What does a subnet calculator tell you?

A subnet calculator takes an IP address and a subnet mask or CIDR prefix and returns the complete network breakdown including network address, first and last usable host, broadcast address, total hosts, wildcard mask, and binary mask. It may also display the IP class and whether the address is private or public.

What is the difference between a subnet mask and a CIDR prefix?

A subnet mask and a CIDR prefix represent the same information in different formats. A subnet mask such as 255.255.255.0 uses dotted decimal notation, while a CIDR prefix such as /24 indicates how many leading bits are set to 1. Both formats are interchangeable.

What is the difference between FLSM and VLSM?

Fixed-Length Subnet Masking (FLSM) divides a network into subnets that are all the same size and use the same subnet mask. Variable-Length Subnet Masking (VLSM) creates subnets of different sizes based on host requirements, which makes more efficient use of IP address space.

When should I use FLSM instead of VLSM?

FLSM is useful when all network segments require the same number of hosts, such as in structured lab environments or simple legacy networks. In most modern networks VLSM is preferred because it uses address space more efficiently.

How does VLSM allocate subnets?

VLSM sorts host requirements from largest to smallest and assigns the smallest possible subnet that can support each requirement. For example, if a network needs 100, 50, and 20 hosts, it may allocate a /25, then a /26, then a /27 to minimize wasted addresses.

Does this calculator find free or unused IP addresses?

Yes. After performing FLSM or VLSM calculations, the calculator can generate a Free or Unused IP Network table that shows which address ranges remain unallocated and available for future subnetting.

What is Supernetting (Route Summarization)?

Supernetting, also called route summarization, combines multiple contiguous smaller networks into one larger summary route. This reduces routing table size, improves router efficiency, and simplifies network management.

What are the rules for supernetting to work correctly?

For supernetting to work correctly, the networks must be contiguous with no gaps, the number of networks must be a power of two such as 2, 4, 8, or 16, and the first network must start on the correct boundary for the summarized prefix.

Do I need to create an account or install software?

No. The calculator runs entirely in your browser and does not require account creation, downloads, or installation. All calculations are performed locally.

Can I calculate IPv6 subnets here?

This page focuses on IPv4 subnetting. A separate IPv6 subnet calculator can be used to analyze IPv6 prefixes and address details.

What other tools are available on this site?

The site offers multiple networking tools including subnet calculators, VLSM and FLSM calculators, supernetting tools, subnet overlap checkers, IPv6 calculators, EUI-64 generators, IP range to CIDR converters, wildcard mask converters, MAC address tools, and other utilities.

Are the results accurate enough to use in a real network design?

Yes. The calculations are based on the same binary and bitwise logic used by routers and networking systems, making them suitable for production network planning, certification study, firewall configuration, and cloud network design.

Route Summarization: A Step-by-Step Supernetting Guide

As enterprise networks grow, so do their routing tables. If a core router has to memorize the individual paths to thousands of small /24 or /28 networks, its CPU and RAM will quickly become overwhelmed, leading to dropped packets and network latency.

The solution is Route Summarization, also known as Supernetting. By mathematically combining multiple smaller, contiguous networks into a single large route, engineers can drastically reduce the size of routing tables. Let's break down exactly how it works.

What is Supernetting?

Supernetting is the exact opposite of subnetting. Instead of borrowing host bits to create more networks, supernetting borrows network bits to create more hosts (and merge networks together). This results in a subnet mask that is smaller than the default classful mask.

The Golden Rules of Route Summarization

Contiguity: The networks you are summarizing must be contiguous (sequential). You cannot summarize 10.0.1.0 and 10.0.5.0 without also including networks 2, 3, and 4.
Powers of Two: You should always try to summarize networks in blocks of 2, 4, 8, 16, etc.
Boundary Alignment: Networks must align on proper binary boundaries. If you summarize networks that cross an irregular boundary, you risk "over-summarization" (advertising routes that don't actually exist).

Calculate Summary Routes Instantly

Don't risk network outages by miscalculating binary boundaries. Paste your network list into our Supernetting tool to find the perfect summary route.

Open the Free Supernetting Calculator →

Step-by-Step Example

Let's say a branch office has four networks that need to be advertised back to the core data center over an OSPF link:

192.168.4.0/24
192.168.5.0/24
192.168.6.0/24
192.168.7.0/24

Step 1: Convert to Binary

Focus on the third octet, since the first two (192.168) are identical across all four networks.

4 = 00000100
5 = 00000101
6 = 00000110
7 = 00000111

Step 2: Find the Matching Bits

Look at the binary values above from left to right. Notice that the first six bits (000001) are exactly the same for all four networks. The bits only start changing at the 7th position.

Step 3: Calculate the New Subnet Mask

The first two octets perfectly matched, which is 16 bits (8+8). We found 6 matching bits in the third octet.
16 + 6 = 22.

Your summarized route is 192.168.4.0/22.

Instead of the router advertising four separate /24 networks, it now only advertises one single /22 network, saving valuable memory and bandwidth!