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.

Public vs. Private IPs: What Every Junior Admin Needs to Know

Have you ever wondered why almost every home Wi-Fi router in the world assigns your laptop an IP address starting with 192.168.x.x or 10.0.x.x? The answer lies in a document published in 1996 called RFC 1918, which completely saved the internet from collapsing.

[Image showing NAT translating private IP addresses to a single public IP address]

Understanding the difference between Public IP addresses and Private IP addresses is one of the most foundational concepts in networking. Let's break down exactly what they are and how Network Address Translation (NAT) makes them work together.

The IPv4 Exhaustion Problem

IPv4 was invented in the early 1980s. It provides roughly 4.3 billion unique IP addresses. Back then, it seemed impossible that we would ever run out. But by the 1990s, with the explosion of personal computers and the early web, engineers realized a terrifying truth: we were going to run out of IPs very quickly.

To delay the internet breaking while they invented IPv6, engineers created a stop-gap solution: Private IP Addresses.

What is a Private IP Address?

A private IP address is an address that is entirely unroutable on the global internet. Think of it like a local extension line in an office building. If you are inside the building, you can dial extension "105" and reach Bob. But someone outside the building cannot call "105" directly—they have to call the building's main phone number first.

The IETF defined three specific ranges of IPv4 addresses to be used strictly for private, internal networks:

Class A Range: 10.0.0.0 to 10.255.255.255 (Used by large enterprises)
Class B Range: 172.16.0.0 to 172.31.255.255 (Used by medium businesses and AWS/Docker)
Class C Range: 192.168.0.0 to 192.168.255.255 (Used by almost all home routers)

Check Your Subnet Class

Want to see if your IP address is Public or Private? Type it into our main subnet calculator and it will instantly tell you its Class and Network Type.

Use the Subnet Calculator →

What is a Public IP Address?

A Public IP address is assigned by your Internet Service Provider (ISP). It is globally unique and completely routable across the public internet. This is the "main phone number" to your building.

How Do They Communicate? (Enter NAT)

If your laptop has a private IP of 192.168.1.50, and private IPs are blocked on the internet, how are you reading this website right now?

The answer is Network Address Translation (NAT). Your home router acts as a middleman. When your laptop requests a webpage, the router takes your packet, strips off your private 192.168.1.50 IP, slaps its own Public IP on it, and sends it to the server. When the server replies, the router remembers that you asked for the page and forwards the data back to your local, private IP.

Because of NAT and Private IPs, thousands of internal devices can share a single Public IP address, effectively saving the IPv4 internet from total exhaustion for the last 30 years!