> CIDR_CALCULATOR

📖 About CIDR Notation

What is CIDR? Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing. It uses a suffix (like /24) to specify how many bits are in the network portion. Introduced in 1993, CIDR replaced the old "classful" addressing system (Class A, B, C) which wasted massive amounts of IP address space.

Understanding CIDR Prefixes: The number after the slash indicates how many bits (out of 32 for IPv4) represent the network portion. The remaining bits represent host addresses. For example, in 192.168.1.0/24, the first 24 bits (192.168.1) identify the network, and the last 8 bits allow for 256 possible addresses (0-255). Smaller prefix numbers mean more addresses: /8 provides over 16 million addresses, while /32 represents a single host.

Common CIDR ranges:

• /8 (255.0.0.0) = 16,777,216 addresses (Class A equivalent) - Used by massive organizations and ISPs
• /16 (255.255.0.0) = 65,536 addresses (Class B equivalent) - Regional networks, large enterprises
• /24 (255.255.255.0) = 256 addresses (Class C equivalent) - Standard small business network
• /28 (255.255.255.240) = 16 addresses - Small office subnets
• /30 (255.255.255.252) = 4 addresses (2 usable) - Point-to-point links between routers
• /32 = Single host address - Specific server or device

Subnetting Basics: Subnetting divides a large network into smaller logical networks for better organization, security, and efficiency. For example, a company with a /24 network could divide it into four /26 subnets (64 addresses each) for different departments. Proper subnetting reduces broadcast traffic, improves security by isolating network segments, and simplifies management.

Use cases: Network planning (designing office LANs), subnet design (segmenting networks by department or function), firewall rules (allowing/blocking specific IP ranges), AWS/Azure/GCP VPC configuration (cloud networking), IP address allocation (efficient address space management), security group rules (cloud access control), and route aggregation (reducing routing table size).

Private IP Address Ranges: RFC 1918 defines private IP ranges that aren't routed on the public internet: 10.0.0.0/8 (10.0.0.0 to 10.255.255.255), 172.16.0.0/12 (172.16.0.0 to 172.31.255.255), and 192.168.0.0/16 (192.168.0.0 to 192.168.255.255). These ranges are perfect for internal networks behind NAT, allowing organizations to reuse addresses without conflicts.

VLSM (Variable Length Subnet Masking): Modern networks use VLSM to assign differently-sized subnets based on actual needs. Instead of wasting a /24 (254 hosts) for a point-to-point link needing only 2 addresses, use a /30. This efficient allocation became possible with CIDR, maximizing usable IP space especially as IPv4 addresses become scarce.

Related: IP Lookup · ASN Lookup · Reverse DNS

❓ CIDR Calculator FAQs

What is CIDR? CIDR represents an IP network as address/prefix (example: 10.0.0.0/8). The prefix length controls how large the range is. CIDR notation is more flexible and efficient than the old classful system, allowing precise allocation of address blocks.

How many IPs are in a /24? A /24 contains 256 total IPv4 addresses. In many cases, 254 are usable hosts (network + broadcast reserved). The first address (e.g., 192.168.1.0) is the network identifier, and the last (192.168.1.255) is the broadcast address for reaching all devices in that subnet.

What are network and broadcast addresses? The network address identifies the subnet (all host bits set to 0), and the broadcast address targets all hosts on that subnet (all host bits set to 1). These are part of traditional IPv4 subnetting. For example, in 192.168.1.0/24, the network address is 192.168.1.0 and broadcast is 192.168.1.255, leaving 192.168.1.1 through 192.168.1.254 (254 addresses) for actual devices.

What should I do with the IP range result? Use it for firewall allowlists (permitting specific IP ranges), subnet planning (designing network architecture), cloud networking (AWS VPC, Azure Virtual Network, GCP VPC configuration), access control lists (restricting service access), load balancer configuration, and IP whitelisting. Pair it with Reverse DNS and IP Lookup when investigating specific addresses within a range.

Why does /31 have 2 usable addresses? RFC 3021 allows /31 subnets for point-to-point links, skipping the traditional network/broadcast reservation. This provides exactly 2 usable addresses, perfect for router-to-router connections, saving precious IPv4 space on links that only need two endpoints.

How do I subnet a larger network? To subnet 192.168.1.0/24 into smaller networks, increase the prefix length. For 2 subnets, use /25 (128 addresses each). For 4 subnets, use /26 (64 each). For 8 subnets, use /27 (32 each). Each time you add one bit to the prefix, you double the number of subnets but halve the addresses per subnet. Use this calculator to see the exact IP ranges for each subnet.

What's the difference between /22 and /24? A /22 contains 4 times more addresses than a /24. Specifically, /22 has 1,024 addresses (equivalent to four /24 networks combined), while /24 has 256. Cloud providers often recommend /22 or /23 for production VPCs to allow growth. You can always subdivide a /22 into smaller subnets, but combining existing /24s requires renumbering.