What is a rainbow table attack and how do you defend against it?

A rainbow table is a precomputed lookup table of hash values for common passwords and their plaintext equivalents. An attacker who obtains a database of hashed passwords can look up each hash in the table to find the matching password almost instantly, bypassing the need to crack each hash individually. The defense is salting: adding a unique random value to each password before hashing so that identical passwords produce different hashes. With a salt, rainbow tables become useless because the attacker would need a separate table for every possible salt value. ---