What is the largest binary number that can be expressed with 12 base or bits?

The answer to your question is 4095 in decimal
In binary 111111111111
If your having trouble with binary here is a link that will help understand it all a bit better.

google.com/k/zach-bacon/counting-in-binary-for-beginners/11ezkn0ytzmdj/3#">http://knol.google.com/k/zach-bacon/counting-in-binary-for-beginners/11ezkn0ytzmdj/3#