# BGP 4-Byte AS Number - Asdot plus notation

BGP uses AS numbers as a fundamental part of its routing process. Because conventional 2-byte public AS numbers were becoming exhausted, the IANA increased the AS numbers by introducing a 4-byte AS numbers. The Asdot+ notation is one of three ways to represent these AS numbers and is described below:

A 4-Byte AS number is represented by 32 bits. To obtain the Asdot+ notation, these 32 bits are separated into two groups of 16 bits. The first 16 bits are converted to decimal, as are the second 16 bits. These two decimal numbers are then separated by a dot to get the notation. For example:

- An AS number of
`00000000 00000000 00000000 11111111`

is represented as 0.255 - An AS number of
`00000000 00000000 11111111 11111111`

is represented as 0.65535 - An AS number of
`00000000 00000001 11111111 11111111`

is represented as 1.65535 - An AS number of
`00000000 11111111 00000000 11111111`

is represented as 255.255

4-Byte AS numbers can also be represented using the Asdot notation as well as the Asplain notation.

Links:

https://networklessons.com/bgp/bgp-4-byte-number/

https://forum.networklessons.com/t/bgp-4-byte-as-number/3184/25?u=lagapides