IPv4 Subnet & CIDR Calculator

Enter any IPv4 address and CIDR prefix length to calculate network boundaries, usable host range, wildcard mask, and binary representation.

Subnet Results

How IPv4 CIDR Subnetting Works

Classless Inter-Domain Routing (CIDR), standardized in RFC 4632, replaces legacy Class A, B, and C addressing with flexible prefix lengths written as /0 through /32. The prefix number defines how many leading bits of a 32-bit IPv4 address identify the network portion, while the remaining 32 - prefix bits identify individual hosts inside that subnet.

To find the network address, routers perform a bitwise AND between the IPv4 address and the subnet mask. The broadcast address sets all host bits to 1. For standard subnets from /8 through /30, two addresses are reserved—the first for the network identifier and the last for directed broadcast—leaving 2^(32 - prefix) - 2 assignable host IPs. Under RFC 3021, /31 prefixes are used on point-to-point router links with 2 usable addresses and no broadcast overhead, while /32 represents a single-host route or loopback interface.

Network engineers configuring Cisco IOS access control lists (ACLs) or OSPF network statements also rely on the wildcard mask, which is the exact bitwise inverse of the subnet mask (for example, 255.255.255.0 inverts to 0.0.0.255). If you need to divide one parent block into differently sized departmental or cloud VPC subnets, use our VLSM Subnet Splitter.