BGP 4-Byte AS Number - Asdot 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 to represent these AS numbers is as follows.

  1. For values between 0 and 65535, Asdot notation is simply the decimal value of the AS number. These values take up to 16 bits to express in binary. Examples include:
  • 5
  • 25
  • 196
  • 65000
  • 65535
  1. For values above 65536, Asdot notation splits the 32-bit binary value into two 16 bit values. These values are represented as two decimal numbers separated by a dot. Examples include:
  • 0.65536
  • 15.418
  • 65535.8520
  • 65535.65535

You will notice that for values of up to 65535, the Asdot is the same as the Asplain notation, and for values of 65536 and above, the Asdot is the same as the Asdot+ notation.