Q:

Length of Version field in IP Datagram header

I have doubt regarding length of "version" field in IP datagram. I have read the possible values are either 4 or 6 but they can represented in binary as 100 and 110, which are 3 bits in length. Then why the field has 4 bit length? 

A:

Simply, that's the way it's been defined. It also leaves some space for future versions. IPv15 - hmm...

