题目内容：
根据下列题干，回答题：Networks can be interconnected by different devices．In the physical layer，networks can be 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 routers 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 can 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．
请选择（1）处最佳答案：
参考答案：
答案解析：