单选题
Networks can be interconnected by different devices. In the
physical layer, networks can be connected by {{U}} {{U}} 20
{{/U}} {{/U}}or hubs, which just move the bits from one network to an
identical network. One layer up we find bridges and switches, which operate at
data link layer. They can accept {{U}} {{U}} 21 {{/U}}
{{/U}},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 {{U}}
{{U}} 22 {{/U}} {{/U}}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 {{U}} {{U}}
23 {{/U}} {{/U}}. As an example, gateways between Internet e-mail and
X.400 e-mail must{{U}} {{U}} 24 {{/U}} {{/U}}the e-mail message
and change various header fields.