Subnetting Guidelines

Introduction

There is always that day when you are called upon to provide a solution to a network problem. 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 solve 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. I've seen some network setups which suffered from all the above, and you would be amazed how frequently I see them at large companies.

Guidelines - Plan for Growth

When creating subnets for your network, answer the following questions:

• 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 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. And if you do happen to have any problems, well, there is always the website's forum where you can post your questions and problems :)