Special:Contact/wiki-name-change
You can change the Sitename and URL of the wiki. An IP address is something completely different, my wiki's is 199.27.72.131, and I have no reason to change it and I don't think they'd let me anyway.
If we're talking about w:c:eragonisle, the sitename is currently "Eragon Isle Wiki" and the URL eragonisle.wikia.com.