No single LAN technology is sufficient to interconnect all the computers in a given plant, campus, or site. Thus it is desirable to combine different types of LANs, using a device called a Bridge, to produce an Extended LAN. Some LANs in the Extended LAN may have incompatible Data Link formats. Thus a bridge may need to encapsulate a frame originating on LAN A inside the Data Link header of another (incompatible) LAN B in order to allow the type A frame to travel over LAN B. In general, frames sent between any pair of LANs in the Extended LAN must be encapsulated across every incompatible LAN in the path between the LANs.Bridges learn their routing information from information contained in frames they forward. Besides the problems of distinguishing various kinds of encapsulated and unencapsulated frames, the encapsulating protocol used by bridges must also solve the learning problem. This leads to a new set of considerations and solutions. We begin with a rough solution, and refine it using informal arguments and examples to lead to the final description. The stages in the description roughly mimic the design process.