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You are here: PacificCable.com Subnetting Tips

Count hosts on each subnet

The most important planning information available to the subnet designer is a count of how many addresses are required per subnet. Don't forget that each router requires a unique IP address, too, and don't overlook the two reserved addresses on each subnet. Of course, if router and reserved addresses can break your subnetting scheme, then it's probably too tight already. Allow for future expansion, both in general by padding all the subnets, and also in specific cases where future growth can be predicted.

Do a mock planning first

Before assigning or even requesting anything, go through the entire planning process using fictitious IP addresses. For example, if you expect to use an overall 20-bit address prefix, do a mock planning with 10.10.16/20. If everything works out, then an almost identical plan will work with any 20-bit prefix. If you're not satisfied with the resulting plan, then try again with a 19-bit prefix (if the mock plan was too tight), or perhaps a 22-bit prefix (if the mock plan was too wasteful). Then you'll be able to go to an Internet provider and say with confidence, "I need a 21-bit address prefix."

Plan ahead for address summarization

In the university example, I could have assigned a prefix of 128.74.29/24 to Einstein Hall, 192.177.13/24 to Wilson Hall, and 208.150.133/24 to Steinbeck Hall. In practice, this situation could come about by using multiple Internet Service Providers for the different halls. If we then wanted to come along later and announce a single summary address for the entire university, we'd be out of luck. The longest matching prefix would be 128/1, and that's simply too short (matching half of all IP addresses!) to be usable.

Another example: consider a company with two offices, each using about a dozen subnets, and an overall 192.177.32/19 address. A naive approach might be to assign even numbered subnets to the Washington office, and odd subnets to the Tampa office, so Washington would use 192.177.32/24, 192.177.34/24, 192.177.36/24, etc, while Tampa was numbered 192.177.33/24, 192.177.35/24, 192.177.37/24, etc. The most logical summarization scheme would be to have one summary for each office, but this addressing design precludes that! A better approach would assign 192.177.32/24, 192.177.33/24, 192.177.34/24 to Washington, and 192.177.48/24, 192.177.49/24, 192.177.50/24 to Tampa, so now we can summarize 192.177.32/20 for Washington and 192.177.48/20 for Tampa.

So, plan ahead for address summarization. Even if you're not presently summarizing, plan your IP addresses as if you were. Develop a likely summarization strategy, if only on paper, then assign subnets based on that design.

Assign summary blocks first

Work from the top down. Assign the large summary blocks first. If you anticipate the need for future summary blocks, allocate them as well. In the last example above, the company might have an additional office in Chicago, so a summarization block should be planned for it, even if there are no immediate plans to connect it:

Washington	192.177.32/21
Chicago	192.177.40/21
Tampa	192.177.48/21
SPARE	192.177.52/21

Assign largest subnets next

Assign larger subnets before smaller ones, since it is much easier to wedge small subnets in between large ones than vice versa.

Assign serial links last

Serial links, which can be assigned 30-bit subnets, should be assigned last.