Sockpuppets don't necessarily have to use the same IP address as the sockpuppeteer. If you, for example, go to an internet cafe and sockpuppet from there, the IP address will be different than when you control the master account at home. Also, if two or more people use the same computer and have seperate accounts, they are not sockpuppets, even though their IP addresses will be the same.
A sockpuppet is a second account controlled by a person already registered on the website. The most common reason for doing this is to get around a ban, though there are other reasons for having a sockpuppet.
To answer your question, yes, there can be sockpuppets in forums as well. Just like in Wikipedia, it is generally frowned upon.