How to Figure a Host Address Range

Identify subnet IP addresses using a single host's IP information.

The Internet has grown at such a fast pace that it has outgrown the number of Internet Protocol (IP) addresses available within the IP version 4 address space. IP address subnetting is a technique used by network engineers and architects to conserve IP version 4 addresses within a network by dividing larger default IP address ranges into smaller ranges of IP addresses. Find the range of IP addresses to which a particular IP address belongs in order to determine the number of hosts possible and address numbers available on the local network.

Instructions

1. Click the "Start" button on your desktop and click "All Programs," then "Accessories." Right-click on the Command Prompt icon and click "Run as Administrator."

2. Type "ipconfig /all" in the command-line window that appears and press the "Enter" key. Note the IP address and subnet mask displayed in the command-line output.

3. Find the starting network number of the host address range by subtracting the number of the last octet in the subnet mask that is not 0 from 256. For example, if the subnet mask were 255.255.255.128, you would subtract 128 from 256 and find that the starting network number of the second host address range is 128 (the first host address range number is always 0).

   The number of hosts in an address range is equal to the starting network number of the second host address range minus 2 (so the number of host addresses in each range is 126 in this example). Note that host addresses reside in octets that do not have a 255 subnet mask for the octet.

4. Match the number in the last octet of the IP address with the IP address ranges found in the previous step to determine within which range the IP address resides. Continuing the example in the previous step, if the IP address is 192.168.1.22, then the IP address belongs to the 192.168.1.1 through 192.168.1.127 IP address range. Note that 192.168.1.0 is the designated network number that starts the first IP address range and 192.168.1.128 is the designated network number that starts the next IP address range.

References

• TechRepublic: Subnetting a Class C network address
• Cisco: IP Addressing and Subnetting for New Users
• Cisco: Working with IP Addresses