11.6 Autonomous System Numbers

CCNP RS 300-101, Establishing Internet Connectivity, Uncategorized

An Autonomous System (AS) is a collection of routing prefixes (and routing policies thereof) under the same administrative control.

AS numbers are used by BGP (path vector) to determine shortest path toward a destination network.

Facts:

The following AS numbers are reserved by the IANA and should never be used:

  • 0
  • 65535
  • 4,294,967,295

Prior to 2007, the ASN was a 16-bit number (0 to 65536), but in 2007, was changed to a 32-bit number (0 to 4,294,967,295) that is represented as two 16-bit ASNs concatenated with a dot (example: 1.61542).

Private ASN range:

  • 64512 through 65534
  • 65536 through 65551 (1.0 – 1.15)

Note: The reserved and private ASNs cannot be assigned to an organization by a RIR or ISP.

Published