Networks can be interconnected by different devices. In the physical layer, networks can be connected by 20 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 21 ,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 22 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 23 . As an example, gateways between Internet e-mail and X.400 e-mail must 24 the e-mail message and change various header fields.