How many bits wide is the Internet Header Length (IHL) field in an IPv4 header?

Concept

An IPv4 header contains fixed-width fields. The Internet Header Length (IHL) field stores the header length as a count of 32-bit words, so its width determines the range of word counts that can be encoded.

An unsigned field of n bits has 2n possible bit patterns, representing values from 0 to 2n − 1.

Application

1. RFC 791 allocates 4 bits to the IHL field.

2. A 4-bit unsigned field represents values from 0 to 15; valid IPv4 headers use IHL values from 5 to 15 because the minimum header has five 32-bit words.

3. Five words multiplied by 4 bytes per word gives 20 bytes, while fifteen words multiplied by 4 bytes per word gives 60 bytes.

Cross-check

The minimum and maximum IPv4 header sizes, 20 bytes and 60 bytes, correspond exactly to IHL values 5 and 15 within this field.

Result

Therefore, the IHL field is 4 bits wide.

Contrast

• A 2-bit width provides only four bit patterns, covering values 0 through 3.

• A 4-bit width provides sixteen bit patterns, covering values 0 through 15.

• An 8-bit width provides 256 bit patterns, covering values 0 through 255.

• A 16-bit width provides 65,536 bit patterns, covering values 0 through 65,535.