IP Subnet Calculator

How CIDR Subnetting Works

This calculator implements standard IPv4 CIDR (Classless Inter-Domain Routing) math. A /n prefix length means the first n bits of the 32-bit address are the network portion and the remaining (32 - n) bits identify hosts within that network. The subnet mask is built by setting the leftmost n bits to 1 and the rest to 0 (e.g. /24 = 255.255.255.0). The network address is found by applying a bitwise AND between the IP address and the subnet mask, and the broadcast address is found by setting every host bit in the network address to 1 (equivalent to the network address OR the wildcard mask, where the wildcard mask is the bitwise complement of the subnet mask).

Usable Hosts and the /31, /32 Special Cases

For a network with h host bits, the total address space is 2^h addresses, but ordinarily two of those are reserved — the all-zeros network address and the all-ones broadcast address — leaving 2^h - 2 usable host addresses. This calculator applies that formula for every prefix from /0 through /30. Two exceptions carved out by RFC 3021 and common routing practice: a /31 network (2 addresses, 0 host bits reserved) is treated as a point-to-point link where both addresses are usable, and a /32 network is a single host route with exactly one usable address and no broadcast address at all.

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