How to restrict a specific user to an IP range he can login from?
Have a look at IP Ranges module.


  IP Ranges is a module that let's you completely ban both single IP-addresses as well as
  full ranges from your site. The ban is triggered already at the early bootstrap phase, so you
  can get rid of unwanted visitors as early as possible without wasting server resources.
  
  You can also define whitelists that override blacklists, both single and ranged.
  The UI is similar to core ip-ban, so you will feel like home immediately.