Designing a VLAN or VPC and figuring out how many hosts a /22 vs /23 will hold (1022 vs 510). Reading a firewall rule like allow 10.42.0.0/16 and confirming the exact range it covers. Splitting a /24 into smaller subnets and checking the boundaries don't overlap. Sanity-checking a cloud security-group rule before deploying. Translating between CIDR and a legacy 255.x.x.x dotted mask in router configs.

"Usable" excludes network and broadcast. A /24 has 256 addresses but only 254 usable for hosts — the first (.0) is the network, the last (.255) is the broadcast. /31 and /32 are special. /31 is used for point-to-point links per RFC 3021 — both addresses are usable. /32 is a single host (used in routing entries). The address part doesn't have to be the network address. 10.5.7.42/24 still means the same /24 as 10.5.7.0/24 — the prefix length is what counts. The tool normalises to the network address in its output. Don't confuse class with CIDR. Classes A/B/C are a legacy concept (pre-1993). A /24 might fall inside a "class A" range and that's fine. RFC 1918 private ranges: 10.0.0.0/8 , 172.16.0.0/12 , 192.168.0.0/16 . Anything else is publicly-routable (or reserved). This tool is IPv4 only. IPv6 CIDR is structurally similar but the prefix can go up to /128 and the address space is much bigger; not handled here.

CIDR Subnet Calculator

Compute IPv4 subnets from CIDR notation. Network, broadcast, mask, host range, and binary view.

CIDR notation

Example: 10.0.0.0/16, 172.16.5.0/24, 203.0.113.42/29

Result

Enter input above to see the result.

What is this for?

CIDR (Classless Inter-Domain Routing) notation packs an IPv4 address and its subnet size into one string: 192.168.1.0/24 means "the address 192.168.1.0 with a 24-bit network prefix" — the same network as the older 255.255.255.0 mask, but written in 12 characters instead of 30. This tool decodes any CIDR into the human-meaningful values: which addresses are in the subnet, the broadcast, the mask in dotted form, and the host range you can actually assign to devices.

When to use it

Designing a VLAN or VPC and figuring out how many hosts a /22 vs /23 will hold (1022 vs 510).
Reading a firewall rule like allow 10.42.0.0/16 and confirming the exact range it covers.
Splitting a /24 into smaller subnets and checking the boundaries don't overlap.
Sanity-checking a cloud security-group rule before deploying.
Translating between CIDR and a legacy 255.x.x.x dotted mask in router configs.

Quick CIDR cheat sheet

/32 — 1 address (a single host route).
/30 — 4 addresses, 2 usable (point-to-point links).
/29 — 8 addresses, 6 usable (small office subnet).
/24 — 256 addresses, 254 usable (classic Class C / typical LAN).
/16 — 65,536 addresses (typical site network or VPC).
/8 — 16.7M addresses (whole-of-organisation).
/0 — every IPv4 address (the default route).

Common gotchas

"Usable" excludes network and broadcast. A /24 has 256 addresses but only 254 usable for hosts — the first (.0) is the network, the last (.255) is the broadcast.
/31 and /32 are special. /31 is used for point-to-point links per RFC 3021 — both addresses are usable. /32 is a single host (used in routing entries).
The address part doesn't have to be the network address. 10.5.7.42/24 still means the same /24 as 10.5.7.0/24 — the prefix length is what counts. The tool normalises to the network address in its output.
Don't confuse class with CIDR. Classes A/B/C are a legacy concept (pre-1993). A /24 might fall inside a "class A" range and that's fine.
RFC 1918 private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Anything else is publicly-routable (or reserved).
This tool is IPv4 only. IPv6 CIDR is structurally similar but the prefix can go up to /128 and the address space is much bigger; not handled here.