What sort of binary representation is %xA and such?

Question

I'm setting up a websocket server to connect with a web browser, while reading the Specification (RFC 6455) for websocket frame exchange I came across these values that are supposed to represent a 4bit op code, they look like this:

%x0 %x1 %x2 ..... %xA %xB .... %xF

I know that %x0 = 0000 and %x1 = 0001

I'd like to know what these values are called and how to convert them to bits. Thank you.

Answer 1

These %xN representations use a single hexadecimal digit to describe a four-bit binary number.

The possible values of a four-bit binary number range from 0 (0000) to 15 (1111), so each value can be expressed as a hexadecimal digit:

     0  %x0
     1  %x1
    10  %x2
    11  %x3
   100  %x4
   101  %x5
   110  %x6
   111  %x7
  1000  %x8
  1001  %x9
  1010  %xA
  1011  %xB
  1100  %xC
  1101  %xD
  1110  %xE
  1111  %xF