CIDR calculator tools turn IP address slash notation into usable network parameters instantly. Enter any CIDR block like 10.0.0.0/8 or 172.16.0.0/12 and this free browser-based tool calculates the network address, subnet mask, broadcast address, and the range of usable host IPs. No sign-up or installation needed.
Subnet Calculator computes all key network parameters from CIDR notation (e.g. 192.168.1.0/24) in one click. Enter an IP address with a prefix length and instantly see the network address, subnet mask, broadcast address, first and last usable hosts, total usable host count, wildcard mask, and IP class. A binary view is available for educational purposes. Subnetting is used by network engineers to divide large IP address blocks into smaller, more manageable segments, and to control routing and broadcast domains. The prefix length (the number after the slash) determines how many bits are used for the network portion of the address, directly controlling the subnet size. For the complementary operation of resolving domain names to IPs, see DNS Lookup. To check what IP address is currently assigned to your connection, see IP Address Lookup.
Subnetting is a foundational networking skill with practical applications in every organization that manages its own IP address space. VLSM (Variable Length Subnet Masking) extends basic subnetting by allowing different parts of an address block to be divided into subnets of different sizes, which avoids wasting addresses by fitting subnet sizes to actual requirements. For example, a point-to-point link between two routers needs only two host addresses, so a /30 is appropriate (2 usable hosts), while a server farm might need a /26 (62 usable hosts) and a large office floor might need a /23 (510 usable hosts). The private address ranges defined in RFC 1918 are the most commonly used: 10.0.0.0/8 is popular in large enterprises and cloud VPCs because its single /8 block contains over 16 million addresses. 192.168.0.0/16 is the default for most home routers. 172.16.0.0/12 is less common but appears frequently in cloud environments. When designing subnet plans, a best practice is to leave room for growth by allocating subnets at least twice as large as current needs. CIDR also enables route summarization: multiple contiguous subnets can be advertised as a single larger prefix, reducing the size of routing tables. For example, 192.168.0.0/24 and 192.168.1.0/24 can be summarized as 192.168.0.0/23.