MD5 crypt hashes the password and salt in a number of different combinations to slow down the evaluation speed. Some steps in the algorithm make it doubtful that the scheme was designed from a cryptographic point of view--for instance, the binary representation of the password length at some point determines which data is hashed, for every zero bit the first byte of the password and for every set bit the first byte of a previous hash computation.
The output is the concatenation of the version identifier ``$1$'', the salt, a ``$'' separator, and the 128-bit hash output.
MD5 crypt places virtually no limit on the size of passwords, while bcrypt has a maximum of 55 bytes. We do not consider this a serious limitation of bcrypt, however. Not only are users unlikely to choose such long passwords, but if they did, MD5 crypt's 128-bit output size would become the limiting factor in security. A brute force attacker could more easily find short strings hashing to the same value as a user's password than guess the actual password. Finally, like DES crypt, MD5 crypt has fixed cost.