Autonomous System

An Autonomous System (AS) is a set of IP networks and routers under one administration entity and with common routing policies.

Each AS supports multiple IGPs. All the networks in an AS are assigned the same AS number and managed by the same administration group. Two types of AS numbers are available: a 2-byte AS number (ranges from 1 to 65535) and a 4-byte AS number (ranges from 1 to 4294967295). Available AS numbers can become exhausted and thereby 2-byte AS numbers need to be extended to 4-byte AS numbers. A 4-byte AS number can also be shown in the X.Y format, where X ranges from 1 to 65535 and Y ranges from 0 to 65535.

AS numbers are classified into public AS number and private AS number, depending on the network where they are used. Table 1 lists the two types of AS numbers and their ranges.

**Table 1** AS number types and ranges
AS Number Type	2-Byte AS Number	4-Byte AS Number
Public AS number	1 to 64511	1 to 64511, 65536 to 4294967295
Private AS number	64512 to 65535	64512 to 65535