ikiwiki supports placing pages in a directory hierarchy. For example, this page, LinkingRules. This is a useful way to add some order to your wiki rather than just having a great big directory full of pages.has some related pages placed under it, like
To add a SubPage, just make a subdirectory and put pages in it. For example, this page is subpage.mdwn in this wiki's source, and there is also a subpage subdirectory, which contains subpage/linkingrules.mdwn. Subpages can be nested as deeply as you'd like.
Linking to and from a SubPage is explained in LinkingRules.