Free subnet calculator
Enter an IPv4 address and a CIDR prefix to instantly get the network address, broadcast address, usable host range, subnet mask, and host count. Great for CompTIA A+, Network+, and Security+ practice — no account needed.
Input
Enter network details
Valid range: 0-32
No results yet
Enter an IP address and CIDR to get started
How subnetting works
An IPv4 address is 32 bits split into a network portion and a host portion. The CIDR prefix (the /24 in 192.168.1.0/24) says how many leading bits belong to the network.
The network address is every host bit set to 0; the broadcast address is every host bit set to 1. Usable hosts sit between them. Every bit you add to the prefix halves the hosts and doubles the subnets — the core skill CompTIA tests.
Subnet calculator — FAQ
What is a subnet mask?
A subnet mask marks which part of an IPv4 address identifies the network and which part identifies the host. For example, /24 (255.255.255.0) uses the first three octets for the network and the last octet for hosts.
How do I calculate the number of usable hosts in a subnet?
Take 2 to the power of the number of host bits, then subtract 2 (one for the network address, one for the broadcast). A /24 has 8 host bits: 2^8 − 2 = 254 usable hosts. (A /31 is a special point-to-point case with 2 usable, and a /32 is a single host.)
What's the difference between /24 and /25?
A /24 has 256 addresses (254 usable). A /25 splits that in half — two subnets of 128 addresses (126 usable) each. Each extra prefix bit halves the number of hosts and doubles the number of subnets.
Is this subnet calculator free?
Yes — it's completely free and needs no account. Everything runs in your browser.
Studying for Network+ or A+?
ExamWizardz turns the official objectives into a guided plan with practice tests, PBQs, and a readiness score. Join the waitlist to get in at launch.