CIDR / Subnet Calculator

The CIDR / Subnet Calculator takes an IPv4 network written in CIDR notation, like 10.0.0.0/24, and works out the range it covers: the first and last address, the total number of addresses, and how many of those are usable hosts. The two reserved addresses, the network and the broadcast, are why a /24 gives you 254 usable hosts rather than 256. Use it when you are planning subnets, checking that two ranges do not overlap, or figuring out how many devices a segment can hold. It is a core part of day to day network configuration. Classless inter domain routing replaced the old fixed size network classes with a prefix length, so a network is written as an address followed by a slash and the number of leading bits that identify the network. Everything practical follows from that number: a twenty four bit prefix leaves eight bits for hosts, which is two hundred fifty six addresses, of which two are reserved, one to name the network itself and one for broadcast, leaving two hundred fifty four usable. Working that out for every prefix by hand is where mistakes creep in, and the consequences are concrete, since overlapping ranges cause routing confusion and an undersized subnet runs out of addresses. This is routine work wherever networks are planned: carving a larger block into segments, checking whether two ranges overlap before they are assigned, confirming that a subnet is large enough for the devices it must hold, and verifying that an address falls inside a given range. The calculation is exact and the reserved address count is the detail people most often forget, which is precisely why a calculator is the right tool for the job.

📘 Related guide: Calculate IP ranges from a CIDR block →