CIDR Calculator

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What this tool does

CIDR Calculator breaks down one or many IPv4 or IPv6 CIDR blocks at once into their network address, broadcast address, subnet mask, usable host range, and total address count, plus an IP-in-range checker and a subnet splitter for dividing a larger block into smaller equal pieces.

How It Works

For IPv4, the /N prefix is converted into a 32-bit subnet mask (the top N bits set to 1, the rest 0), and a bitwise AND between the address and that mask gives the network address. The broadcast address is the network address with every host bit set to 1 instead, found by OR-ing the network address with the wildcard mask (the bitwise inverse of the subnet mask). Total addresses is 2^(32−N); usable hosts is that minus 2 for the reserved network and broadcast addresses, except /31 (2 usable, no reservation) and /32 (1 usable, a single host).

IPv6 works the same way but over 128 bits instead of 32, using exact big-integer arithmetic (not floating point, since 2^128 is far larger than JavaScript's safe integer range) so even the largest blocks are computed precisely. Splitting a block into smaller subnets repeats the same idea in reverse: moving the prefix from /N to /(N+k) divides the block into exactly 2^k equal, contiguous, non-overlapping pieces.

Problems it solves

• Planning a network: figuring out how many usable hosts a subnet actually has before assigning it.
• Checking a batch of CIDR blocks from a firewall rule or cloud security group all at once instead of one at a time.
• Dividing a larger allocated block (like a cloud VPC CIDR) into smaller subnets for different environments or availability zones.
• Quickly checking whether a specific IP address falls inside an allowlist or blocklist range.

Frequently asked questions