Very confuse with Header Length field in IPv4

[satyaz]

Just starting my CCNP Route journey and already confused with Header Length thing.

In the CCNP OCG book, it's said that Header Length is a 4 bit filed indicating the number of 4 byte words in the ipv4 header.

I understand the part of 4 bit field, but what does it mean with "number of 4 byte words in the ipv4 header"?

How can from 4 bit, it becomes 4 byte?

What the meaning of term "word" in this case?

If don't mind, could you please explain to me with step by step and "down to earth" english?

English isn't my native so mayber i'm missing something here.

[Harold Ritter]

A 4 byte word is a group of 4 bytes. So for example a IP header is 20 bytes by default, which means that the value in the 4 bit field would 5 (5*4=20).

Regards,

[rasmus.elmholt]

Hi

As Harold said the length defines the header in 4 byte words. The reason for the IHL field is the support for options in IPv4. The IHL field will tell the software where the IPv4 header with options stops and the next header starts.
Note that all options are 4 bytes in length or have a padding to make them exactly 4 bytes.

Take a look at this header drawing for IPv4 and the options fields.
https://luv.asn.au/overheads/ipv6/ipv6_headers.html