How to identify private IP address ranges?

[todayodibet]

I came to the conclusion that bookmakers are identifying player's accounts by checking the IP range of the player, and not only the unique IP used to login to the account.
Could someone explain how these ranges work, and how could a player find out the range of IPs that he or she is using?
The question is important for many members I suppose.
Thank you.

 

[betb2live]

Thank you guys. This is problematic, most of the time it's not so easy to get IP range to change. Do you have strong evidence that bookies check the range, not only the full IP?