Norfolk, Va. – Effective immediately, anyone sending mail to Navy mobile units must use a nine-digit ZIP code to ensure delivery, according to a message released by Secretary of the Navy Ray Mabus.

A postal code is a five digit code used to help postal services deliver mail and packages. Here's what the numbers mean and how they determine yours.

The 54915 ZIP code can indicate Calumet and Outagamie counties, which have a 0.5% sales tax, but also Winnebago County, which doesn't have the tax.