What is a subnet mask? How to work out IP addresses and subnets

Subnet masks are used by many organizations and businesses. Using a subnet mask makes network allocation more orderly and logical. Let's look at what a subnet mask is? How does a subnet mask work? Let MONA Media walk you through the details of the subnet mask in the article below.

About the subnet mask

Today's smart devices each have their own IP address with two parts: the host/client address and the network address. IP addresses are assigned by a DHCP server or set manually (static IP address). The subnet mask divides an IP address into the host address and the network address. It tells the system exactly which parts of an IP address belong to the network and which belong to the device.

What is a subnet mask?

First, let's get the definition of a subnet mask so we have a clear picture of the term. A subnet mask is a 32-bit binary number that lets you split an IP address into a network address and a host address. Certain numeric addresses cannot be used for hosts.

For example, address 255 is always assigned as the broadcast address, while address 0 is assigned by default as the network address. The subnet mask, IP address, gateway or router make up the basic structure most networks use to communicate between devices over the Internet Protocol.

When required, sub-networking automatically divides the host portion of an IP address into smaller subnets, which enables the subnetting process.

What a subnet mask does

When a business uses a subnet mask to divide its network, the system can tell the private network from the company-wide network. The network is also split into smaller sub-networks with separate IP addresses. As a result, the network's connectivity reaches its maximum.

Here is a more concrete example of what a subnet mask does. Say a large company has hundreds of thousands of devices accessing its LAN. On paper, the network structure can handle every connection. In practice, the structure limits the number of machines that can connect: some can get on the network, others cannot.

In worse cases, when all devices share one network, the system becomes harder to manage and control. The subnet mask was created to solve these difficulties.

Benefits of a subnet mask

A subnet mask brings users several benefits, such as:

• It cuts the time spent processing data transfers, which frees up CPU time.
• Each computer network can work with its own configuration, without depending on the LAN.
• It helps direct the data flow and limits the range of broadcast messages, which noticeably reduces congestion on the network.
• The shared LAN system runs more efficiently, with no downtime. Problems or network errors can be fixed easily by the administrator without affecting the wider network.

How a subnet mask works

It is worth understanding how this number works before you use it. A subnet mask divides a network into several parts, called subnetworks or subnets.

Users do not need to install or add a new network to reduce network traffic, which makes using the internet easier. You only need one main network, and the sub-units divide it into smaller networks for their own use. This way the network does not get overloaded.

For organizations that use a single local LAN and need to split it into several segments, subnetting is essential, and it gives these benefits:

• Reduces network load by cutting the volume of broadcast traffic
• Users can work from home and still use the local network.
• Organizations can get around LAN limits and exceed the cap on the number of hosts.

How to subnet

By the subnet concept, a network has 256 addresses in total (a class C network). Of these, one address identifies the network and another identifies the broadcast address. That leaves 254 addresses available for hosts.

Divide the 256 addresses evenly into 8 subnets, and each subnet has 32 addresses. Each new subnet needs 2 addresses for the subnet itself and for broadcast. The end result is 8 subnets, each with 30 network addresses available for hosts. Managing addresses is then easier and more efficient.

IP address classes and subnet masks

Address schemes for a range of existing networks are based on how an IP address is divided into octets. As the picture below shows, the class is determined by the three leftmost high-order bits of any given IP address, across the five network classes A to E.

The classes are divided as follows:

• Class D is reserved for multicast, and class E is not used on the internet because it is reserved for research by the Internet Engineering Task Force (IETF).
• Class A uses the first octet for the network portion. Octets 2, 3 and 4 are divided by the network administrator into hosts and subnets as needed. Class A is used for networks with more than 65,536 hosts.
• Class B uses the first two octets for the network. The remaining 16 bits, octets 3 and 4, are for subnets and hosts. Class B is for networks with 256 to 65,534 hosts.
• Class C uses the first three octets for the network, and the remaining 8 bits of octet 4 for hosts and subnets. Class C is used for smaller networks with fewer than 254 hosts.

Class A, B and C networks also have a natural mask, or default subnet mask:

• Class A: 255.0.0.0
• Class B: 255.255.0.0
• Class C: 255.255.255.0

You can determine the number of hosts and the type of an IP address from its default subnet mask.

How to calculate IP addresses and subnet masks

Normally, a 32-bit IP address represents one unique device on the network. Through the subnet mask, the 32 binary bits are split into the host and network parts, in 4 groups of 8 bits. Binary is hard for many people to calculate with, so developers solved this by converting each octet to decimal numbers separated by dots.

Example: 172.16.254.5 is a dotted decimal address whose values range from 0 to 255, representing 00000000 to 11111111 in binary.

What is a subnet mask calculator?

The calculation methods have many uses, such as widening the range or searching with tools known as subnet mask calculators. A subnet mask calculator gives you information such as the IP range, IP addresses and so on.

Some popular subnet mask calculators are:

• IPv6 Subnet calculator: helps map hierarchical subnets.
• IPv4/IPv6 Calculator/Converter: an IP subnet calculator alternative for IPv6, or for converting IP numbers from IPv4 to IPv6.
• IPv4 CIDR Calculator: helps adjust subnet masks and convert hex.
• IPv4 Wildcard calculator: uses wildcard mask calculation to identify available IP addresses.
• HEX Subnet Calculator: calculates end-point subnet addresses and the hexadecimal notation of multicast addresses.
• IP Subnet Mask Calculator: a basic tool for identifying subnets and the matching subnet masks.
• Subnet Range/Address Range Calculator: gives the most accurate start and end addresses.

The article above covered what a subnet mask is? How a subnet mask works. It also pulled together how to divide a network with a subnet mask and what subnet mask calculators are. We hope this useful information helps you understand the subnet mask, a very important part of any network system.