Autonomous System (AS) Ranges

You often heard about AS (Autonomous System) since the beginning of your BGP study. By now, you know that when we talk about AS we are referring to set or group of routers operating with same routing policy under a single administrative domain/control. The AS number should be unique to identify them in the internet. It is 16-bit in size allowing 65536 AS to be used in BGP. However, not all are available for use. Some are reserved for documentation purposes and the rest are divided for private and public use.

As of now, the 2 bytes AS that we used to know are already depleted so that’s why IETF introduced 4 bytes (0-4294967295) AS. In the router implementation, an ASdot notation or ASplain format will be used to identify a 32-bit AS number. The ASplain format is just writing the whole 0-4294967295 numbers. The problem with this is that it is prone to typo errors. The ASdot format will use this number: <upper 16 bits>.<lower 16 bits>. The first set of 65,536 numbers will be written as 0.0 to 0.65535 and the second set from 1.0 through to 1.65535, and so on.

AS 23456 will play a major part as it is reserved as interface between 16-bit and 32-bit AS. The 32-bit AS numbers will appear as AS 23456 in the new BGP attribute AS4 Path (AS Path on 16-bit). The 32-bit AS number is backward compatible with the 16-bit so one thing that is required is making sure the router supports 32-bit AS under the BGP command process.

If you are having a problem converting the ASplain format to ASdot format, you may try this conversion tool provided by APNIC: 32-Bit Conversion Tool. This tool converts Autonomous System numbers from dot notation to decimal or from decimal to dot notation.

For example: 100500 ASplain is equivalent to 1.34964 ASdot notation.

Below is the IANA assignment for the 32-bit notation: