I've got a MD5 32 char password hash which I looking to crack (NO funny business, working towards a certification in Network Security).

I have no idea of the length or combination of characters used so initially I used a dictionary attack which did not work. I then started to use a brute force from characters 1 to 6, on my dual core this will take 1.7days. On another computer I'm running 7 and 8 characters of upper, lower and numbers which will again take a while.

To test, I created a simple raw-MD5 password of 'admin' which is 21232f297a57a5a743894a0e4a801fc3. I put this in a txt file and ran John on BT3, after a few hours I cancelled it and saw that it was processing 6 characters and did not pick up the password, any idea how come?

I've started looking into rainbow tables and was wondering if someone could explain the process of using them to crack MD5 and point me in some direction of some to start testing.

Also, any pointers on why John the Ripper did not work?

Thanks in advance