Content Distribution Networks (CDNs) are a rapidly§growing service on the Internet. CDNs distribute and§co-locate content to be geographically close to the§users (of the content). This content mirroring 1)§reduces the load on the origin server, 2) reduces§traffic on the Internet, and 3) improves response§time to the users. For CDNs to be feasible, methods§of routing of HTTP requests originating from users§are needed. URL routers need routing tables similar§to IP routing tables in IP routers. However, the§methods used for IP routing cannot scale to URL§routing. In URL routing the number of destination§addresses (URLs) greatly exceeds the number of IP§addresses. In addition, URLs are not stable - new§ones are constantly created and old ones disappear. §The large size of URLs, their very large number, and§their dynamic nature makes URL routing a difficult§problem. This is the problem that is addressed in§this book.