Subnet Calculator -- CIDR, IP Range & Hosts

Calculate Subnet Details

A subnet calculator takes an IPv4 address and subnet mask (or CIDR prefix) and returns the network address, broadcast address, usable host range, wildcard mask, and total host count. All calculations run in your browser.

Worked Examples

CIDR Cheat Sheet

Quick reference for all common IPv4 subnet sizes. Use this to find the right prefix length for your network.

* /31 subnets have no network or broadcast address per RFC 3021, so both addresses are usable for point-to-point links.

Private IPv4 Address Ranges (RFC 1918)

These address ranges are reserved for private networks and are not routable on the public internet. They are used in homes, offices, and data centers behind NAT.

Understanding Subnetting

What Is a Subnet?

A subnet (subnetwork) is a logical subdivision of an IP network. Subnetting divides a larger network into smaller, isolated segments. Each subnet has its own network address (the first IP, identifying the subnet), broadcast address (the last IP, used to reach all hosts), and a range of usable host addresses in between.

Why Subnet?

• Security: Isolate sensitive systems (e.g., servers, IoT devices) from general traffic
• Performance: Smaller subnets reduce broadcast domain size, decreasing unnecessary traffic
• Organization: Map network structure to physical or organizational layout (floors, departments, sites)
• Efficient IP use: Allocate only the addresses each segment needs, avoiding waste

How Subnetting Works

Every IPv4 address is 32 bits. A subnet mask divides those 32 bits into two parts: the network prefix (identifies the subnet) and the host portion (identifies the device within the subnet).

For example, with a /24 mask, the first 24 bits are the network prefix and the last 8 bits are the host portion. That gives 2⁸ = 256 total addresses, minus 2 reserved (network + broadcast) = 254 usable hosts.

The formula for usable hosts is: 2^(32 − prefix) − 2

Subnet Mask vs. Wildcard Mask

A subnet mask like 255.255.255.0 has 1-bits for the network portion and 0-bits for the host portion. A wildcard mask is its bitwise inverse: 0.0.0.255. They encode the same information in opposite ways.

• Subnet mask: Used in operating system network configuration, routing tables, and DHCP settings
• Wildcard mask: Used in Cisco ACLs (access control lists), OSPF area definitions, and some firewall rules to match IP address ranges

To convert: subtract each octet of the subnet mask from 255. For 255.255.255.240: wildcard = 0.0.0.15.

VLSM — Variable Length Subnet Masking

VLSM allows different subnets within the same network to use different prefix lengths. Without VLSM (classful networking), every subnet must be the same size, which wastes addresses.

Example: You have the 10.0.0.0/24 block (256 addresses) and need to serve three groups:

• Office A: 100 devices → /25 (126 usable hosts) → 10.0.0.0/25
• Office B: 50 devices → /26 (62 usable hosts) → 10.0.0.128/26
• Router link: 2 devices → /30 (2 usable hosts) → 10.0.0.192/30

With VLSM, you use only 196 addresses instead of 768 (3 × 256) under classful subnetting. The remaining space (10.0.0.196 – 10.0.0.255) stays available for future needs.

Frequently Asked Questions

What is a subnet?

A subnet (subnetwork) is a logical subdivision of an IP network. Subnetting divides a larger network into smaller segments, each with its own network address, broadcast address, and usable host range. Subnets improve security by isolating traffic, reduce broadcast domain size for better performance, and allow more efficient use of IP address space.

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its network prefix length. It is written as an IP followed by a slash and the prefix length — for example, 192.168.1.0/24. The prefix (0–32 for IPv4) indicates how many leading bits identify the network. A /24 means the first 24 bits are network and the remaining 8 bits are for hosts.

How do I calculate the number of usable hosts?

The formula is 2^(32 − prefix) − 2. You subtract 2 because the first address (network address) and last address (broadcast address) are reserved. For example, a /24 has 2⁸ − 2 = 254 usable hosts. A /30 has 2² − 2 = 2 usable hosts. The exception is /31, which per RFC 3021 allows both addresses as usable for point-to-point links.

What is the difference between a subnet mask and a wildcard mask?

They are bitwise inverses. A subnet mask like 255.255.255.0 uses 1-bits for the network portion and 0-bits for the host portion. The wildcard mask is 0.0.0.255 — 0-bits for network, 1-bits for hosts. Subnet masks are used in OS network configuration. Wildcard masks are used in Cisco ACLs, OSPF, and certain firewall rules.

What are private IP address ranges?

RFC 1918 defines three private ranges not routable on the public internet: 10.0.0.0/8 (~16.7 million addresses), 172.16.0.0/12 (~1 million), and 192.168.0.0/16 (~65,000). These are used for internal networks behind NAT (Network Address Translation).

What is VLSM?

VLSM (Variable Length Subnet Masking) lets you use different prefix lengths for different subnets within the same network. This avoids wasting addresses — you can use a /24 for a large office and a /30 for a router link, all from the same block.

What is a broadcast address?

The broadcast address is the last address in a subnet. Packets sent to it are delivered to all hosts on that subnet. In 192.168.1.0/24, the broadcast is 192.168.1.255. It cannot be assigned to a host.

When should I use a /30 or /31 subnet?

A /30 gives exactly 2 usable IPs — ideal for point-to-point links between routers. A /31 (RFC 3021) uses both addresses for hosts with no network or broadcast address, saving 1 IP per link. Most modern routers support /31 on point-to-point interfaces.

Does this calculator store my data?

No. All calculations run in your browser using JavaScript. No data is sent to any server.

Related Tools

• CIDR Calculator — calculate network ranges from CIDR notation with visual subnet breakdown
• IP Address Converter — convert between IPv4 decimal, binary, hex, and integer formats
• IP to Binary Converter — see the binary representation of any IPv4 address
• Base Converter — convert numbers between binary, decimal, hex, and other bases
• Binary to Decimal Converter — convert binary numbers to decimal

Subnet Calculator FAQ