Appendix B IP Addresses and Subnetting

The following figure shows the company network after subnetting. There are now two sub-networks, A and B.

Figure 86 Subnetting Example: After Subnetting

In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of 27 – 2 or 126 possible hosts (a host ID of all zeroes is the subnet’s address itself, all ones is the subnet’s broadcast address).

192.168.1.0 with mask 255.255.255.128 is subnet A itself, and 192.168.1.127 with mask 255.255.255.128 is its broadcast address. Therefore, the lowest IP address that can be assigned to an actual host for subnet A is 192.168.1.1 and the highest is 192.168.1.126.

Similarly, the host ID range for subnet B is 192.168.1.129 to 192.168.1.254.

Example: Four Subnets

The previous example illustrated using a 25-bit subnet mask to divide a 24-bit address into two subnets. Similarly, to divide a 24-bit address into four subnets, you need to “borrow” two host ID bits to give four possible combinations (00, 01, 10 and 11). The subnet mask is 26 bits (11111111.11111111.11111111.11000000) or 255.255.255.192.

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GS1510 Series User’s Guide