When citing an Internet source using the MLA format can you just cite the main page or do you have to site each individual page on the site?

Because websites show up differently depending upon how and when they are accessed, one need only site the main website. Because writing styles change every now and then, you might want to check nuances like this every few years. Technological citation is still in it's infancy, and will change.

The Owl Purdue site, easybib.com, or a quick search on Yahoo/in a style guide can assure you that you're using the most up-to-date citation styles, depending upon how important this is, in context.

Usually, however, a citing the whole site is fine.