CIDR Calculator

CIDR (Classless Inter-Domain Routing) describes a network range by appending a /prefix to an IP address, e.g. 192.168.0.0/24. This calculator parses your CIDR input and instantly shows the network address, broadcast address, subnet mask, wildcard mask, and the usable host range and count.

Use it as the first step in firewall rules, cloud VPC/subnet design, routing table checks and ACL setup. To turn an arbitrary start/end IP into a CIDR list, pair it with the IP Range to CIDR converter. Everything runs in your browser, so it is fast and private.

What each field means

• Network address: the first address of the range, usually not assigned to a host.
• Broadcast address: the last address, reserved for sending to the whole subnet.
• Subnet mask: the bitmask splitting network and host parts (e.g. 255.255.255.0).
• Wildcard mask: the inverted subnet mask, used in Cisco ACLs and OSPF.
• Usable hosts: addresses left for devices after excluding network and broadcast.

Hosts per prefix

• /24 = 256 addresses (254 usable) — the typical small LAN block
• /16 = 65,536 — large private networks
• /30 = 4 (2 usable) — router point-to-point links
• /32 = 1 — a single host (loopback, allowlist)

To compress or expand IPv6 notation, use the IPv6 Compress / Expand tool; to check whether a range is private or reserved, use the Private IP Checker.

Prefix ↔ mask ↔ hosts quick reference

Subnet mask, wildcard mask, total addresses and usable hosts for common IPv4 prefixes./31 and /32 are computed as special cases and shown separately.

Worked example: 192.168.10.130/26

You can enter any host address and the calculator applies the mask to align it to the network address. A/26 carves the last octet into blocks of 64, so 192.168.10.130 falls in the128–191 block.

• Network address: 192.168.10.128 (input IP & mask)
• Broadcast: 192.168.10.191
• Subnet mask: 255.255.255.192 / wildcard 0.0.0.63
• Usable host range: 192.168.10.129 – 192.168.10.190 (62 hosts)

Common pitfall

• Don't be surprised when a host IP changes the network address. Entering10.0.0.45/24 returns 10.0.0.0/24 — that is correct, not a bug. The tool masks your input to reveal which network it belongs to.
• Each extra prefix bit halves the host count. Confusing /25 (126 hosts) with /24 (254 hosts) is a frequent sizing mistake that packs too many devices into a small block.
• Using /24 instead of /30 on a router link wastes ~250 addresses.Point-to-point links should use /30 (or /31).