What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation is a compact way to specify an IP address and its associated network mask. It is written as an IP address followed by a slash and a number (e.g., 192.168.1.0/24), where the number indicates how many leading bits of the address are the network prefix.

What is the difference between the network address and the broadcast address?

The network address is the first address in a subnet and identifies the network itself (all host bits are 0). The broadcast address is the last address in a subnet and is used to send data to all hosts on that network (all host bits are 1). Neither can be assigned to a host.

What are usable hosts?

Usable hosts are the IP addresses in a subnet that can be assigned to devices. The total number of addresses minus the network address and broadcast address gives the usable host count. For a /24 network, there are 256 total addresses but only 254 usable hosts.

What is a wildcard mask?

A wildcard mask is the inverse of a subnet mask. Where the subnet mask has 1-bits, the wildcard mask has 0-bits, and vice versa. Wildcard masks are commonly used in access control lists (ACLs) on Cisco routers and in OSPF configurations.

What are IP address classes?

IP address classes (A, B, C, D, E) are a legacy system for categorizing IPv4 addresses. Class A uses the range 1-126, Class B uses 128-191, Class C uses 192-223, Class D (224-239) is for multicast, and Class E (240-255) is reserved. CIDR has largely replaced classful addressing.

Subnet Calculator - Free Online Tool

How Subnet Calculation Works

Subnetting divides a larger network into smaller, more manageable sub-networks. The CIDR prefix length determines how many bits of the IP address are used for the network portion, and the remaining bits are used for host addresses. A /24 prefix means 24 bits for the network and 8 bits for hosts, allowing 256 total addresses (254 usable).

This tool performs all calculations using bitwise operations on the IP address octets entirely in your browser. It computes the subnet mask, network address, broadcast address, host range, and wildcard mask from the given IP and prefix length.

Common CIDR Sizes

A /24 gives you 256 addresses (254 usable) — the standard for a small office or home network. A /16 provides 65,536 addresses for larger organizations. A /28 carves out just 16 addresses (14 usable), common for small server clusters or point-to-point links. Cloud providers typically let you create VPCs with /16 and subdivide into /24 or smaller subnets per availability zone.

Private IP Ranges

Three ranges are reserved for private networks and won't be routed on the public internet: 10.0.0.0/8 (16.7 million addresses), 172.16.0.0/12 (1 million addresses, covering 172.16.x through 172.31.x), and 192.168.0.0/16 (65,536 addresses). Your home router almost certainly uses 192.168.x.x. AWS VPCs default to the 10.x.x.x range. Pick a range that won't conflict with other networks you might need to peer with.

Why Subnetting Matters for Cloud VPCs

In AWS, GCP, or Azure, your VPC's CIDR block determines how many resources you can deploy. Running out of IP addresses in a subnet means you can't launch new instances without restructuring. Plan for growth: a /16 VPC with /24 subnets gives you 256 subnets of 254 hosts each. Also note that cloud providers reserve a few addresses per subnet (typically 5 in AWS) for internal use, so your usable count is slightly lower than the math suggests.

Subnet Calculator

How to Use Subnet Calculator

How Subnet Calculation Works

Common CIDR Sizes

Private IP Ranges

Why Subnetting Matters for Cloud VPCs

Frequently Asked Questions

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