Enter a CIDR to get network, broadcast, usable hosts and masks for IPv4 and IPv6.
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The CIDR Subnet Calculator takes a CIDR (such as 192.168.1.0/24) or "IP + mask" and instantly returns the network address, broadcast address, subnet mask, wildcard mask, first and last usable address, total addresses and usable hosts, with the binary network/host bits. It handles IPv4 and IPv6, /31 (RFC 3021) and /32, and adds subnet splitting, VLSM planning, IP range to the fewest CIDRs, containment and overlap checks, CIDR aggregation, and the reserved-address rules of AWS, Azure and Google Cloud.
The tool clears the host bits and shows "the actual network is 192.168.1.0/24". Network address = IP address AND subnet mask, so any address inside that block gives the same network as long as the first 24 bits match.
/32 is a single host address (a host route), so it has 1 usable host. /31 is used for point-to-point links under RFC 3021: both addresses are usable and there is no network or broadcast address, so it has 2. For other prefixes usable hosts = 2^(32 − prefix) − 2.
AWS and Azure reserve 5 addresses per subnet (the first 4 and the last 1), while Google Cloud reserves 4 in a primary IPv4 range (network, default gateway, second-to-last and broadcast). Switching the "Cloud rules" selector subtracts them from the usable count and shows the subnet-size range the platform allows. Rule sources and verification dates appear under the result.
A subnet mask sets network bits to 1 and host bits to 0 (for example 255.255.255.0); a wildcard mask is the opposite (0.0.0.255) and is common in Cisco ACLs and OSPF configuration. The tool shows both, and if you enter a wildcard mask it converts it to a prefix length automatically.
IPv6 has no broadcast address and every address in a block can be assigned; each network segment is normally a /64, and the remaining bits are used to divide the space into subnets. For example a /48 holds 65,536 /64 networks. Address counts are shown as powers of 2 because the numbers become unreadably large.
VLSM (variable-length subnet masks) lets different segments use subnets of different sizes so addresses are not wasted. You list the hosts each segment needs; the tool allocates from largest to smallest, aligns every block to its own size, and tells you which segment does not fit when space runs out.
Some systems read a number with a leading zero (010) as octal (8), so the same text can mean different addresses in different programs; the tool asks you to write 10.1.1.1 to avoid ambiguity. All calculation happens in your browser and the addresses you type are not uploaded.
Binary (network bits / host bits)
Special-use ranges
