See MLA Handbook (7th ed.) page 137 (5.4.2)
Important Note on the Use of URLs in MLA
MLA no longer requires the use of URLs in MLA citations. Because Web addresses are not static (i.e., they change often) and because documents sometimes appear in multiple places on the Web (e.g., on multiple databases), MLA explains that most readers can find electronic sources via title or author searches in Internet Search Engines.
For instructors or editors who still wish to require the use of URLs, MLA suggests that the URL appear in angle brackets after the date of access. Break URLs only after slashes.
Aristotle. Poetics. Trans. S. H. Butcher. The Internet Classics Archive. Web Atomic and Massachusetts Institute of Technology, 13 Sept. 2007. Web. 4 Nov. 2008. ‹http://classics.mit.edu/›.
See MLA Handbook (7th ed.) page 192 (5.6.4)