Im hoping somebody can point me in the right direction with this one !

Im have created a page for my companies website that is going to have an input box on, in this box the customer will enter there customer number.

I want it so if the customer number is recognized then the customer is directed to one page but if its not recognized then they are directed to a different page..

I know this is possible but I dont know how to explain it or what its called so I cant just google it !

Any help much appreciated !