Gateway protocols are necessary for exchanging routing information. Consider for a moment the analogy of sending a letter via the U.S. Post Office. Once you have addressed your letter, applied your postage, and placed it into the mailbox for pickup it will go to your nearest post office for routing. Your local post office will review a zip code table and based on the destination zip code it will know which major mail clearinghouse it should be sent to so that it can be forward to the post office that will ultimately deliver your letter to its destination.

How does the post office keep the zip code table up to date and reflective of route improvement? Well in this analogy, the post office would have some mechanism to update the zip code table and communicate the changes to each post office. This is where we bridge our analogy to routing packets on a network. How do routers update the information they use to route packets. Two mechanisms used today include the exterior gateway protocol and the interior gateway protocol.