Alt Checker

Alt Checker is a minecraft plugin that allows you to see your player's IP address, as well as all the accounts that have logged into your server with the same IP address.

The plugin stores IP addresses and ip address lists in the config, so do not mess with it. Upon logging in, a player will have their IP stored and if their IP does not already exist in the config, it will create a directory. If it does exist, it will add the player to an already existing list directory. Due to the nature of YAML files, the plugin will log IPs in an odd way. The way they are logged does not matter though, so the plugin will work fine even if IPs stored in the list in the config are done oddly.

Commands:
/alts <player>: Displays the players IP adress and then will display all the accounts that have logged into the server with the same IP.

Permissions:
altcheck.check: Allows a player to use /alts command. Given to OPs by default.
altcheck.bypass: Will not track the IP of a player with this permission node.

The plugin will work on offline players, so long as they have logged onto your server before while the plugin is installed.
Example:

 Versions: 

The current version is working on test servers up to 1.11.2.