 The 20771 ZIP Code is assigned exclusively to post-office box (P.O.Box) addresses, while 20768 is the designated ZIP Code for Goddard Space Flight Center, situated on federal government owned land that is contiguous with a portion of Greenbelt's eastern border.NASA's Goddard Space Flight Center, located directly adjacent to Greenbelt's eastern boundary, utilizes a Greenbelt postal address (Greenbelt, MD 20768), as well.