The last two octets contain 0s because those 16 bits are for host numbers and are used
for devices that are attached to the network. The IP address in the example (172.16.0.0) is
reserved for the network address; it is never used as an address for any device that is
attached to it. An example of an IP address for a device on the 172.16.0.0 network would
be 172.16.16.1. In this example, 172.16 is the network-address portion and 16.1 is the
host-address portion.
If you wanted to send data to all the devices on a network, you would need to use a network
broadcast address. Broadcast IP addresses end with binary 1s in the entire host part of the
address (the host ?¬?eld), as shown in Figure 1-33.
For the network in the example (172.16.0.0), in which the last 16 bits make up the host ?¬?eld
(or host part of the address), the broadcast that is sent out to all devices on that network
includes a destination address of 172.16.255.255.
Figure 1-33 Network Broadcast Address
The network broadcast is also known as a directed broadcast and is capable of being routed,
because the longest match in the routing table would match the network bits. Because the
host bits would not be known, the router would forward this out all the interfaces that were
members of the major 172.
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