What Is the Purpose of a Subnet Mask in IP Addressing?


The subnet mask is part of the Internet Protocol version 4 (IPv4) system. IPv4 has been around since 1973; even before the World Wide Web. It marks the standard addressing scheme used by all networks around the world. IPv4 is made up of two parts, the IP address and the subnet mask. One cannot work without the other. The architecture of the system looks disarmingly simple; but it is not.

What is the Structure of an IP Address

  • An IP (Internet Protocol) address is based on the numbers 0 to 255. The IP number has four parts, called octets. For example, 154.37, 39, 210, is an IP number. Each octet, can only use the numbers from 0 to 255.

    Another feature of the IP number is that there are five classes that define what type of network the IP number resides in. Those classes are:

     0 -126: Class A

    128 - 191: Class B
    192 - 223: Class C
    224 - 239: Class D
    240 - 255: Class E

    Notice that the number 127 is not included; that is because it is a self reflecting number called a loopback address. Another factor is that only three classes are used by network administrators: A, B and C. The other two, D and E are reserved.

    One more thing. The Class that was mentioned above only applies to the first octet. This is important because in order to go from one network to another, The Router, will only look at the first octet to determine where to forward the data.

What is a Router

  • Think of a traffic light at an intersection. It directs traffic. It has red, amber and green lights. It also has directional signals. You can turn right, or left, or stop, or go.

    But it only directs traffic. That is what a router does; it directs traffic, in this case network traffic. It could be LAN traffic, WAN traffic or Internet traffic.

    You cannot put applications on a router; it just has an operating system designed for only one job: control traffic.

    In relation to the subnet mask and the IP address. The router will look at the IP number, and decide if it is class A, B, C, D or E. From that information, the router can direct the traffic to the proper location, defined by the subnet mask.

What is the Structure of a Subnet Mask

  • The subnet mask also has only four octets. However, unlike the IP number, there are only certain numbers that can be used in the mask. The numbers it can use are: 255, 254, 252, 248, 240, 224, 192, 128 and 0.

    Here are three examples: or or

    These masks are used to define how many networks are available to the IP address. That is because each IP address is part of a network of addresses.

What Is the Purpose of the Subnet Mask

  • Think of the following: there are four buildings, each with their own address. One building has 32 floors, one has 16 floors, the two others have 64 and 128 floors.

    The address on one is; another is; is the third; and the fourth.

    Each IP can have the same subnet mask as the others; or it can have a separate subnet mask from the others. But the important point is that subnet mask is used to specify what "floor" the IP address is on. It will also specify how many "desks" are in that "floor." In other words, an IP address does not exist independently of other IP addresses, it is part of a network of similar IP addresses. That is what makes it a network.

    The IP network address identifies the "building," but the subnet mask identifies the "floor" in the building; it can also specify how many "desks" are on that "floor."

    That is the relationship between the mask and the IP address. They are not independent of each other.

How Does the IP Address Work with the Subnet Mask

  • Make no mistake, working with a subnet mask is among the most complicated concepts in networking. This will give you an example. Take the IP address 192.168, 45.61 with a subnet mask of The math is intricate but there are 16 "floors" and in each floor there are 16 "desks."
    In network speak, there are 16 networks, and 16 hosts (i.e., computers) in each network.

    The location of the IP address is important, it is located on the 4th network, with a network ID of There are 16 addresses starting with and ending with

    This is only one of 16 networks. The rest are:


  • In networking, the subnet mask helps define where an IP number will appear in a network. The relation between the mask and the IP number is intricate. Each needs the other to create a network of numbers.

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