What is a CIDR prefix, and how do I know which one to use?

A CIDR prefix (like /24 or /16) tells you how many bits of the IP address are the network portion. /24 means 256 addresses (254 usable), /16 means 65,536 addresses. Your network design or ISP allocation will tell you the correct prefix.

How many usable hosts does a /24 subnet give me?

A /24 subnet has 256 total addresses. Two are reserved — the network address (first) and the broadcast address (last) — leaving 254 usable host addresses.

What is the wildcard mask and when do I need it?

The wildcard mask is the inverse of the subnet mask. It's used in Cisco ACL and OSPF configurations to specify which bits of an address should be ignored when matching. For a /24 (255.255.255.0 mask) the wildcard is 0.0.0.255.

Does this work for IPv6 subnets?

The current version focuses on IPv4 CIDR calculations. IPv6 subnetting uses the same CIDR notation but with 128-bit addresses.

Subnet Calculator

WEB

Calculate network and broadcast addresses, masks, host ranges and counts from an IP and CIDR — free in your browser.

IPv4 address

Prefix length

CIDR192.168.1.0/24

Network192.168.1.0

Broadcast192.168.1.255

Netmask255.255.255.0

Wildcard0.0.0.255

First host192.168.1.1

Last host192.168.1.254

Usable hosts254

Total addresses256

What it does

Subnet Calculator takes an IP address and a CIDR prefix length — for example 192.168.1.0/24 — and instantly works out everything you need to know about that network: the network address, broadcast address, subnet mask, wildcard mask, usable host IP range, and the total number of hosts that fit. It's the tool network engineers, students, and sysadmins use when planning IP address space, configuring firewalls, or troubleshooting routing. All the math is done locally in your browser — no sign-up, no internet connection required, and completely free. Whether you're subnetting a class C for a small office or dividing a /16 into VLANs, the calculator gives you the answers in one step.

How to use the Subnet Calculator

Open Subnet Calculator in Carbide.
Enter an IP address in the IP field — for example 10.0.0.0.
Enter the CIDR prefix length in the second field — for example 22 — or use the slash notation like 10.0.0.0/22 if the tool accepts it.
The calculator instantly displays the network address, broadcast address, subnet mask, wildcard mask, usable host range, and host count.
Adjust the prefix length to compare different subnet sizes for your network design.

Frequently asked questions

What is a CIDR prefix, and how do I know which one to use?: A CIDR prefix (like /24 or /16) tells you how many bits of the IP address are the network portion. /24 means 256 addresses (254 usable), /16 means 65,536 addresses. Your network design or ISP allocation will tell you the correct prefix.
How many usable hosts does a /24 subnet give me?: A /24 subnet has 256 total addresses. Two are reserved — the network address (first) and the broadcast address (last) — leaving 254 usable host addresses.
What is the wildcard mask and when do I need it?: The wildcard mask is the inverse of the subnet mask. It's used in Cisco ACL and OSPF configurations to specify which bits of an address should be ignored when matching. For a /24 (255.255.255.0 mask) the wildcard is 0.0.0.255.
Does this work for IPv6 subnets?: The current version focuses on IPv4 CIDR calculations. IPv6 subnetting uses the same CIDR notation but with 128-bit addresses.

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