Subnet Calculator

The IP subnet calculator provides a quick and easy way to find information about subnets for IPv4 and IPv6, including details like network addresses, host ranges, and subnet masks.

Using the calculator is simple. If you are using the IPv4 subnet calculator, choose the network class (or select ‘Any’), choose the subnet mask in CIDR notation, then input the IP address and hit calculate. For IPv6 networks, select the network prefix length and enter the IP address before hitting calculate.

For either network type, just those few basic details will provide relevant information about the actual or possible subnet.

Some useful tips

• You can use any IP address in your subnet’s IP range. The calculator will provide all the details for that range, so you don’t need to work out where each subnet starts and finishes when you are gathering details.
• If you need to recalculate when you get the results, the calculator is also on the results page. However, it will be under the results and a list of all the possible subnets, so you might need to scroll to find it. It will have your previous calculation pre-filled, making it easy to adjust.
• If you are using the subnet calc to set up a network, think carefully about your requirements. You might think you need small subnets now, but how easy will it be to re-assign all those fixed host IP addresses if your needs grow?

Why you should use the subnet calculator

Although it is possible to calculate it manually, it is not easy. The process requires converting IP addresses into binary, adding the subnet mask, and then converting them back into decimal or hexadecimal. Using a subnet calculator eliminates the risk of errors that come with manual calculations, which may result in overlapping subnets and difficulty in identifying networking problems later.

And while most people who use a subnet mask calculator will be IT professionals who understand networking, our version only requires basic information. This makes it accessible to those who might occasionally need to manage a network but are not networking experts.

IP subnet basics

One of the main reasons that subnetting was developed was the relative shortage of IP addresses. Traditional IPv4 addresses can run out quickly, especially in large organizations. Subnetting can make the use of network IP addresses more efficient, reducing the number of unused addresses. But they also bring other benefits because each subnet is independent.

One benefit is performance. Any node on a network can see all the traffic on that network. And as networks grow larger, this creates a big performance hit.

It also improves security by limiting nodes and users to just their subnetwork. For example, customer service staff would not be able to see finance department data and vice versa, helping to protect information and confidentiality.

It can also help IT staff. Because the network is divided into subnets, it limits the potential causes of problems, making troubleshooting easier.

How subnetworks work

Subnets divide parts of a network into smaller networks using what is known as a subnetmask. An IP address consists of a network prefix and a host ID. The first part identifies the network, and the second part identifies a single node.

You can identify the network prefix in two ways. Its class, A, B, or C, refers to the groups of numbers, or octets, that form the network prefix. A more recent method uses Classless Inter-Domain Routing or CIDR. This is denoted by a slash and a number following the IP address. The number refers to the number of bits that form the network prefix.

As an example of CIDR, Google owns the IP range 173.194.0.0 to 173.194.255.255, which could also be expressed as 173.194.0.0/16. The 16 identifies the first 16 binary bits, 10101101.11000010 or 173.194 in decimal, as the network prefix. IPv6 addresses, with 128 bits, will only use CIDR notation.

Subnetmasks use the same notation. Therefore, a subnet mask of 255.255.255.0/24 means that the final group or numbers, or octet, can be anything from zero to 255, using up one more bit in the subnet mask 255.255.255.128/25 halves the available range to either zero to 127 or 128 to 255.

Using both the subnet mask and the host ID, a router can determine which network a host is on and direct traffic accordingly.

The calculator in action

To understand how the calculator can help, let’s imagine a small business with about 100 employees working in four different departments that each need a network.

One solution would be to have four separate class C networks, which would waste over 900 IP addresses. Instead, we can use the subnetting calculator to calculate our subnets.

In our first example, we’ll use 192.168.0.0 as the starting point for our calculations. We know that a CIDR notation of /24 would be a network with 256 addresses. Using up two more bits and using /26, we further limit subnets to just 64 addresses.

In our IP range calculator, select the relevant subnet, in this case, 255.255.255.192 /26 (the 26 is the crucial part), then in the IP address box, type 192.168.0.0. Then press calculate.

The results will be displayed instantly, presenting detailed information about the first network. Underneath, it will also have basic details about all four possible networks using that subnet mask.

To see fuller details for each subnet, enter an IP address from the range and calculate again.

Using the IP calculator for IPv6 is even easier. Input the prefix length and the IPv6 address. The design of IPv6 overcomes many of the limitations in IPv4, not least the number of available addresses. Because IPv6 addresses have 128 bits, an IPv6 network can easily have billions of nodes, although most network administrators want something more manageable.

IPv6 only uses CIDR to identify the network prefix and host ID and does not use a subnet mask. Input the prefix length and address into the IP address range calculator. For example, to get a network range with 256 hosts, use a prefix of /120. The calculator will provide the details of the network and host range.