that only work if you had two pieces of information. username and password works because you can find the salt value associated with that username and then use that for the password hash. an ip would still require an unhashed thing to lookup to get the hash if you did it per ip address. for this you might be able to get away with using a sole salt value for all ip addresses but even then if you get hacked it would be trivial to write a script to compute the rainbow table when you steal the salt value.