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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.

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