Subnet Calculator

CIDR (Classless Inter-Domain Routing) notation expresses a network as an IP address followed by a forward slash and a prefix length — for example 192.168.1.0/24. The number after the slash indicates how many bits of the 32-bit IP address are used for the network portion. /24 means the first 24 bits are the network, leaving 8 bits for hosts — giving 254 usable addresses.

A wildcard mask is the inverse of a subnet mask — every 0 in the subnet mask becomes a 1, and every 1 becomes a 0. Wildcard masks are used in Cisco ACL (Access Control List) configurations and OSPF area definitions to specify which bits of an IP address must match. For a /24 subnet (mask 255.255.255.0), the wildcard mask is 0.0.0.255.

The network address is the first address in a subnet — it identifies the subnet itself and cannot be assigned to a host. The broadcast address is the last address in the subnet — packets sent to it are delivered to all hosts in the subnet and it also cannot be assigned. All addresses between network and broadcast are usable host addresses.

VLSM (Variable Length Subnet Masking) is the practice of using different prefix lengths on different parts of a network. Instead of wasting a /24 (254 hosts) on a point-to-point link that only needs 2 hosts, VLSM lets you use a /30 (2 hosts) for that link and save the rest of the address space. The Subnet Split table in this calculator shows how your parent network divides at each prefix length from your current prefix down to /30.

The binary visualization shows all 32 bits of your IP address, subnet mask, and network address. Network bits are highlighted in navy and host bits in orange, making the boundary between them visually obvious. This is the same technique used in CCNA certification training. Seeing the bit boundary makes it immediately clear why certain IP addresses belong to a given subnet and others do not — and it makes the math behind subnetting intuitive rather than abstract.

Private IP addresses are defined in RFC 1918 and can only be used inside private networks — they are not routable on the public internet. The three private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Any IP outside these ranges is a public address assigned by your ISP and routable on the internet. This calculator detects which type your entered IP belongs to.

Use the Required Hosts input. Enter the number of hosts you need and the calculator will automatically select the smallest subnet prefix that can accommodate that count, accounting for the 2 reserved addresses (network and broadcast). For example, entering 50 hosts selects a /26 subnet with 62 usable host addresses — the smallest prefix that fits 50 hosts without wasting space.