IP subnet and CIDR calculator

What the prefix actually does

An IPv4 address is 32 bits. The prefix says how many of those bits identify the network; the rest identify a host inside it. A /24 fixes the first 24 bits, leaving 8 host bits and therefore 2⁸ = 256 addresses.

address 192.168.  1. 10  = 11000000 10101000 00000001 00001010
mask    255.255.255.  0  = 11111111 11111111 11111111 00000000
AND ---------------------------------------------------------
network 192.168.  1.  0  = 11000000 10101000 00000001 00000000

Every other figure follows from that AND. The broadcast address is the network with all host bits set; the wildcard mask is the mask inverted; the host range is everything in between.

The reference table

Why the cloud column is different

AWS, Azure and GCP each reserve five addresses in every subnet rather than two. In an AWS VPC subnet 10.0.0.0/24:

• 10.0.0.0 — network address
• 10.0.0.1 — the VPC router
• 10.0.0.2 — DNS resolver
• 10.0.0.3 — reserved for future use
• 10.0.0.255 — broadcast address (reserved even though VPCs do not support broadcast)

AWS also refuses subnets smaller than /28, which after reservations leaves eleven usable addresses. Turn on cloud mode above to see the corrected count for your own block.

Planning with VLSM

Variable-length subnet masking means allocating each subnet only as much space as it needs instead of splitting everything evenly. The method:

1. List every subnet you need with its host count, largest first.
2. Round each up to the next power of two, remembering to add two for network and broadcast — or five in a cloud VPC.
3. Allocate the largest from the start of the block, then the next, and so on.
4. Leave the tail unallocated for growth; expanding later is far easier than renumbering.

The split table above lists every subnet at a chosen prefix, which is the working document for that process.

IPv6 is different in one important way

There is no broadcast address and no host-count arithmetic to worry about, because every LAN gets a /64 regardless of how many devices are on it — that is what SLAAC requires. Site allocations are typically /48, which contains 65,536 of those /64s. The IPv6 panel above uses BigInt throughout, so a /32 reports its 79,228,162,514,264,337,593,543,950,336 addresses exactly rather than as a rounded float.

No lookups, ever

This is arithmetic on the bits you typed. There is no WHOIS, no geolocation and no reverse DNS, because all three require a network call and this site makes none. Everything here works with the network disconnected — including on a plane, which is when subnet planning tends to happen.