Subnet Calculator

IP Subnetting Fundamentals

Subnetting is the practice of dividing a large network into smaller, more manageable segments called subnets. Each subnet is a contiguous range of IP addresses sharing a common network prefix. Subnetting is fundamental to network design because it improves performance by reducing broadcast traffic, enhances security by isolating network segments, and makes IP address allocation more efficient.

Every IPv4 address consists of 32 bits, conventionally written in dotted-decimal notation as four 8-bit octets (e.g., 192.168.1.0). The subnet mask divides this address into two parts: the network portion (which identifies the subnet) and the host portion (which identifies individual devices within that subnet). A subnet mask of 255.255.255.0 means the first 24 bits are the network portion, leaving 8 bits for up to 256 addresses (254 usable, since the first is the network address and the last is the broadcast address).

Understanding CIDR Notation

CIDR (Classless Inter-Domain Routing) notation provides a compact way to express both an IP address and its subnet mask. Written as an IP address followed by a slash and a prefix length (e.g., 192.168.1.0/24), CIDR notation tells you exactly how many bits are in the network portion of the address.

The prefix length (the number after the slash) directly determines the size of the subnet:

• /32 — A single host. Contains exactly 1 address. Used for loopback interfaces and host routes.
• /30 — 4 addresses, 2 usable. Standard for point-to-point links between routers.
• /24 — 256 addresses, 254 usable. The most common subnet for small office and home networks (a single "Class C" block).
• /16 — 65,536 addresses, 65,534 usable. Common for medium to large enterprise networks.
• /8 — 16,777,216 addresses. A "Class A" block, typically allocated to very large organizations or ISPs.

Network Design with Subnetting

When designing a network, subnet planning is one of the first steps. Engineers determine how many subnets are needed, how many hosts each subnet must support, and which IP address ranges to allocate. The private IP ranges defined in RFC 1918 are the standard building blocks: 10.0.0.0/8 (over 16 million addresses), 172.16.0.0/12 (over 1 million addresses), and 192.168.0.0/16 (65,536 addresses).

A practical design example: a company needs subnets for its office LAN (200 hosts), server VLAN (50 hosts), guest WiFi (30 hosts), and management network (10 hosts). A network engineer would allocate 192.168.1.0/24 for the office LAN, 192.168.2.0/26 for the server VLAN (64 addresses, 62 usable), 192.168.3.0/27 for guest WiFi (32 addresses, 30 usable), and 192.168.4.0/28 for management (16 addresses, 14 usable). This allocates addresses efficiently without wasting the large contiguous blocks that older classful networking would have required.

The wildcard mask (the inverse of the subnet mask) is used in access control lists (ACLs) on Cisco routers and firewalls. A wildcard mask of 0.0.0.255 for a /24 subnet means "match any address where the first three octets match". Our calculator displays both the subnet mask and the wildcard mask for use in firewall and router configurations.