A Spoonful Of CIDR Helps the Routing Tables Go Down - Page 3

 By Michael Burton
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VLSM is not just useful for major ISPs and Fortune 500 companies. An administrator with more than one subnet can use VLSM to utilize his or her allocated space more efficiently. Consider the example map below:

The ISP assigns you with a subnet mask of You now own the lower half of the network, up to You can then further break down your networks as follows:

Network Name


Subnet Mask

Host Range

Number of maximum hosts

Link to ISP Net to


Engineering Net


Finance Net to


Sales Net to


You'll notice that the maximum number of hosts is two shy of the total possible addresses. This is because the first available number designates the network, and the last possible number is the special "all hosts" broadcast address for that subnet. When you are setting up your VLSM spaces, it is best to double the number of hosts on each subnet and use that as your target maximum hosts number. That way you can allow for proper expansion. Finally, the "point-to-point" link for the ISP net really only requires a single address for each "point." You are effectively wasting 27 IP addresses. To fix this, you could further sub-divide the subnet.

There are many great free "IP address calculators" available which can help you work through the details of setting up your network. Routers across the board support VLSM as a standard feature. When figuring out how IP addresses you need for your clients, don't forget that one of the addresses in the range needs to be reserved for the router gateway.

CIDR and VLSM together not only saved the Internet from completely failing, they can also be a useful tool for you to more effectively manage your own company's address space!

Michael Burton is a project manager for Intel's Automated Networking Group, deploying infrastructure network management tools globally. He resides in Portland, Oregon.
This article was originally published on Jul 23, 2004
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