Subnetting a Network

Exam: Microsoft 70-642 - Windows Server 2008 Network Infrastructure, Configuring


In case of large organizations, where the number of computers is very large or the systems are geographical locations of the systems are not at the same place, it is recommended that the network be divided into smaller networks and interconnect these smaller networks using routers. The smaller networks are known as subnets. The article discusses the benefits of subnetting and implementing subnetting.

Benefits of Subnetting

The main benefits of subnetting are:

  • Reduced amount of network traffic which improves performance of the network. Most of the traffic is able to stay on the local network and only packets that are destined for other networks cross the router to reach another subnet.
  • Simplified Management as identification and isolation of networks becomes easier in smaller groups.

An organization that has a single network address can have subnet address for each and every smaller network. Each subnet while remaining a part of the shared network address gets an additional identifier known as the subnet address.

The original design of Internet Protocol were made for limited number of networks and hosts and the addressing scheme was based on network address for each physical network. The unexpected growth of the Internet resulted in creating problems like insufficient addresses and huge routing tables. The solution to these problems was seen in subnetting.

Implementing Subnetting

The first step for implementing subnetting is to determine the current needs and draw out a plan on how subnetting would be implemented.

  • Determining Subnetting Requirements:

    Go through the following steps in order to determine the needs of your subnet.

    1. Identify the number of network IDs that are required;
    2. Identify the number of host IDs that are required for every subnet;
    3. Create a subnet mask for the complete network, a unique subnet for every physical segment and a range of host IDs for every unique subnet.

  • Implementing Subnetting:

    Subnetting is the process of assigning a subnet address to every machine that is present on a particular physical network. No alteration is possible in the default part of the IP address without an encroachment on the address space of another administrative domain. The only exception is where multiple consecutive classful addresses are assigned.

    For maximizing the efficiency of assigned address space, machines that are on the same network share the same network address. In the case of subnetting, the host address is manipulated while the network address remains constant. The subnet address scheme picks a portion of the host address and recycles it as a subnet address. Bit positions are picked from the host address for use as a subnet identifier.

    Implementation of subnetting requires certain kind of hardware to be installed on the network. One way of doing so is by using a router and the other method is to use a Windows Server 2008 R2 machine with multiple NIC adapters which are configured with the routing enabled on the server. Routers of these kinds are known as multi-homed routers.

    A Sample Subnet

    A Sample Subnet

  • Use of Subnet Masks:

    For ensuring that the subnet address scheme works, every machine that is on the network is required to know the portion of the host address that has to be utilized as a network address. This is achieved by the assignment of a subnet mask to each machine.

    A 32 bit subnet mask is created comprising of 0s and 1s by the network administrator. Each of these bits have their own meaning in this arrangement. 1s denote the position in the IP address that refers to the network and the subnet address, while 0s represent the positions that refer to the host part of the address. The subnet mask can be shown in two ways - by using decimal equivalents or binary patterns.

    All networks are required to have subnets and there is no need to have customized subnet masks as default masks are allocated. Once the subnet masks are created assigned to the various machines by the administrator.

  • Calculating the Number of Subnets

    The formula that is resorted to for calculating the highest number of subnets and the hosts for every subnet are:

    • 2 × number of masked bits in subnet mask = maximum number of subnets
    • 2 × number of unmasked bits in subnet mask - 2 = maximum number of hosts per subnet

    In the above mentioned formulas:

    • masked: means the bit position of 1 and
    • unmasked: means the bit position of 0.

    The disadvantage of using an entire byte of a node address as the subnet address is that it reduces the number of node addresses that are possible on each subnet.


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