Subnet Mask

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Subnet Mask Definition

Every device has an IP address with two pieces: the client or host address and the server or network address. IP addresses are either configured by a DHCP server or manually configured (static IP addresses). The subnet mask splits the IP address into the host and network addresses, thereby defining which part of the IP address belongs to the device and which part belongs to the network.

The device called a gateway or default gateway connects local devices to other networks. This means that when a local device wants to send information to a device at an IP address on another network, it first sends its packets to the gateway, which then forwards the data on to its destination outside of the local network.

Diagram depicts a subnet mask architecture.

What is Subnet Mask?

A subnet mask is a 32-bit number created by setting host bits to all 0s and setting network bits to all 1s. In this way, the subnet mask separates the IP address into the network and host addresses.

The “255” address is always assigned to a broadcast address, and the “0” address is always assigned to a network address. Neither can be assigned to hosts, as they are reserved for these special purposes.

The IP address, subnet mask and gateway or router comprise an underlying structure—the Internet Protocol—that most networks use to facilitate inter-device communication.

When organizations need additional subnetworking, subnetting divides the host element of the IP address further into a subnet. The goal of subnet masks are simply to enable the subnetting process. The phrase “mask” is applied because the subnet mask essentially uses its own 32-bit number to mask the IP address.

IP Address and Subnet Mask

A 32-bit IP address uniquely identifies a single device on an IP network. The 32 binary bits are divided into the host and network sections by the subnet mask but they are also broken into four 8-bit octets.

Because binary is challenging, we convert each octet so they are expressed in dot decimal. This results in the characteristic dotted decimal format for IP addresses—for example, The range of values in decimal is 0 to 255 because that represents 00000000 to 11111111 in binary.

IP Address Classes and Subnet Masks

Since the internet must accommodate networks of all sizes, an addressing scheme for a range of networks exists based on how the octets in an IP address are broken down. You can determine based on the three high-order or left-most bits in any given IP address which of the five different classes of networks, A to E, the address falls within.

(Class D networks are reserved for multicasting, and Class E networks not used on the internet because they are reserved for research by the Internet Engineering Task Force IETF.)

A Class A subnet mask reflects the network portion in the first octet and leaves octets 2, 3, and 4 for the network manager to divide into hosts and subnets as needed. Class A is for networks with more than 65,536 hosts.

A Class B subnet mask claims the first two octets for the network, leaving the remaining part of the address, the 16 bits of octets 3 and 4, for the subnet and host part. Class B is for networks with 256 to 65,534 hosts.

In a Class C subnet mask, the network portion is the first three octets with the hosts and subnets in just the remaining 8 bits of octet 4. Class C is for smaller networks with fewer than 254 hosts.

Class A, B, and C networks have natural masks, or default subnet masks:

  • Class A:
  • Class B:
  • Class C:

You can determine the number and type of IP addresses any given local network requires based on its default subnet mask.

An example of Class A IP address and subnet mask would be the Class A default submask of and an IP address of

How Does Subnetting Work?

Subnetting is the technique for logically partitioning a single physical network into multiple smaller sub-networks or subnets.

Subnetting enables an organization to conceal network complexity and reduce network traffic by adding subnets without a new network number. When a single network number must be used across many segments of a local area network (LAN), subnetting is essential.

The benefits of subnetting include:

  • Reducing broadcast volume and thus network traffic
  • Enabling work from home
  • Allowing organizations to surpass LAN constraints such as maximum number of hosts

Network Addressing

The standard modern network prefix, used for both IPv6 and IPv4, is Classless Inter-Domain Routing (CIDR) notation. IPv4 addresses represented in CIDR notation are called network masks, and they specify the number of bits in the prefix to the address after a forward slash (/) separator. This is the sole standards-based format in IPv6 to denote routing or network prefixes.

To assign an IP address to a network interface since the advent of CIDR, there are two parameters: a subnet mask and the address. Subnetting increases routing complexity, because there must be a separate entry in each connected router’s tables to represent each locally connected subnet.

What Is a Subnet Mask Calculator?

Some know how to calculate subnet masks by hand, but most use subnet mask calculators. There are several types of network subnet calculators. Some cover a wider range of functions and have greater scope, while others have specific utilities. These tools may provide information such as IP range, IP address, subnet mask, and network address.

Here are some of the most common varieties of IP subnet mask calculator:

  • A IPv6 IP Subnet Calculator maps hierarchical subnets.
  • An IPv4/IPv6 Calculator/Converter is an IP mask calculator that supports IPv6 alternative and condensed formats. This network subnet calculator may also allow you to convert IP numbers from IPv4 to IPv6.
  • An IPv4 CIDR Calculator is a subnet mask adjustment and Hex conversion tool.
  • An IPv4 Wildcard Calculator reveals which portions of an IP address are available for examination by calculating the IP address wildcard mask.
  • Use a HEX Subnet Calculator to calculate the first and last subnet addresses, including the hexadecimal notations of multicast addresses.
  • A simple IP Subnet Mask Calculator determines the smallest available corresponding subnet and subnet mask.
  • A Subnet Range/Address Range Calculator provides start and end addresses.

What Does IP Mask Mean?

Typically, although the phrase “subnet mask” is preferred, you might use “IP/Mask” as a shorthand to define both the IP address and submask at once. In this situation, the IP address is followed by the number of bits in the mask. For example:

These are equivalent to

IP address: with subnet mask of

IP address: with a subnet mask example of

However, you do not mask the IP address, you mask the subnet.

For more on the actual implementation of load balancing, security applications and web application firewalls check out our Application Delivery How-To Videos.