190

In Figure 6-9, the addresses between the network address of 10.0.0.0 and the broadcast address of 10.0.0.255 are the host addresses. This means that the addresses 10.0.0.1 to 10.0.0.254 can be assigned to the hosts in this logical network.

Network Prefixes

When you examine a network address, you might ask, “How do you know how many bits of this address represent the network portion and how many bits represent the host por-tion?” The answer is the prefix mask. When an IPv4 network address is expressed, you add a prefix length to the network address. This prefix length is the number of bits in the address that gives the network portion. This prefix length is written in slash format. That is a forward slash (/) followed by the number of network bits. For example, in 172.16.4.0 /24, the /24 is the prefix length. This tells you that the first 24 bits are the network address. The remaining 8 bits, the last octet, are the host portion.

Networks are not always assigned a /24 prefix. Depending on the number of hosts on the network, the prefix assigned can be different. Having a different prefix number changes the host range and broadcast address for each network. Notice that the network addresses in Table 6-9 remain the same, but the host range and the broadcast address are different for the different prefix lengths. You can also see that the number of hosts that can be addressed on the network changes as well.

Table 6-9	Using Different Prefixes for the 172.16.4.0 Network
	Network		Broadcast
Network	Address	Host Range	Address
172.16.4.0 /24	172.16.4.0	172.16.4.1–172.16.4.254	172.16.4.255
172.16.4.0 /25	172.16.4.0	172.16.4.1–172.16.4.126	172.16.4.127
172.16.4.0 /26	172.16.4.0	172.16.4.1–172.16.4.62	172.16.4.63
172.16.4.0 /27	172.16.4.0	172.16.4.1–172.16.4.30	172.16.4.31

Subnet Mask: Defining the Network and Host Portions of the Address

Another question you might ask is, “How do the network devices know how many bits are the network portion and how many bits are the host portion?” The answer to this question is the subnet mask.

The prefix and the subnet mask are different ways of representing the same information: the network portion of an address. The prefix length tells you the number of bits in the address that are the network portion in a way that is easier to communicate to humans. The subnet mask is used in data networks to define this network portion for the devices.