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.

IPv6 Subnetting Basics: Why /64 is the Golden Rule

If you have spent your entire IT career mastering IPv4 VLSM to save precious IP addresses, IPv6 is going to require a massive shift in mindset. With 340 undecillion available addresses, the concept of "conserving hosts" is entirely dead.

In IPv6, we do not subnet to save addresses; we subnet strictly to organize our routing architecture and define administrative boundaries. The core of this new philosophy revolves around one magical number: the /64 prefix.

The Structure of an IPv6 Address

An IPv6 address is 128 bits long, represented as eight groups of four hexadecimal digits. For the vast majority of enterprise deployments, this 128-bit address is split cleanly down the middle:

First 64 bits (Network Prefix): Used for global routing and site subnetting.
Last 64 bits (Interface ID): Used to uniquely identify the specific host/device on that network.

Why is /64 the Golden Rule?

In IPv4, you might assign a /29 to a small remote office because they only need 5 IP addresses. In IPv6, you will assign that same office a /64, giving them 18.4 quintillion addresses. Why?

Because IPv6 relies heavily on a feature called SLAAC (Stateless Address Autoconfiguration). SLAAC allows devices like smartphones, laptops, and IoT sensors to automatically generate their own IP addresses without needing a DHCP server.

However, the SLAAC protocol (specifically the EUI-64 process) hard-codes the requirement that the Interface ID must be exactly 64 bits long. If you assign a prefix smaller than a /64 (like a /80 or /96) to a local LAN, SLAAC will completely break, and your modern devices will fail to connect to the network.

Calculate IPv6 Address Scopes

Need to expand compressed IPv6 addresses or find the exact start and end ranges of a massive IPv6 block? Use our dedicated tool.

Open the Free IPv6 Calculator →

How ISPs Assign IPv6 Blocks

Because every single VLAN or LAN segment in your company will receive a /64, ISPs do not hand out single IP addresses anymore. They hand out massive blocks:

/48 (Enterprise Site Allocation): Gives a business exactly 65,536 /64 subnets to deploy across their buildings.
/56 (Residential Allocation): Given to home users, providing 256 /64 subnets (more than enough for a home network and guest Wi-Fi).

The only time you should ever violate the /64 rule is for point-to-point router links, where RFC 6164 recommends using a /127 to prevent neighbor discovery exhaustion attacks.