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 2^{16} = 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 2^{32} = 4,294,967,296 Possible AS Numbers

Any numbers ranging from 65536 to 4294967295 are considered 32Bit AS Numbers.

**ASPLAIN**

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”

**ASDOT**

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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