We have already covered the topic of network addresses. The first （1） in a block (in class A, B, and C) defines the network address. In classes A, B, and C, if the hosted is all 1 s, the addressis called a direct broadcast address. It is used by a （2）to send a packet to all hosts in a specificnetwork. All hosts will accept a packet having this type of destination address. Note that thisaddress can be used only as a （3） address in an IP packet. Note also that this special addressalso reduces the number if available hosted for each netid in classes A, B, and C.
In classes A, B, and C, an address with all 1 s for the netid and hosted (32 bits) define a （4） address in the current network. A host that wants to send a message to every other hosts can use this address as destinationaddress in an IP packet. However, a router will block a packet having this type of address to confine thebroadcasting to the （5） network. Note that this address belongs to class E.

（3）是（）