17. A tangible computer-readable medium storing computer instructions, wherein the computer instructions, when executed by a computer system, cause the computer system to perform a method comprising: receiving a plurality of masks, each mask comprising a string of one or more characters; accessing a list of URLs; for each URL in the list of URLs: identifying any portions of the URL that match the one or more characters in the plurality of masks, and removing from the URL the identified portions to create a resultant URL; and collapsing all identical resultant URLs into one URL.
18. The computer-readable medium of claim 17 , wherein the computer-readable medium further stores instruction for: prior to comparing each URL and collapsing all identical resultant URLs into one URL, sorting the plurality of URLs into one or more bins, wherein each URL comprises at least one field and each bin comprises at least one URL, each URL in a bin having common contents for at least one field.