210

Figure 6-14 shows the network address for the 172.16.20.0 /25 network.

Figure 6-14     Network Address for the 172.16.20.0 /25 Network

172								16										20									0
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	0	0	0		0	0	0	0
												Network																			Host

Host bits are all 0s: 0+0+0+0+0+0+0+0=0. So, the network address is 172.16.20.0. This makes the last octet of the address 0. Therefore, the network address is 172.16.20.0.

Calculating the Lowest Host Address

Next, you should calculate the lowest host address. This is always 1 greater than the net-work address. Therefore, using binary counting, you increment the 1s bit, making the last host bit a 1. Figure 6-15 shows the lowest host address for the network 172.16.20.0 /25.

Figure 6-15     Lowest Host Address for the 172.16.20.0 /25 Network

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

All host bits except the least significant address are all 0s: 0+0+0+0+0+0+0+1=1. With the lowest bit of the host address set to a 1, the address is 172.16.20.1. So, the lowest host address is 172.16.20.1.

Calculating the Broadcast Address

Although it can seem a little out of sequence, it is often easier to calculate the broadcast address before calculating the highest host address. The broadcast address of a network is the highest address in the address block. It requires all the host bits to be set. Therefore, all seven host bits used in this example network are 1s, as shown in Figure 6-16.

Figure 6-16     Broadcast Address for the 172.16.20.0 /25 Network

172								16										20									127
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	1
												Network																			Host

All host bits are 1s: 64+32+16+8+4+2+1=127. From the calculation, the value of the last octet is 127. This gives you a broadcast address of 172.16.20.127 for the network 172.16.20.0 /25.