# 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
AS numbers range from 6553*4 byte only***6**– 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 6553**6** – 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.