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.
（1）是（）
A.datagram
B.function
C.address
D.service
参考答案：C
解析：
我们已经讲述了有关网络地址的内容。第一种块地址(A类、B类和C类)定义了网络地址。在A类、B类和C类地址中，如果主机ID部分都是“1”，这种地址叫作直接广播地址。通常被路由器用于把分组发送给某特定网络中的所有主机。所有主机都接收具有这种目标地址的分组。值得注意的是，这种地址只能在IP分组中被用作目标地址。此外，这种特殊地址也减少了每一个A类、B类和C类网络ID中可用的主机ID数量。
在A类、B类和C类地址中，如果所有网络ID和主机ID部分(32位)全为“1”，则这种地址定义了当前网络中的广播地址。一个主机如果想要发送报文给每一个其他主机，则可以使用这个地址作为IP分组的目标地址。然而，路由器通常会阻挡具有这种地址的分组，以限制向本地网络的广播。注意，这类地址属于E类地址。