根据下面的文字资料回答 71～75 题
connected by（ 1 ）or Hubs,which just move the bits from one network to an identical network. One layer up we find bridgas and swiehes,which operate at data link layer. They can accept （ 2 ） , examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer,we have touters that can connect two netwoks. If two networks have（ 3 ）network layer,the router may be able to translate between the packet formats. In the transport layer we find transport gateway, which cart interface between the two transport connections. Finally, in the application layer, application gateways translate message（ 4 ）. As an example,gateways between Internet e-mail and X.400 e-mail must （ 5 ）the e-mail message and change various header fields.
第6题：文中（ 1 ）处正确的答案是（ ）。
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