CIDR / Subnet Calculator

How to use the CIDR calculator

Type an address block in any of the accepted forms and every figure updates live. You can enter classless inter-domain routing (CIDR) notation such as 192.168.1.0/24, an address with a dotted subnet mask such as 10.0.0.0 255.0.0.0, or a bare address such as 172.16.5.4 (treated as a single host, /32). IPv6 works the same way: enter a prefix like 2001:db8::/48 and the first address, last address and total address count are computed with full 128-bit precision. The network and broadcast rows are highlighted because they are the two addresses you usually cannot assign to a host.

What CIDR notation means

CIDR notation writes an address followed by a slash and a prefix length — the number of leading bits that identify the network. In 192.168.1.0/24 the /24 means the first 24 bits are the network portion and the remaining 8 bits are available for hosts, giving 2⁸ = 256 addresses. The prefix length maps directly to a subnet mask: /24 is 255.255.255.0, /16 is 255.255.0.0, and so on. Shorter prefixes mean larger blocks; a /8 covers over sixteen million addresses, while a /30 covers just four. Because the boundary can fall anywhere, CIDR replaced the old fixed class A/B/C system and lets address space be allocated in right-sized blocks.

What each result means

The calculator reports the complete picture of the block so you can plan or audit a subnet at a glance:

• Network address — the first address, with all host bits set to zero; it names the subnet and is not assigned to a device.
• Broadcast address — the last address, with all host bits set to one; traffic sent here reaches every host on the subnet.
• Usable host range — the first and last addresses you can actually assign, i.e. everything between the network and broadcast.
• Total addresses — 2 raised to the number of host bits, including network and broadcast.
• Usable hosts — total addresses minus the two reserved ones (with the /31 and /32 exceptions below).
• Subnet mask & wildcard mask — the mask in dotted-decimal and its bitwise inverse, the latter used in router access control lists.
• Class & type — the legacy class (A–E) and whether the block is private (RFC 1918), loopback, link-local, carrier-grade NAT, multicast or public.

The /31 and /32 edge cases

Two prefix lengths break the simple "subtract two" rule, and the calculator handles both correctly. A /32 describes exactly one address — a single host or route — so the network, broadcast and host are all the same address and the usable count is one. A /31 describes two addresses; under RFC 3021 both are usable on a point-to-point link, because such links have only two endpoints and need no separate network or broadcast address. Getting these right matters when you are counting addresses for point-to-point WAN links or loopback interfaces, where an off-by-one assumption wastes scarce space.

IPv6 prefixes

IPv6 uses the same prefix idea over a 128-bit address, so a /64 — the standard size for a single LAN — contains 2⁶⁴ addresses, a number far too large for ordinary integers. This tool uses JavaScript BigInt so the first address, last address and total count are exact rather than rounded. It expands :: shorthand, re-compresses the result to canonical form, and identifies whether the prefix falls in the global unicast, unique-local (fc00::/7), link-local (fe80::/10), loopback or multicast range. Because IPv6 subnets are so large, the concept of "usable hosts minus two" is generally not applied; the address count is reported in full.

Why subnet locally

Subnetting is pure, deterministic bit arithmetic, which is exactly the kind of task where a real calculator beats asking an AI assistant — language models routinely make off-by-one errors on host counts and miscompute broadcast addresses, and they present those mistakes with full confidence. This tool applies the same exact operations a router does, every time. It also runs entirely in your browser, so your internal addressing plan — often considered sensitive infrastructure information — never travels to a third-party server. Paste a whole allocation table, work offline, and close the tab when you are done.

Frequently asked questions

What is the difference between the network and broadcast address?

The network address is the first address (host bits all zero) and names the subnet; the broadcast is the last address (host bits all one) and reaches every host. Neither is normally assignable, so a /24 has 256 addresses but 254 usable hosts.

Why does a /31 show two usable hosts?

RFC 3021 allows /31 on point-to-point links where both addresses are usable endpoints. A /32 is a single host.

What is a wildcard mask?

The bitwise inverse of the subnet mask — /24 gives 0.0.0.255 — used to match address ranges in router access control lists.

Are my IP addresses sent anywhere?

No. Everything is computed locally with bitwise and BigInt math, so your network details never leave your device.

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