IP Address Subnet Design Example  – Internet Protocol Version 4 (IPv4) Design

The development of an IP address plan or IP address subnet design is an important concept for a network designer. You should be capable of creating an IP address plan based on many factors, including the following:

• Number of locations
• Number of devices per location
• IP addressing requirements for each individual location or building
  • Number of devices to be supported in each communication closet
  • Site requirements, including VoIP devices, wireless LAN, and video
• Subnet size

Let’s look at an example of subnetting for a small company. Suppose the company has 200 hosts and is assigned the Class C network 195.10.1.0/24. The 200 hosts need to be in six different LANs.

You can subnet the Class C network using the mask 255.255.255.224. Look at the mask in binary: 11111111 11111111 11111111 11100000. The first 3 bytes are the network part, the first 3 bits of the fourth byte determine the subnets, and the 5 remaining 0 bits are for host addressing.

Table 1-16 shows the subnetworks created with the mask 255.255.255.224. Using this mask, 2n subnets are created, where n is the number of bits taken from the host part for the subnet mask. This example uses 3 bits, so 23 = 8 subnets. The first column of the table lists the LANs. The second column shows the binary of the fourth byte of the IP address. The third column shows the subnet number, and the fourth and fifth columns show the first host and broadcast address of the subnet.

Table 1-16 Subnets for Network 195.1.1.0

We use the formula 2n − 2 to calculate the number of hosts per subnet, where n is the number of bits for the host portion. The preceding example has 5 bits in the fourth byte for host addresses. With n = 5, there are 25 − 2 = 30 hosts. For LAN 1, host addresses range from 195.10.1.33 to 195.10.1.62 (30 addresses). The broadcast address for the subnet is 195.10.1.63. Each LAN repeats this pattern, with 30 hosts in each subnet.