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Note

When planning a network implementation, a subnet calculator like the VLSM (CIDR) subnet calcula-tor at http://vlsm-calc.net can be useful. However, no calculators are permitted during certification

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ANDing Process

Figure 6-13 shows the ANDing of 192.0.0.1 with a subnet mask of 255.255.255.0. For the first two octets, the subnet mask bits are all 1s (255). For the AND of each of these bits to the host address, the result is the host address bit. For the last two octets, the subnet mask bits are 0s (0). For these AND processes, all the resultants are 0.

Figure 6-13     Applying the Subnet Mask

		Dotted Decimal					Binary Octets
Host	192	0	0		1	11000000	00000000	00000000	00000001
Mask	255	255	0		0	11111111	11111111	00000000	00000000
AND
Network	192	0	0		0	11000000	00000000	00000000	00000000

Calculating Network, Hosts, and Broadcast Addresses

Now you might be asking, “How do you determine the network, hosts, and broadcast addresses for a network?” This calculation process requires you to examine these addresses in binary. Calculating the address assignments requires the following steps:

How To

Step 1.      Calculate the network address.

Step 2.      Calculate the lowest host address.

Step 3.      Calculate the broadcast address.

Step 4.      Calculate the highest host address.

Step 5.      Determine the host address range.

In the following sections, you examine address assignments for the 172.16.20.0 /25 network.

Calculating the Network Address

First, determine the network address. The network address is the lowest address in the address block. To represent a network address, all the host bits are 0. With a 25-bit prefix, the last 7 bits are host bits and are 0s.