Most default passwords are not alphanumeric and in my experience, 90% of passwords given out by the ISP's are never changed by the user. You need to research what each ISP uses.
Most ISP's use
8 characters lower
8 characters upper
8 characters HEX
10 characters HEX
10 characters digits
I've had success with all of the above in reasonable timeframes. Below is a very good site for calculating time taken, just input your custom keys per second and calculate.
http://calc.opensecurityresearch.com/
Example:
4x 290X's at 200k/s would crack 8 characters lower OR upper in 3 days max. 1.5 days average.
Most ISP's use
8 characters lower
8 characters upper
8 characters HEX
10 characters HEX
10 characters digits
I've had success with all of the above in reasonable timeframes. Below is a very good site for calculating time taken, just input your custom keys per second and calculate.
http://calc.opensecurityresearch.com/
Example:
4x 290X's at 200k/s would crack 8 characters lower OR upper in 3 days max. 1.5 days average.