CIDR Calculator
About the CIDR Calculator
The CIDR Calculator turns a network in CIDR notation, such as 192.168.1.0/24, into the full picture of that address block: the network address, broadcast address, usable host range, subnet mask, and total number of addresses. CIDR (Classless Inter-Domain Routing) replaced the old rigid Class A/B/C system by letting the prefix length — the number after the slash — define exactly how many bits belong to the network versus the hosts.
The math is bitwise: the prefix length determines the subnet mask, which the calculator applies to the input address to find where the network begins and ends. A /24 sets the first 24 bits as network, leaving 8 bits for hosts, which yields 256 total addresses (256 minus 2 usable for the network and broadcast addresses in IPv4). Smaller prefixes like /16 cover far larger ranges, while larger ones like /30 carve out tiny point-to-point links with just two usable addresses.
Network engineers and developers use CIDR calculation when planning subnets, writing firewall and security-group rules, configuring VPCs in cloud environments, and avoiding overlapping address ranges across networks. It is essential for verifying that an allocation is large enough for the hosts you need and for splitting a block into smaller subnets without collisions.
A practical tip is to remember that each step in prefix length halves or doubles the block: a /25 is half a /24, a /23 is two /24s combined. Reserve room for growth rather than allocating the exact size you need today, and double-check that new subnets do not overlap existing ones. For IPv6 the same prefix logic applies but the address space is vastly larger, so the usable-host minus-2 rule does not carry the same weight. Pair this with the CIDR ranges you see when inspecting What's My IP results.
Frequently asked questions
- What does the number after the slash mean in CIDR?
- It is the prefix length: the count of leading bits that identify the network. A /24 means the first 24 of 32 bits are the network portion, leaving 8 bits (256 addresses) for hosts.
- Why are two addresses subtracted from the host count?
- In an IPv4 subnet the first address is the network identifier and the last is the broadcast address, neither of which can be assigned to a host. So a /24 has 256 total but 254 usable addresses.
- How many addresses does each prefix cover?
- Each decrease of one in the prefix doubles the block. A /32 is one address, /31 is two, /30 is four, /24 is 256, and /16 is 65,536. The formula is 2 to the power of (32 minus the prefix).
- How do I know if two subnets overlap?
- Compare their network and broadcast addresses. If one block's range falls inside or straddles another's, they overlap. The calculator's start and end addresses make this comparison straightforward.