Networks face another switch

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

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.

The registries began issuing the new numbers on a request- only basis in January 2007, with the two-byte numbers remaining the default standard. In January 2009, registries will begin issuing the four-byte numbers by default, and the old format will be issued only by request until January 2010. Present plans call for registries to cease issuing two-byte numbers completely at that point.

That policy could change, Jimmerson said, 'but it is believed that by that time, everyone will have hardware and firmware in place that will recognize the new numbers.'

Most routing and network management equipment being sold now is ready for the new format or is capable of accepting a firmware upgrade to make it ready. However, network administrators will have to make sure their infrastructure is ready to accept the new numbers as they go into use. The federal government has not yet established a procurement policy for four-byte-ready networking equipment or requirements for checking readiness of current equipment.

New equipment still will handle two-byte AS numbers, which will remain in use. If older, unprepared equipment encounters new numbers, it probably will not be a fatal problem, Jimmerson said. 'You will eventually get through.' But it could create problems with using the Border Gateway Protocols as the traffic works around the problem.

Although they have been available for 18 months, 'there has not been much uptake yet in the four-byte numbers,' Jimmerson said. Unlike the new IPv6 protocols, there are no operational advantages to using the extended AS numbers; they only extend the life of the system.

About the Author

William Jackson is a Maryland-based freelance writer.


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