A piece of mail sent to a recipient without the addition of a postcode, also referred to as a postcode or zip code, is unlikely to reach its intended destination. Postcodes are used to all over the world, from the U.S. to the U.K. and New Zealand, to identify what area a street address is located in. This is necessary because many cities or towns often have more than one street with similar names such as 17th Street and 17th Avenue. The U.S. Postal Service offers a tool that will allow you to look up a postcode.
Go to USPS' Zip Code Lookup Tool (see Resources).
Enter the street address into the "Address 1" line.
Enter the city and state into the corresponding fields.
Press "Submit." The tool will display the zip code for that address.