Protocols, Lesson 10: Subnetting guidelines

The last lesson in our IP series describes creating subnets for your network.

The number of problems that can occur in a network are numerous, and -- believe it or not -- most of them can be avoided if the initial design and installation of the network are done properly.

When I say "done properly," I don't just mean connecting the correct wires into the wall sockets. Looking at it from an administrator's point of view, I'd say that a properly done job is one that has had a lot of thought put into it to avoid silly routing problems and meet today's and any future needs.

This page contains all the information you need to know in order to design a network that won't suffer from any of the above problems. You would be amazed at how frequently I see networks suffering from all the above at large companies.

Guidelines - Plan for growth

  • How many subnets are needed today?
    Calculate the maximum number of subnets required by rounding up the maximum number to the nearest power of two. For example, if an organization needs five subnets, 2 to the power of 2 or 4 will not provide enough subnet addressing space, so you must round up to 2 to the power of 3 = 8 subnets.
  • How many subnets are needed in the future?
    You must plan for future growth. For example, if 9 subnets are required today, and you choose to provide for 2 to the power of 4 = 16 subnets, this might not be enough when the seventeenth subnet needs to be deployed. In this example, it might be wise to provide for more growth and select 2 to the power of 5 = 32 as the maximum number of subnets.
  • What are the maximum number of hosts on a given segment?
    You must ensure that there are enough bits available to assign host addresses to the organization's largest subnet. If the largest subnet needs to support 40 host addresses today, 2 to the power of 5 = 32 will not provide enough host address space, so you would need to round up to 2 to the power of 6 = 64.
  • How many hosts will there be in the future?
    Besides planning for additional subnets, you must also plan for more hosts to be added to each subnet in the future. Make sure the organization's address allocation provides enough bits to deploy the required subnet addressing plan.

    When developing subnets, class C addresses present the greatest challenge because fewer bits are available to divide between subnet addresses and host addresses. If you accommodate too many subnets, there may be no room for additional hosts and growth in the future.

    All the above points will help you succeed in creating a well designed network which will have the ability to cater for any additional future requirements.

    Return to the introduction.



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  • When creating subnets for your network, answer the following questions:
    This was last published in October 2004

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