Network Subnet Calculator
The Network Subnet Calculator is a free, private tool for planning IPv4 and IPv6 addressing without sending data to a server. Enter a CIDR or an address with a dotted mask to see the network and broadcast addresses, first and last usable hosts, usable host counts (including RFC 3021 /31 links and /32 host routes), wildcard and binary masks, legacy class, and whether the address is private, public, CGNAT, loopback, or link-local. Split a parent block by subnet count or hosts per subnet into a table you can copy as CSV, or feed a VLSM list of named host requirements and allocate largest-first with a clear error when the plan does not fit. IPv6 mode compresses and expands addresses, shows the prefix network, first and last address, an exact BigInt address count, and the type — global, ULA, link-local, or multicast — so lab and production planning stay in the browser.
How to use
- On IPv4, enter a.b.c.d/nn or an IP plus mask; tap Copy beside any result field.
- Use Splitter to divide by subnet count or hosts per subnet, or VLSM with name,hosts lines; Copy CSV from the bar.
- Switch to IPv6 for compress/expand, prefix range, exact address count, and type — Share or Copy the summary anytime.
FAQ
Does the subnet calculator send my addresses to a server?
No. IPv4, IPv6, VLSM, and splitter math all run in your browser. Nothing is uploaded or logged.
How are /31 and /32 networks handled?
A /31 follows RFC 3021 with two usable addresses and no reserved network or broadcast. A /32 is treated as a single host route.
What does the VLSM planner do?
You list named host requirements; the planner sorts largest-first, allocates aligned blocks inside the parent CIDR, and errors if the plan does not fit.
Is IPv6 address counting exact?
Yes. Counts use BigInt so prefixes from /0 through /128 stay exact, including huge ranges that exceed JavaScript Number precision.