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BGP 4 Byte AS Numbers

We use to just have 2 byte AS numbers, ranging from 0-65535 (with 64512-65535 reserved for private use) available for distribution. However, since the growth of the internet, 4 byte AS numbers are now being issued out to consumers. The increased size, gives us a total of range of 0 – 4294967295 useable AS’s.

There are now three naming conventions for defining an AS:

  • ASPLAIN = 65540
  • ASDOT+ = 1.4
  • ASDOT = explained later

 

ASPLAIN

This is the format everyone is use to.

  • 2 byte numbers range from 0 – 65535
  • 4 byte only AS numbers range from 65536 – 4294967295
  • 4 byte AS numbers range from 0 – 4294967295

ASDOT+

This is way more fun, to write an AS number. Let’s take a look at an example. Below is an ASPLAIN number, then underneath, is the ASDOT+ representation of it.

  • ASPLAIN number = 65540
  • ASDOT+ representation = 1.4

How did I work that out?

The largest integer from 65540/ 65535(the highest ASPLAIN number) = 1. So this is the number we use on the left of the dot (called the high order bit). To work out the number on the right of the decimal (the low order bit), take the high order bit (1) away from the remainder (the remainder was 5 in my case), which gives us 4. This is placed to the right of the dot. Easy peasy!

The ASDOT+ is the naming convention used for numbers above the ASPLAIN range, which is anything in the range of 65536 – 4294967295

ASDOT

ASDOT uses same naming convention as ASDOT+, however, it is used on numbers within the ASPLAIN range (0-65535). This obviously causes the number before the dot to always be 0. For example, 65534 = 0.65534.

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