This is AntiMulti. It is a plugin designed to prevent the use of alternate accounts from the same IP address. To put it simply, a player cannot use multiple accounts on the same IP address and likewise, will not be able to use a ton of IPs.
AntiMulti is a plugin which has undergone multiple changes from its original intent to be a simple plugin to prevent alternate accounts from being used on a server by blocking too many accounts from sharing an IP to a plugin which can also prevent session stealing using this same concept and also supporting admin protection. This is done simply by using an IP system to prevent the unauthorized use of an account by using IPs to prevent session stealing from working in the first place. All of this is completely toggle-able in the configuration file in case one does not use want to use certain features or not.
There are a few things you must understand when using this plugin. This is not a one-hit wonder in that it can stop all alternate accounts, as proxies would be able to get around this system, however common proxies would eventually be blocked after they are used enough. This does its best though to stop a member from simply using 2 accounts to gain an advantage over another player.
There are 2 groups this plugin will understand: Players and Admins. Players are normal players and are not forced to use the protection system as strictly as admins. Players are more loosely held initially by allowing more connections per name and IP, however this can be changed. Admins are more protected to prevent the taking of the account to cause problems by forcing registration and using a smaller range of names and IPs that can those accounts can share. This is all permission based in that there is a perm that designates whether the player is an admin or a player, which is just antimulti.admin, which if given, marks the player as an admin and handles them accordingly.
We do have Metrics with this plugin, so stats will be sent to our mcstats page if you are opt-ed in, where that config is located in the PluginMetrics folder.
If you have any issues, please create a ticket here with the log file and as much information as you can provide so the issue can be investigated and handled. If you have a question, feel free to ask here but please read everything first before you ask since your question may have already been answered elsewhere.