Free Subnet Calculator (CIDR)

Enter an IP address and CIDR prefix length to instantly calculate the network address, broadcast address, usable host range, subnet mask, wildcard mask, and total number of hosts. Essential for network planning, firewall rules, and understanding how IP subnetting works.

How to Use This Tool

1. Enter an IP address – any valid IPv4 address in dotted-decimal notation (e.g., 192.168.1.0 or 10.0.0.50). This can be a network address or any host address within the subnet you want to calculate.
2. Select a prefix length – choose a CIDR prefix from /1 to /32 using the dropdown or slider. The most common values are /24 (256 addresses, typical home or small office network), /16 (65,536 addresses), and /8 (16.7 million addresses). The tool defaults to /24.
3. View the calculated results. All values update instantly as you change the inputs – no need to click a button.

Understanding Your Results

Why This Matters

Subnetting is how networks are divided into smaller, manageable segments. Every network engineer, system administrator, and IT professional deals with subnetting regularly – whether designing a new office network, writing firewall rules, configuring VPN tunnels, or troubleshooting connectivity problems.

Getting the math wrong has real consequences. Assign an IP outside the subnet range and the device can’t communicate. Miscalculate the wildcard mask in a firewall rule and you either block legitimate traffic or allow traffic you meant to block. Use an incorrect network address in a routing table and packets go nowhere.

Common use cases:

• Network planning. Determine how many usable addresses a subnet provides before allocating it to a department or VLAN. A /24 gives you 254 hosts – enough for a small office. Need 500 devices? You’ll need a /23 (510 usable hosts).
• Firewall rules. Calculate the correct network address and wildcard mask to permit or deny traffic for a specific subnet.
• VPN configuration. When configuring split tunneling or site-to-site VPNs, you need the exact network address and prefix for each subnet that should route through the tunnel.
• Cloud infrastructure. AWS VPCs, Azure VNets, and Google Cloud VPCs all require CIDR notation when defining subnets. Overlapping subnets cause routing conflicts that are painful to debug.

Frequently Asked Questions

What CIDR prefix should I use for my home network?

Most home routers default to /24 (255.255.255.0), which provides 254 usable addresses. This is more than enough for a typical household with phones, laptops, smart TVs, and IoT devices. You’d only need a larger subnet if you had more than 254 devices on a single network segment, which is uncommon outside of enterprise environments.

What’s the difference between a subnet mask and a wildcard mask?

They’re inverses of each other. A subnet mask uses 1-bits to mark the network portion and 0-bits for the host portion (e.g., 255.255.255.0). A wildcard mask flips this – 0-bits for the network portion and 1-bits for the host portion (e.g., 0.0.0.255). Subnet masks are used in most operating systems and network devices. Wildcard masks are primarily used in Cisco IOS access control lists (ACLs) and OSPF configurations.

What is a /31 subnet used for?

A /31 subnet contains exactly 2 addresses with no network or broadcast address (per RFC 3021). It’s used for point-to-point links between two routers where you don’t need a broadcast address. This saves one IP address compared to using a /30, which matters when you have hundreds or thousands of router-to-router links. A /32 represents a single host address and is commonly used in loopback interfaces and host routes.