There are two basic models for interconnecting local-area networks and wide-area (or long-haul) networks in the Internet. In the first, the local-area network is assigned a network prefix and all routers in the Internet must know how to route to that network. In the second, the local-area network shares (a small part of) the address space of the wide-area network. Routers that support this second model are called address sharing routers or transparent routers. The focus of this memo is on routers that support the first model, but this is not intended to exclude the use of transparent routers.
The basic idea of a transparent router is that the hosts on the local-area network behind such a router share the address space of the wide-area network in front of the router. In certain situations this is a very useful approach and the limitations do not present significant drawbacks.
The words in front and behind indicate one of the limitations of this approach: this model of interconnection is suitable only for a geographically (and topologically) limited stub environment. It requires that there be some form of logical addressing in the network level addressing of the wide-area network. IP addresses in the local environment map to a few (usually one) physical address in the wide- area network. This mapping occurs in a way consistent with the { IP address <-> network address } mapping used throughout the wide-area network.
Multihoming is possible on one wide-area network, but may present routing problems if the interfaces are geographically or topologically separated. Multihoming on two (or more) wide-area networks is a problem due to the confusion of addresses.
The behavior that hosts see from other hosts in what is apparently the same network may differ if the transparent router cannot fully emulate the normal wide-area network service. For example, the ARPANET used a Link Layer protocol that provided a Destination Dead indication in response to an attempt to send to a host that was off- line. However, if there were a transparent router between the ARPANET and an Ethernet, a host on the ARPANET would not receive a Destination Dead indication for Ethernet hosts.