Bukkit

This plugin lets you set the number of account allowed to connect from the same IP.

You could also use this plugin to cross-reference what accounts log in from what IPs. To do this you would look in the config file next to the names and IPs, and see what matches.

Installation:

• To use this plugin, download the latest version and put it in your server plugins file. After this is done, then reload/restart your server. This will then generate a config file. Open this file in your text editing software, I recommend NotePad++. Then, change the "MaxAlts" to how many accounts you would like to be able to connect to from the same IP. Bear in mind that people in the same household will most likely have the same IP, so you should leave it at around 2.
  
• Once this is set, reload/restart your server once more, and StopAlts is setup.

Commands:

• None

Permissions:

• StopAlts.Exempt
  This permission node makes you exempt from the kick on that account only. For example, you already had 2 accounts on your IP, and the MaxAlts was set to 2. Normally if you would join on another account, you would be kicked. If your account has this permission node, you would be able to join as normal.

Currently you have to use the permission node to make people exempt to the kick. I am working on stopping OPs from being kicked as well.