CIDR Calculator
Calculate network range, netmask, and host counts from any CIDR.
What is CIDR?
CIDR, or Classless Inter-Domain Routing, is a method for allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the previous classful network addressing architecture. CIDR notation represents an IP address and its associated network mask in a compact format: `A.B.C.D/n`, where `n` is the prefix length—the number of leading 1s in the subnet mask.
Understanding the Results
- Network Address: The first IP address in a subnet. It represents the entire network and cannot be assigned to a specific device.
- Netmask: A 32-bit mask used to divide an IP address into its network and host portions. The prefix (`/n`) is a shorthand for this mask.
- Usable Host Range: The range of IP addresses that can be assigned to individual devices within the subnet. It excludes the Network and Broadcast addresses.
- Broadcast Address: The last IP address in a subnet. It is used to send a message to all devices on the network simultaneously.
- Total Hosts: The total number of IP addresses in the subnet, including the network and broadcast addresses.
- Usable Hosts: The number of IP addresses available for assignment to devices.
Common CIDR Prefixes
| Prefix | Netmask | Hosts |
|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 |
| /16 | 255.255.0.0 | 65,536 |
| /24 | 255.255.255.0 | 256 |
| /28 | 255.255.255.240 | 16 |
| /32 | 255.255.255.255 | 1 |
The Ultimate Guide to CIDR Notation and Subnetting
Whether you are an AWS cloud architect provisioning Virtual Private Clouds (VPCs) or a network engineer configuring on-premise routers, understanding Classless Inter-Domain Routing (CIDR) is arguably the most important foundational skill in IP networking. Our free calculator allows you to instantly visualize network boundaries, host ranges, and broadcast addresses.
What Exactly is CIDR?
Before 1993, the internet used a "Classful" networking model (Classes A, B, and C). This system was incredibly inefficient. A Class C network only allowed 254 hosts (too small for many businesses), while a Class B network allowed 65,534 hosts (massively too large, leading to millions of wasted IP addresses). As the internet rapidly expanded, the world was quickly running out of IPv4 addresses.
CIDR (Classless Inter-Domain Routing) was introduced as a brilliantly flexible solution. Instead of rigidly locking networks into three arbitrary sizes, CIDR allows network administrators to allocate IP addresses in blocks of almost any size. It does this by appending a "suffix" or "prefix length" to the IP address—for example, 192.168.1.0/24.
How to Read CIDR Notation
An IPv4 address consists of 32 bits (represented as four 8-bit numbers, or octets, separated by periods). In traditional CIDR notation, the number after the slash simply tells the router exactly how many of those 32 bits are locked as the network identifier.
Example: 10.0.0.0/16
The 16 means the first 16 bits (the first two octets, 10.0) are the fixed network address. The remaining 16 bits (32 total - 16 network = 16 available) can be used to assign IP addresses to individual computers, servers, or devices (yielding 65,536 possible addresses).
Understanding the Network Data Points
- Subnet Mask: The traditional dotted-decimal representation of the CIDR suffix. A /24 suffix directly translates to a 255.255.255.0 subnet mask.
- Network Address (First IP): The very first IP in the subnet. It fundamentally represents the network itself and cannot be assigned to a computer.
- Broadcast Address (Last IP): The very last IP in the subnet block. When a router sends a packet to this address, the switch blasts copies of that packet to every single connected machine simultaneously. It cannot be assigned to a computer.
- Usable Host Range: all the IP addresses resting between the Network and Broadcast addresses. These are the IPs you can actually give to your laptops and servers.
Common Subnets Cheat Sheet
Here are the most frequently encountered CIDR blocks used in cloud architecture like AWS VPCs and Docker networks:
| CIDR Prefix | Subnet Mask | Usable Hosts | Common Use Case |
|---|---|---|---|
| /16 | 255.255.0.0 | 65,534 | Standard starting size for an AWS VPC or massive corporate network. |
| /24 | 255.255.255.0 | 254 | The most ubiquitous subnet size. Standard home router allocation. |
| /28 | 255.255.255.240 | 14 | Small isolated subnet for a clustered database or load balancer array. |
| /32 | 255.255.255.255 | 1 | Represents exactly one single computer. Used explicitly in firewall whitelists. |