Transparent Routers
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 n ) 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.
有两种方式把局域网和广域网连接到一起。第一种是每个局域网都必须分配一个网络前缀，并且互联网中的路由器必须知道怎样路由寻找到该网络。另一种是，局域网共享一小部分广域网的地址空间。支持第二种模式的路由器叫做地址共享路由器或者透明路由器。
透明路由器的基本思想是在这个透明路由器后面的主机共享在该透明路由器前的广域网的地址空间。在某些状况下，这是一个很有用的方法，其限制也没有呈现出多大的缺点。
前端和后端表明了这种方法的不足：这种交互的模式只适合于地理上（或者拓扑上）受限的stub enviroment(不知道是什么意思）。它需要某种网络层广域网寻址的逻辑寻址方式。本地环境的IP地址映射到广域网的一些物理地址。这种映射方式类似于广域网的映射方式（IP地址<->网络地址)
多寻址是广域网的一种可能方式，但是如果接口在物理或者拓扑上分离的话，可能会有许多的问题。在两个广域网使用多寻址可能会引起地址的混淆。