Sometime people create multiple fake accounts in your WordPress site like they do in Facebook. You can fight back by logging their IP addresses and this can be done easily by using Register IP plugin in your site.
Single Site View (User’s List):
Multisite View (User’s List):
Register IP – Multiste is a free WordPress plugin. If you have this plugin activated in your site and a user registers, their IP get logged in the wp_usermeta under the signup_ip key. And you can look to their profile or the users table to see what it is. For security purposes their IP is not displayed to them when they look to their profiles. It works both in single and multisite WordPress installation. IPs can only be seen by Admins and Network Admins on Users List.