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.

Understanding IPv4-Mapped and 6to4 Addresses

The transition from IPv4 to IPv6 has been happening for over a decade, but it is not a "flip the switch" event. In reality, modern networks must operate in a dual-stack environment where IPv4 and IPv6 coexist. A major challenge in this environment is allowing pure IPv6 applications to process legacy IPv4 addresses.

To solve this, the IETF created specific transition mechanisms that embed an IPv4 address directly inside an IPv6 address format. Let’s break down the two most common types: IPv4-Mapped Addresses and 6to4 Prefixes.

IPv4-Mapped IPv6 Addresses

An IPv4-Mapped address is by far the most common transition address you will see as a developer or systems administrator. It is used primarily by dual-stack operating systems to represent an IPv4 address to an application that expects an IPv6 address (like modern Node.js or Python server bindings).

How it is structured:

An IPv6 address is 128 bits. To map a 32-bit IPv4 address inside it, the system does the following:

The first 80 bits are set to 0.
The next 16 bits are set to 1 (which is FFFF in hexadecimal).
The last 32 bits contain the original IPv4 address.

Example: The IP address 192.168.1.1 becomes ::ffff:192.168.1.1 (Dot-Decimal hybrid format) or ::ffff:c0a8:0101 (Pure Hexadecimal format).

Need to Map an Address Quickly?

Our dedicated tool instantly converts any standard IPv4 address into its proper IPv4-Mapped and 6to4 Hexadecimal formats.

Open the IPv4 to IPv6 Converter →

6to4 Transition Mechanism (RFC 3056)

While IPv4-Mapped addresses are mostly used internally by software, the 6to4 mechanism is an actual routing technique. It allows IPv6 packets to be transmitted over an IPv4 core network without needing explicit IPv6 tunnels configured by the ISP.

How 6to4 works:

The Internet Assigned Numbers Authority (IANA) reserved the massive 2002::/16 prefix specifically for 6to4 routing. To create a 6to4 prefix for your network:

Start with the reserved prefix: 2002:
Convert your public IPv4 address to hexadecimal (e.g., 192.168.1.1 becomes c0a8:0101).
Combine them to get your unique /48 IPv6 prefix: 2002:c0a8:0101::/48.

This provides your site with a massive /48 block of IPv6 addresses that your local routers know how to encapsulate and send out over your IPv4 ISP connection.

What about IPv4-Compatible Addresses?

If you read older Cisco documentation, you might see "IPv4-Compatible" addresses (e.g., ::192.168.1.1). These are simply IPv4 addresses padded with 96 bits of zeros. However, this format has been officially deprecated by the IETF (RFC 4291) because it was causing routing confusion. Modern networks should strictly use the IPv4-Mapped format (with the ::ffff: prefix) instead.