IP Subnet Calculator – CIDR, Network & Host Range

How IP Subnetting Works

An IPv4 address is a 32-bit number written as four octets separated by dots (e.g., 192.168.1.100), uniquely identifying every device on an IP network. Each octet ranges from 0 to 255, giving roughly 4.3 billion possible addresses in total. Because networks vary enormously in size, the 32 bits are split into two portions: the network portion, which identifies the specific network segment, and the host portion, which identifies individual devices within that segment.

A subnet mask determines where the boundary falls between these two portions. The mask is also a 32-bit value, but it uses a contiguous run of 1-bits on the left (network) followed by 0-bits on the right (host). For example, the mask 255.255.255.0 has twenty-four 1-bits and eight 0-bits, meaning the first three octets identify the network and the last octet identifies hosts. By performing a bitwise AND between the IP address and the subnet mask, you obtain the network address. The broadcast address is found by setting all host bits to 1, and usable host addresses fall between the network address and the broadcast address.

Subnetting lets network administrators divide a single large network into smaller, more manageable segments. This improves security by isolating traffic between departments or floors, reduces broadcast domain size for better performance, and makes IP address management more organized. Without subnetting, every device in an organization would share one flat broadcast domain, resulting in congestion and security vulnerabilities.

CIDR Notation Explained

CIDR (Classless Inter-Domain Routing) is an IP addressing method, defined in RFC 4632, that replaced the older classful addressing system in 1993. Under the classful system, networks were rigidly divided into Class A (/8), Class B (/16), and Class C (/24), which led to massive address waste. A company needing 300 addresses would receive an entire Class B with 65,534 host addresses, wasting over 65,000 IPs.

CIDR notation appends a prefix length to the IP address, written as a slash followed by a number from 0 to 32. The number represents how many bits belong to the network portion. For example, /24 means 24 network bits and 8 host bits (256 addresses), /25 gives 128 addresses, and /26 gives 64 addresses. This flexibility allows administrators to allocate exactly the right number of addresses for each subnet, minimizing waste.

Common CIDR prefix lengths include /8 (16.7 million addresses, used by major ISPs), /16 (65,536 addresses, suitable for large campuses), /24 (256 addresses, the standard for small networks), /28 (16 addresses, ideal for small server groups), and /30 (4 addresses, 2 usable, used for point-to-point router links). The /32 prefix represents a single host address and is used in routing tables and loopback configurations.

Key IP Subnetting Terms

Common Subnet Masks Reference Table

The following table lists the most commonly used CIDR prefixes from /8 through /30, along with their dotted-decimal subnet mask, wildcard mask, total addresses, and usable host count. Use this as a quick reference when planning network segments.

Practical Subnetting Examples

Example 1: Splitting a /24 Into Four /26 Subnets

Suppose you have the network 192.168.10.0/24 and need to create four equal subnets for four departments. Moving from /24 to /26 borrows 2 additional bits from the host portion, creating 2^2 = 4 subnets, each with 64 addresses (62 usable).

Example 2: Planning a Small Office Network

A small office has 40 workstations, 5 printers, 3 servers, a router, and 2 access points. That is 51 devices. The smallest subnet that fits is a /26 with 62 usable addresses, leaving room for 11 future devices. If you chose a /27 (30 usable hosts), it would be too small. The network plan would look like this:

• Network: 10.0.1.0/26
• Gateway (router): 10.0.1.1
• Servers: 10.0.1.2 - 10.0.1.4
• Printers: 10.0.1.5 - 10.0.1.9
• Access points: 10.0.1.10 - 10.0.1.11
• Workstations: 10.0.1.12 - 10.0.1.51 (DHCP range)
• Spare: 10.0.1.52 - 10.0.1.62
• Broadcast: 10.0.1.63

If the office grows beyond 62 devices, you would need to move to a /25 (126 usable hosts) or add a second /26 subnet with a VLAN and inter-VLAN routing.

IPv4 vs IPv6 Overview

IPv4 has been the dominant Internet Protocol since the early 1980s, but its 32-bit address space (about 4.3 billion addresses) is now fully exhausted according to IANA. IPv6, specified in RFC 8200, was developed to solve this limitation with a vastly larger 128-bit address space, providing approximately 3.4 x 10^38 unique addresses -- enough to assign trillions of addresses to every person on Earth.

IPv6 addresses are written as eight groups of four hexadecimal digits separated by colons, such as 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Leading zeros can be omitted, and consecutive groups of zeros can be replaced with :: (double colon) once per address. The standard subnet prefix for IPv6 is /64, which allocates 64 bits for the network and 64 bits for the host, giving each subnet 2^64 (about 18.4 quintillion) addresses.

While this calculator focuses on IPv4 subnetting, the core concepts of dividing addresses into network and host portions apply equally to IPv6. Most modern networks run dual-stack configurations, supporting both IPv4 and IPv6 simultaneously during the transition period.