A lot of forums have restrictions on amount of accounts. Using this plugin you can effectively track multiple accounts that are linked to one and the same IP address.

The plugin provides an overview of duplicate IP users in the admin control panel. Selecting view details on one specific user allows the administrator to find users with the same IPs.

Not only does this plugin look at the last used IP, but also the registration IP addresses. Even better, it cross-references the registration IP with any of the last used IPs.

Performance tested on a database with 5000+ users and the plugin is able to load the pages in about 1 second.