Exel Help

I deal with department that have many accounts #s under them(example finance-accts#1234,acct#5467#9856,ect) on my Spread sheet I would like to type the account number and then it would in a separate cell input the correct department name.  Can anyone explain how to do this or is there a function I should be using.   I have 10 different depts and each has 10 to 15 different account numbers.

Thanks 

Tim