Editing Border Gateway Protocol (section)

=== AS number depletion and 32-bit ASNs<span class="anchor" id="AS23456"></span> ===
The RFC 1771 BGP-4 specification coded AS numbers on 16 bits, for 64,510 possible public AS numbers.{{efn|ASN 64512 to 65534 were reserved for private use and 0 and 65535 are forbidden.}} In 2011, only 15,000 AS numbers were still available, and projections<ref>[http://www.potaroo.net/tools/asn16/ ''16-bit Autonomous System Report''], Geoff Huston 2011 (original archived at https://web.archive.org/web/20110906085724/http://www.potaroo.net/tools/asn16/)</ref> were envisioning a complete depletion of available AS numbers in September 2013.

RFC 6793 extends AS coding from 16 to 32 bits,{{efn|The 16-bit AS range 0 to 65535 and its reserved AS numbers are retained.}} which now allows up to 4 billion available AS. An additional private AS range is also defined in RFC 6996.{{efn|ASN 4200000000 to 4294967294 are private and 4294967295 is forbidden by RFC 7300.}} To allow the traversal of router groups not able to manage those new ASNs, the new attribute '''AS4_PATH''' (optional transitive) and the special 16-bit ASN '''AS_TRANS''' (AS23456) is used.<ref>{{Cite RFC|rfc=4893|title=BGP Support for Four-octet AS Number Space|last1=Vohra|first1=Quaizar|last2=Chen|first2=Enke||publisher=[[IETF]]|date=May 2007}}</ref> 32-bit ASN assignments started in 2007.