We have already covered the topic of network addresses. The first （） 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.（） 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 （） 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 is for the netid and hostid (32 bits) defines a（） 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 ofaddress to confine the broadcasting to the （） network. Note that this address belongs to class E.
It is used by a.（） to send a packet to all hosts in a specific network.
A．router
B．switch
C．hub
D．firewall