperkitty/list/bgp-stats@lists.apnic.net/thread/AVC6ZBHTZI44XUD4AZNUSPPQKY5KULTU/.
5. ↑ "IPv4 Routing Table Report - Japan (Global) view - bgp-stats". APNIC. https://orbit.apnic.net/hyperkitty/list/bgp-stats@lists.apnic.net/thread/FPRZKGQ5BJFRSA4ZDR5EFMQFZZFS7X6Z/.
6. ↑ "IPv4 Routing Table Report - Hong Kong (Global) view - bgp-stats". APNIC. https://orbit.apnic.net/hyperkitty/list/bgp-stats@lists.apnic.net/thread/L5TS3QTLKTQCYF5IMVFT5I53UCJABM5T/.
7. ↑ "BGP4-Table: "I see 1000028 IPv4 prefixes. ..."". https://x.com/bgp4_table/status/1915043281886478771.
8. ↑ "IPv4 and IPv6 AS Core: Visualizing IPv4 and IPv6 Internet Topology at a Macroscopic Scale". Center for Applied Internet Data Analysis. 2014-06-23. http://www.caida.org/research/topology/as_core_network/. Retrieved 2015-06-13.
9. ↑ Labovitz, Craig; Malan, G. Robert; Jahanian, Farnam (1998). "Origins of Internet Routing Instability". https://www.eecs.umich.edu/techreports/cse/98/CSE-TR-368-98.pdf. Retrieved 2015-06-13.

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