Subnet math,
made painless.
CIDR (Classless Inter-Domain Routing) notation describes an IP range as address/prefix, where the prefix says how many leading bits are fixed as the network portion. This calculator returns the network, broadcast, host range, mask and binary for any IPv4 or IPv6 block, plus VLSM and a prefix cheat sheet.
Corrections welcome — see our editorial policy.
IPv4 prefix lengths
| Prefix | Mask | Addresses | Usable | Class |
|---|
IPv6 prefix lengths
| Prefix | Typical use |
|---|
| Name | Hosts needed | Allocated | Network | Prefix | Range | Broadcast | Mask |
|---|
CIDR (Classless Inter-Domain Routing) is a compact notation for
describing an IP range as address/prefix. Where the prefix is a number
from 0 to 32 (IPv4) or 0 to 128 (IPv6) that tells you how many bits are fixed as
the network portion and how many are free for host addresses.
The prefix length determines the block size. A /24 fixes 24 bits,
leaving 8 for hosts (28 = 256 addresses, 254 usable after subtracting
network and broadcast). A /16 fixes 16 bits: 65,536 addresses.
Each step down in prefix length doubles the block: /23 is twice as
large as /24, /22 four times, and so on. Before CIDR
(pre-1993), the internet used a rigid "classful" system (Class A = /8, Class B = /16,
Class C = /24) which wasted enormous swathes of address space.
VLSM (Variable Length Subnet Masking) is what you get when you
subdivide a single allocation into smaller subnets of different sizes. Suppose
you have 192.168.1.0/24 and need three networks: 50 hosts, 25 hosts, and
10 hosts. The math is: pick the smallest prefix that fits each, allocate them in
decreasing order from the start of the block, and advance the pointer by the size of
each allocation. The VLSM tab above does this automatically and shows exactly how the
address space is carved up, including the remainder that's still free.
IPv6 CIDR works identically but with 128-bit addresses. A /48
is a typical site allocation: it leaves 80 bits for internal use, enough for 65,536
/64 subnets, each containing more addresses than the entire IPv4 internet. The
/64 boundary is significant: SLAAC (stateless address autoconfiguration)
requires it, so almost every end network is a /64 regardless of how many devices it
actually has. IPv6 also has no broadcast address, which is why the "usable hosts"
field is hidden for v6 results.
The address range type for IPv6 matters too. 2000::/3 is globally routable
unicast (the "real" internet). fe80::/10 is link-local: valid only on the
local segment, never routed. fc00::/7 is ULA (Unique Local Addresses) —
the IPv6 equivalent of RFC 1918 private ranges. ff00::/8 is multicast,
::1/128 is loopback. We label the range type in the breakdown so you don't
accidentally assign link-local addresses to a public-facing service.