Subnet Calculator

What Is a Subnet Calculator?

A subnet calculator is a tool that computes all subnetting details from an IP address and CIDR prefix length. This tool supports both IPv4 (like 192.168.1.0/24) and IPv6 (like 2001:db8::/48). For IPv4, get the network address, broadcast address, subnet mask, wildcard mask, first and last usable hosts, total addresses, and usable host count. For IPv6, get the expanded and compressed address forms, network prefix, first and last addresses, total addresses, address type, and scope.

Whether you're a network engineer designing enterprise architectures, a system administrator configuring servers, or a student learning subnetting, this free IP subnet calculator eliminates manual binary math and gives you accurate results instantly. Use it as a subnet mask calculator to convert between CIDR and dotted-decimal notation, as a CIDR calculator that shows all 32 bits colored by network and host, as a VLSM calculator that splits a block by the hosts each subnet needs, or as an IP range to CIDR converter. All calculations happen in your browser — no data is sent to any server.

How CIDR Notation Works

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network prefix in the format IP/prefix. For IPv4, the prefix ranges from 0–32 (out of 32 bits). For IPv6, it ranges from 0–128 (out of 128 bits). The prefix indicates how many bits belong to the network portion — the remaining bits identify individual hosts or interface identifiers within that network.

Visual CIDR Calculator: Flip Bits and Watch the Range Change

Most IP calculators hide the binary. This one shows it. The grid above the results lays out all 32 bits of the IPv4 address in four octets. Blue bits are the network portion (the first n bits for a /n prefix) and purple bits are the host portion. Click any bit to flip it and the address, network, broadcast, and range recalculate instantly. Drag the prefix slider or press the − and + buttons to move the boundary between network and host bits, and use the ▲ ▼ arrows on an octet to step through neighbouring addresses.

Try it with a few common blocks: 192.168.1.0/24, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/25, or a single host such as 203.0.113.7/32.

Subnet Mask Cheat Sheet: Every CIDR Prefix from /8 to /32

Use this subnet cheat sheet when converting between subnet mask and CIDR notation. Every prefix from /8 to /32 is listed with its subnet mask, wildcard mask, total addresses, and usable hosts — the same numbers the calculator produces, in one CIDR chart you can bookmark. Click a prefix to load it into the calculator.

Subnet Splitter and VLSM Calculator

Open the Split / VLSM tab to divide a block into smaller networks. Split into N equal subnets (a /24 into four /26s), split to a target prefix (a /16 into /20s), or use VLSM (Variable Length Subnet Masking) and list the hosts each subnet needs. The VLSM calculator sorts the requirements largest first and assigns the smallest prefix that fits each one, so no address space is wasted and every subnet starts on a valid boundary.

The example above splits 10.0.0.0/24 for four departments that need 100, 50, 20 and 2 hosts. Open this VLSM plan in the calculator, then copy the table as CSV for your IP address management (IPAM) sheet. The remaining space (10.0.0.228 – 10.0.0.255) stays free for growth.

IP Range to CIDR Converter

Firewalls, cloud security groups, and router ACLs accept CIDR blocks, not arbitrary start and end addresses. The Range → CIDR tab takes any IPv4 range and returns the smallest set of CIDR blocks that covers it exactly. A range that lines up with a power of two collapses to a single block (192.168.0.0 – 192.168.0.255 is 192.168.0.0/24); an unaligned range such as 192.168.0.1 – 192.168.0.100 needs nine blocks, from a /32 up to a /27. Copy the list straight into an AWS security group, a Cloudflare IP list, or an ip route statement.

Going the other way (CIDR to IP range) is what the main tab already does: the first and last address of any block are the network and broadcast addresses shown in the results. To see which organization owns a public block, run the network address through the IP WHOIS lookup or the ASN lookup.

Wildcard Mask Calculator for Cisco ACLs and OSPF

Every IPv4 result includes the wildcard mask, the bitwise inverse of the subnet mask that Cisco IOS uses in access lists and OSPF network statements. A 0 bit in the wildcard means "must match" and a 1 bit means "ignore". So 192.168.1.0/24 becomes 192.168.1.0 0.0.0.255, 10.0.0.0/8 becomes 10.0.0.0 0.255.255.255, and a single host is 0.0.0.0 (or the host keyword). To convert by hand, subtract each octet of the subnet mask from 255.

IPv6 CIDR Prefix Reference

IPv6 uses 128-bit addresses, providing a vastly larger address space than IPv4. The IPv6 subnet calculator handles prefix lengths from /8 to /128. Here are the most common IPv6 prefix lengths used in real-world network planning and ISP allocations.

Unlike IPv4, IPv6 does not use broadcast addresses — it uses multicast and anycast instead. The standard subnet size for LANs is /64, which provides 2⁶⁴ addresses for Stateless Address Autoconfiguration (SLAAC). Our IPv6 calculator also detects the address type (Global Unicast, Link-Local, Unique Local, Multicast, Loopback, or IPv4-Mapped).

Understanding Subnet Calculation Results

When you enter an IP address in CIDR notation into the subnet calculator, it computes several key fields. Understanding each field is essential for proper network configuration and troubleshooting.

Private IP Address Ranges (RFC 1918)

RFC 1918 defines three blocks of private IPv4 addresses that are not routable on the public internet. These are used for internal networks behind NAT (Network Address Translation). Our subnet calculator automatically detects whether the entered IP falls within a private or public range.

Who Uses a Subnet Calculator?

Subnet calculators are essential tools for anyone working with IP networks. From enterprise network architects to home lab enthusiasts, understanding subnetting is a fundamental networking skill.

Subnetting Best Practices

Proper subnetting is crucial for efficient network design. Follow these best practices to avoid common mistakes and plan your IP addressing effectively.

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Frequently Asked Questions