211

Calculating the Highest Host Address

After determining the broadcast address, you can easily determine the highest host address.

It is 1 less than the broadcast address. In Figure 6-17, with a broadcast address of

172.16.20.127, the highest host address would be 172.16.20.126. To determine the highest host address, make the lowest host bit a 0 and all other host bits a 1.

Figure 6-17     Highest Host Address for the 172.16.20.0 /25 Network

172								16										20									126
1	0	1	0	1	1	0	0	0		0	0	1	0		0	0	0	0		0	0	1	0	1	0	0	0	1	1	1		1	1	1	0
												Network																			Host

All host bits, except the lowest address, are all 1s: 64+32+16+8+4+2+0=126. This makes the highest host address in this example 172.16.20.126.

Note

Although, for this example, all the octets are expanded, you only need to examine the content of a divided octet, that is, an octet that contains network bits and host bits.

Determining the Host Address Range

Finally, you need to determine the host range for the network. The host range of the net-work includes all the addresses from the lowest host address to the highest host address inclusive. Therefore in this network, the address range is

172.16.20.1 to 172.16.20.126

These IPv4 unicast addresses can be assigned to the hosts in the logical network 172.16.20.0 /25. A host that is assigned any other address will be in a different logical network.

Tip

Notice that the least significant bit of the network address and the least significant bit of the highest host address are always 0. That makes these values even.

Similarly, the least significant bit of the lowest address and the least significant bit of the broadcast address are always 1. This makes these values odd.

This is a quick check to help you determine whether you have the correct values.

Basic Subnetting

Another IPv4 addressing skill helpful for a network associate is the ability to plan the sub-netting of a network. The address range used in an internetwork needs to be divided into networks. Each of these networks must be assigned a portion of these addresses called a subnet. Many factors and techniques are used to create a subnetting plan. These sections will present some of these factors and techniques.