A glue that holds the whole Internet together is the network layer protocol, (1) .Unlike most older network layer protocols, it was designed from the beginning with internetworking in mind. Its job is to provide a (2) way to transport data grams from source to destination, without regard to whether these machines are on the same network or whether there are other networks in between them.

Communication in the Internet works as follows. The (3) layer takes data streams and breaks them up into data grams. Each datagram is transmitted through the Internet, possibly being fragmented into smaller units as it goes. When all the pieces finally get to the destination machine, they are reassembled by the (4) layer into the original datagram. This datagram is then handed to the transport layer, which inserts it into the receiving process’ input stream.

An IP datagram consists of a header part and a text part. The header has a (5) part and a variable length optional part.

3（）

A．data link

B．transport

C．network

D．application

互感器如发生故障停用时，应先停电后切除故障互感器，不宜直接去拉开故障互感器。