IP Subnet Calculator

A subnet mask is a 32-bit number that divides an IP address into a network portion and a host portion. It works by performing a bitwise AND operation with the IP address to determine the network address. For example, a mask of 255.255.255.0 (/24) means the first 24 bits identify the network and the last 8 bits identify individual hosts.

CIDR (Classless Inter-Domain Routing) notation expresses a subnet mask as a prefix length — the number of leading '1' bits in the mask. For example, /24 corresponds to a subnet mask of 255.255.255.0, and /16 corresponds to 255.255.0.0. CIDR replaced the older classful addressing system to allow more flexible allocation of IP ranges.

The network address is the first address in a subnet (all host bits set to 0) and identifies the subnet itself — it cannot be assigned to a host. The broadcast address is the last address (all host bits set to 1) and is used to send data to all hosts on the subnet simultaneously. Both addresses are reserved and not usable for individual devices.

A wildcard mask is the inverse of the subnet mask — bits that are 0 in the subnet mask become 1 in the wildcard mask and vice versa. Wildcard masks are commonly used in Access Control Lists (ACLs) on routers and firewalls to specify which bits of an IP address must match a rule. For example, a /24 subnet mask (255.255.255.0) has a wildcard mask of 0.0.0.255.

The number of usable hosts equals 2^(host bits) − 2. The subtraction accounts for the network address and broadcast address, which cannot be assigned to hosts. For a /24 subnet (8 host bits), that gives 2^8 − 2 = 254 usable hosts. For a /30 (2 host bits), you get 2^2 − 2 = 2 usable hosts — common for point-to-point links.

IPv4 addresses are divided into five classes: Class A (1–126 first octet, supports ~16 million hosts per network), Class B (128–191, supports ~65,000 hosts), Class C (192–223, supports 254 hosts), Class D (224–239, used for multicast), and Class E (240–255, reserved for experimental use). Modern networking primarily uses CIDR rather than strict classful addressing.

Subnetting is the process of dividing a single IP network into smaller sub-networks (subnets). It improves network efficiency by reducing broadcast traffic, enhances security by isolating network segments, and allows more efficient use of available IP address space. Organizations subnet their networks to logically separate departments, floors, or device types.

IPv4 uses 32-bit addresses (e.g. 192.168.1.1) providing about 4.3 billion unique addresses, which are largely exhausted. IPv6 uses 128-bit addresses (e.g. 2001:0db8::1) providing a virtually unlimited address space (~3.4 × 10^38 addresses). IPv6 also includes built-in support for auto-configuration, improved security, and more efficient routing.