Skip to content

4 Byte BGP Autonomous System Numbers

Like IPv4 Address space depletion, the 2-Byte (16 bit) BGP AS number is also running out. As per RFC4893 (BGP Support for Four-octet AS Number Space) 4-Byte Autonomous Systems (AS) numbers have been issued by the Regional Internet Registry (RIR).

2-Byte (16 Bit) Autonomous System Numbers
We have a total of 216 = 65536 Possible AS Numbers
Private AS Numbers: 64512 – 65534
Reserved AS Numbers: 59392 – 64511, 65535

4-Byte (16 Bit) Autonomous System Numbers
We have a total of 232 = 4,294,967,296 Possible AS Numbers
Any numbers ranging from 65536 to 4294967295 are considered 32Bit AS Numbers.

This is the IETF preferred notation of AS Numbers, where a 2-Byte AS Number such as 65535 is represented in the form of text in both command and CLI. Where a 4-Byte As number such as 65546 will be represented in the form of 65546

As mentioned above, the ASDOT notation for the 2-Byte AS Numbers are represented in decimal format.
4-Byte AS Numbers is represented in the following format.

For example, if we take the 4-Byte AS Number 65546, it will be 1.10

How it is derived is pretty straight forward and done in the following order…

A: 4-Byte AS Number Divided by 65535.
B: Note down the quotient and the reminder.
C: Subtract quotient from the reminder and note that down.

So the answer will be B.C

Here are some examples below…
65547 –> 1.11
65549 –> 1.13
175254 –> 2.44182

If you would like to know more about it, do check out RFC5396 and if you are looking to do conversion on the fly, do check out from APNIC.

comments powered by Disqus