About SubnettingCalculator.com

SubnettingCalculator.com was built by networking professionals who spent years doing subnet maths by hand, flipping between half a dozen different tools mid-config, and hunting for a plain English explanation of why VLSM always places the largest subnet first. We got tired of tools that were either too basic to be useful or too complicated to use quickly under pressure.

The goal was simple: build one place with every tool a network engineer or student actually needs, make each tool fast and accurate, explain every concept in plain English alongside it, and keep the whole thing completely free — no accounts, no subscriptions, no ads obscuring the results.

Networking knowledge should not be locked behind expensive software or scattered across dozens of half-finished web pages. Whether you are a CCNA student working through subnetting for the first time, a senior network engineer who just needs a quick wildcard mask, or a developer trying to understand why two AWS VPCs are refusing to peer — you deserve a tool that gives you a correct answer immediately and explains what it means.

Our flagship tools cover every IPv4 subnetting scenario from a quick single-subnet lookup to complex multi-department network designs.

1. → All-in-One Subnet Calculator — the homepage combines four tools in a single interface. Enter an IP and mask and get live subnet details instantly. Switch to FLSM, VLSM, or Supernetting mode without leaving the page. The fastest way to go from an address to a complete network plan.
2. → Subnet Calculator — dedicated single-subnet tool. Enter any IP and prefix and get the complete breakdown: network address, broadcast, first and last usable host, wildcard mask, binary representation, usable host count, IP class, and more. Results update live as you drag the CIDR slider.
3. → FLSM Calculator — Fixed Length Subnet Masking. Divide a network into a set number of equal-size subnets in one click. Enter your base network, quantity, and hosts-per-subnet and get a complete subnet table with every network address, usable range, broadcast, mask, and wildcard — plus a Free/Unused IP table showing remaining address space.
4. → VLSM Calculator — Variable Length Subnet Masking. Enter comma-separated host requirements (e.g. 100, 50, 20) and the calculator sorts them largest-first, allocates the minimum-waste subnet for each, and outputs a complete table with free blocks. The most efficient way to design a real-world network addressing scheme.
5. → Supernetting / Route Summarization — combine multiple networks into a single summary route to reduce routing table size. Paste two or more networks (one per line) and get the summary prefix, the exact address range it covers, and confirmation that all input networks are contained within it.
6. → Subnet Overlap Checker — enter two networks or IP addresses and instantly know whether they overlap, whether one is a subset of the other, or whether they are completely safe to route together. Essential before configuring VPC peering, VPN tunnels, or merging two networks.
7. → Subnet List Generator — expand any CIDR block into a complete list of every usable host address. Copy the output directly into firewall rules, audit documentation, or DHCP pool planning spreadsheets.
8. → IP Address Class & Type Detector — enter any IPv4 or IPv6 address and instantly identify its classful category, default mask, private/public status, and scope. Works for both IPv4 (Classes A–E) and IPv6 (Global Unicast, Link-Local, Unique Local, Multicast, and more).
9. → IPv4 Subnet Cheat Sheet — a pre-calculated reference table of all 33 prefix lengths from /0 to /32. Every row shows the subnet mask, wildcard mask, total addresses, usable hosts, and common use. Includes the three RFC 1918 private ranges. Bookmark it and keep it open during exams and firewall configurations.

IPv6 addressing is fundamentally different from IPv4 — the numbers are incomprehensibly large, there is no dotted decimal subnet mask, and the prefix lengths that matter follow entirely different patterns. Our IPv6 tools are built to close that knowledge gap quickly.

1. → IPv6 Subnet Calculator — enter any IPv6 address and prefix length from /0 to /128. Instantly get the expanded address, compressed address, network prefix, host portion, first and last addresses in the block, total address count, address type and scope, and the number of /64 subnets contained within the block.
2. → IPv6 SLAAC / EUI-64 Generator — enter a MAC address and an optional /64 prefix and see the complete EUI-64 conversion step by step: the FF:FE insertion, the 7th-bit flip, and the final Interface Identifier. Shows the full SLAAC-generated IPv6 address when a prefix is provided.
3. → IPv6 Address Compression Tool — paste any IPv6 address in any format (expanded, partially compressed, or messy) and get back the correctly compressed form, the fully expanded form, and the raw hex string. All three formats are instantly copyable.
4. → IPv6 Subnet Cheat Sheet — three reference tables in one page: common prefix lengths from /0 to /128 with /64 subnet counts and typical use, all 11 IPv6 address types with scope and routability, and well-known multicast addresses. The most comprehensive IPv6 quick-reference available.
5. → IPv4 to IPv6 Converter — converts any IPv4 address into its three transition-mechanism representations: IPv4-Mapped (::ffff:), 6to4 (2002::), and the deprecated IPv4-Compatible format. Used when working with dual-stack applications, database schemas that store IPv6 addresses, and legacy tunnel configurations.

Networking work generates a constant stream of format conversion tasks — wildcard masks for ACLs, reverse DNS for PTR records, IP integers for database queries, MAC addresses for switch configs. Our converters handle all of it in one place.

1. → IP Range to CIDR Converter — enter a start IP and end IP and get the minimum set of CIDR blocks that covers the range exactly with no gaps and no excess. Essential when configuring firewalls and cloud security groups (AWS, Azure, GCP) that require CIDR notation rather than arbitrary ranges.
2. → IP Address Converter — converts any IPv4 or IPv6 address into binary, hexadecimal, and integer (decimal) formats. Auto-detects the input type. Used for packet analysis, database storage optimisation, and decoding obfuscated IP addresses in security research.
3. → Wildcard Mask Converter — enter a subnet mask or CIDR prefix and instantly get the wildcard mask used in Cisco IOS ACLs and OSPF network statements. Shows the full binary breakdown so you can see exactly which bits are being matched versus ignored.
4. → Reverse IP Converter — generates the reverse DNS (PTR record) notation for any IPv4 or IPv6 address. Essential for configuring DNS zone files, setting up mail server rDNS, and verifying PTR records during network troubleshooting.
5. → MAC Address Converter — converts any MAC address between all four common formats: colon-separated (Linux/macOS), hyphenated (Windows), Cisco dot notation, and plain hex. Accepts any input format and outputs all four simultaneously. Includes a Random MAC generator for lab and VM testing.
6. → IP Address Cleaner — paste any block of text containing IP addresses — log files, firewall exports, emails, spreadsheets — and get back a clean, deduplicated, sorted list of every valid IPv4 and IPv6 address found. All surrounding text and noise is stripped automatically.

Every tool on the site has its own how-to section written from scratch in plain English. We do not assume you already know what VLSM stands for or why wildcard masks have inverted bits. Each guide explains what the concept is, why it matters, and exactly what to enter and what the output means. All guides include worked examples with real numbers, reference tables, and tip boxes for non-obvious details.

If you want everything in one place, our complete documentation page covers all 19 tools with step-by-step guides, output field explanations, and reference tables — organised with a quick navigation index so you can jump to any tool instantly.

A calculator that gives you a correct answer without helping you understand it is only half useful. Our blog exists to fill in the gaps — to explain why the maths works the way it does, where each concept is used in real networks, and how networking topics connect to things you may already encounter in your work.

Articles are written at a level that works for both CCNA students and working engineers who want a quick refresher without having to wade through academic language or vendor marketing.

Every tool on the site runs its calculations in JavaScript or PHP using the same bitwise logic a router uses. Results are verified against known-good outputs across hundreds of test cases covering edge cases like /0, /31, /32, the private RFC 1918 ranges, IPv6 link-local addresses, and VLSM allocations that push right up to the edge of the available address space. If you find a calculation error, please use the contact page to report it — we take accuracy seriously and will fix it.

All calculations run entirely in the browser — nothing is sent to a server and nothing waits on a network request. The CIDR slider on the subnet calculator updates every field in real time as you drag it. Forms are designed to accept input in whatever format you naturally type — dotted decimal, CIDR notation, and any common IP format are all handled without requiring a specific format string.

We do not require an account to use any tool on the site. We do not store IP addresses you enter into calculators. We do not sell data to third parties. Our complete Privacy Policy is published and written in plain English. You can read our Terms of Use and Disclaimer on their respective pages.

We continue to add tools, improve existing calculators, and expand the article library based on what the networking community actually needs. If there is a tool that does not exist yet but should, or an article topic that is not covered, we want to hear about it. Reach us through the contact page — feedback from real users is how the site grows.

Whether you have found a calculation error, have a suggestion for a new tool, spotted a bug, or just want to say the site helped you pass your CCNA — we read every message and genuinely appreciate the feedback. It is how the site improves.

You can reach us two ways:

1. → Contact Form — visit the Contact Page and fill in the form. We aim to respond within one to two business days.
2. → Email — send a message directly to contact@subnettingcalculator.com. Include as much detail as possible — if you are reporting a calculation issue, the IP address and prefix you entered helps us reproduce it quickly.