Does this work with the Restrict Login add-on?
Yes. However, if an IP isn't in the list of allowed IP addresses, it won't login automatically. Thus, the Restrict Login add-on takes precedence.

Is it possible to add an IP range?
Yes, you can add CIDR ranges.

What's the Description field for?
Just to add a note. It's espcially useful if you have a lot of IP addresses.

I want to know more about IP spoofing and the risks, do you have some more info?
Here are some interesting links you may want to check out:

• https://security.stackexchange.com/questions/55279/how-easy-is-it-really-to-do-ip-spoofing
• https://superuser.com/questions/619477/how-do-i-spoof-the-ip-that-my-computer-sends-a-server-without-using-something-li