从供选择的答案中，选出最确切的解答。 Networks can be interconnected by different devices. In the phys

从供选择的答案中，选出最确切的解答。

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 fine bridges and switches, 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 networks. If two networks have(3)network layer, the router may be able to translate between the packer formats. In the transport layer we find transport gateway, which can interface between two transport connections. Finally, in the application layer, application gateways translate message(4). As an example, gateways between Intemet E-mail and X.400 E-mail must(5)the E-mail message and change various header fields.