IPv4 Subnet and CIDR Calculator

IPv4 subnetting and Classless Inter-Domain Routing (CIDR) calculation form the foundation of modern IP network design and routing architecture. Every IPv4 address consists of 32 bits written as four dotted-decimal octets ranging from 0.0.0.0 to 255.255.255.255. A CIDR prefix length such as /24 specifies how many leading bits from the left identify the network segment, while the remaining trailing bits (32 minus the prefix length) identify individual host interfaces within that broadcast domain. This calculator determines the network address using a bitwise AND operation between the IPv4 address and the netmask, and computes the broadcast address using a bitwise OR between the network address and the wildcard mask. For standard prefixes from /0 through /30, two addresses are reserved in every block—the first address identifies the network itself and the last address serves as the directed broadcast address—leaving two to the power of the host bits minus two usable host addresses. Special handling applies to /31 point-to-point router links defined in RFC 3021, which provide two usable addresses with no broadcast waste, and /32 host routes, which lock onto a single interface address. Common operational mistakes include sizing a VLAN subnet too tightly without room for future DHCP growth, inverting netmasks and wildcard masks when configuring OSPF network statements or firewall access control lists, and overlapping RFC 1918 private ranges across VPN tunnels. Inspecting the exact network boundary, usable host range, binary mask structure, and RFC address classification helps network engineers prevent routing overlaps and interface misconfigurations before deploying changes to production switches and routers.