IPv4 CIDR Notation Calculator
Enter a CIDR block like 192.168.1.10/24, or enter the IP and prefix separately.
Tip: Press Enter in any field to calculate instantly.
What Is CIDR Notation?
CIDR stands for Classless Inter-Domain Routing. It is the modern way to represent IP networks and subnet sizes using the format
IP/prefix, such as 192.168.1.0/24. The number after the slash is the number of bits used for the network portion.
A CIDR notation calculator helps you quickly determine important subnet details, including network address, broadcast address, subnet mask, wildcard mask, and usable host range.
How to Use This CIDR Notation Calculator
- Enter a full CIDR value (example:
172.16.40.12/22) or enter IP address + prefix separately. - Click Calculate.
- Review subnet details like host range, total addresses, and mask values.
- Use Reset to clear all fields and run a new subnet calculation.
What the Results Mean
Network Address
The first address in the subnet. It identifies the network and cannot be assigned to a regular host in traditional subnets.
Broadcast Address
The last address in the subnet. It is used for broadcasting traffic to all hosts in that subnet (except special cases like /31 and /32).
Subnet Mask and Wildcard Mask
The subnet mask is the dotted-decimal form of your prefix (for example, /24 = 255.255.255.0).
The wildcard mask is the inverse (0.0.0.255 for /24), commonly used in ACL rules.
Usable Host Range
This is the practical assignable range for devices. For most subnets, it excludes the network and broadcast addresses.
For /31, both addresses can be used in point-to-point links; for /32, only one host exists.
Quick CIDR Cheat Sheet
/8→ 255.0.0.0 → 16,777,216 total addresses/16→ 255.255.0.0 → 65,536 total addresses/24→ 255.255.255.0 → 256 total addresses/25→ 255.255.255.128 → 128 total addresses/26→ 255.255.255.192 → 64 total addresses/27→ 255.255.255.224 → 32 total addresses/28→ 255.255.255.240 → 16 total addresses/29→ 255.255.255.248 → 8 total addresses/30→ 255.255.255.252 → 4 total addresses
Why CIDR Is Important
CIDR replaced old class-based IP design (Class A/B/C) and enables efficient IP allocation. Instead of forcing fixed block sizes, network engineers can assign precisely sized subnets to reduce waste and simplify route aggregation.
- Better address utilization
- Cleaner routing tables with summarization
- Flexible subnet design for cloud and enterprise networks
- Easier VLSM (Variable Length Subnet Masking) planning
Common Subnetting Mistakes
- Confusing prefix length with number of hosts
- Forgetting network and broadcast reservations on standard subnets
- Mixing public and private IP ranges unintentionally
- Using the wrong wildcard mask in firewall/ACL rules
Final Thoughts
Whether you are preparing for CCNA, configuring routers, designing VLANs, or working in cloud networking, this IP subnet calculator gives you fast and accurate CIDR results. Keep it handy for network address planning, route summaries, and troubleshooting.