It produces an output of fixed length for an input of arbitrary length. So there is an infinit set of possible inputs mapped to a finit set of checksums - so the algorithm can't produce a unique checksum for every url you feed it. I suggest you read the article on wikipedia and then have a look at SHA1 as possibly the better solution - that nonetheless will not be bijective either (It will produce collisions!) - it depends on your problem at hand if this is a hindrance.