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GCN Home > 08/25/08 issue

Networks face another switch

Autonomous System numbers expand to four bytes as current numbers run out

By William Jackson

Now that government network administrators have the IPv6 transition under their belts, a change in another Internet numbering system is under way. Faced with the imminent exhaustion of the current pool of two-byte Autonomous System (AS) numbers used with the Border Gateway Protocol, the Internet’s Number Resource Organization is moving to a new four-byte format.

Unlike the switch to larger IPv6 addresses, which is voluntary, the move to expanded AS numbers will occur by default.

An Autonomous System is a network or collection of networks administered as a single entity that share a common set of routing policies. Each organization announcing a routing policy on the Internet is assigned an AS number as a unique identifier. It forms an IP routing prefix used with the Border Gateway Protocol, which indicates the way traffic moves and among multiple autonomous networks.

The world’s five Regional Internet Registries, which comprise the Number Resource Organization, began issuing new four-byte numbers in 2007 and plans call for no more two-byte numbers to be issued after 2009. Without support from vendors, network operators risk having routers and network administration systems that will not accept the new number format.

“We want to make sure that when it happens, folks are ready for this,” said Richard Jimmerson, chief information officer of the American Registry for Internet Numbers, the regional registry for North America.

“Network operators should be checking with vendors now to make sure they’re ready for four-byte numbers,” said ARIN Chief Executive Officer Raymond Pizak. “If they’re not, they should be planning to get capable routers as soon as possible.”

AS numbers are managed centrally by the Internet Assigned Numbers Authority, which assigns them to the five Regional Internet Registries for assignment to users. In addition to ARIN in North America, the other regional registries are:
  • The RIPE Network Coordination Centre for Europe, the Middle East and Central Asia.
  • The Asia-Pacific Network Information Centre.
  • Latin American and Caribbean Network Information Centre.
  • African Network Information Centre.
IANA still holds about 25 percent of the original 65,536 two-byte AS numbers for distribution, but that pool is estimated to run dry by early 2011. The regional registries and the Internet Corporation for Assigned Names and Numbers are getting ahead of the issue by introducing a new four-byte format that will make more than 4 billion numbers available.

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