In every ISO8583 message, there is a bit map to indicate which fields are included in the message. Any ISO8583 message must contain at least one bitmap which is called Primary Bitmap (16 hexadecimal digits). Here is an example of such bitmap:

4210001102C04804

This bitmap indicates that the message contains these fields: 2,7,12,28,32,39,41,42,50,53,62.

If the most significant bit (MSB) of the first digit of the bitmap is set as 1, means that there is a secondary bitmap. Here is an example.

C0000000000000010000000000000003

In this example, the first hexadecimal digit is 'C' (1100 in binary) and its MSB is 1, and it means there is a secondary bitmap. This bitmap indicates that the message contains fields: 2,64 y 127 and 128.

Tool Kit - Show fields of a BITMAP

BitMap:
  Input what you see:

Show Fields