We have already covered the topic of network addresses. The first (1) in a block (in classes A, B, and C) defines the network address. In classes A, B, and C,if the hostid is all ls,the address is called a direct broadcast address. It is used by a(2) to send a packet to all hosts in a specific network. All hosts will accept a packet having this type of destination address. Note that this address can be used only as a (3) address in an IP packet. Note also that this special address also reduces the number of available hostid for each netid in classes A,B，and C． In classes A, B, and C, an address with all ls for the netid and hostid (32bits) define a (4) address in the current network.A host that wants to send a message to every other host can use this address as a destination address in an IP packet. However, a router will block a packet having this type of address to confine the broadcasting to the (5) network. Note that this address belongs to class E.
空白（1）处应选择（）
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"，这种地址叫做直接广播地址。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。还要提及的是，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，这种地址属于E类地址。