What Is CIDR Notation?
CIDR (Classless Inter-Domain Routing) writes an IPv4 prefix as an address followed by a slash and a number from 0 to 32. In 192.168.1.0/24, the first 24 bits identify the network and the remaining 8 bits identify addresses inside that block.
CIDR replaced classful address allocation and allows networks to be sized in powers of two. It is used in routing tables, firewall rules, cloud VPCs, VPNs, Kubernetes clusters, and on-premises network plans.
How to Use This IPv4 Subnet Calculator
- Enter an IPv4 address such as
10.20.30.40. - Choose a CIDR prefix or enter a contiguous dotted-decimal subnet mask.
- Review the canonical network, upper boundary, usable range, host counts, binary bits, and optional equal-size subnet split.
Use Copy Report for quick sharing or download TXT and JSON files for documentation and infrastructure planning.
Subnet Address and Host Calculations
The block contains 2^(32 − prefix) addresses. For prefixes from /0 through /30, the conventional usable-host count excludes the network and directed-broadcast addresses. The wildcard mask is the bitwise inverse of the subnet mask.
A /31 is different: RFC 3021 allows both addresses on a point-to-point link. A /32 identifies one host address or host route. This calculator applies those modern rules instead of incorrectly returning zero usable addresses.
AWS IPv4 Subnet Capacity
Standard AWS IPv4 subnets accept prefix sizes from /16 through /28. AWS reserves the first four addresses and the last address in each subnet, so a /24 has 251 assignable addresses and a /28 has 11.
Prefixes outside the supported AWS range are shown as N/A instead of presenting a misleading capacity. Other cloud platforms can use different limits and reservations, so check the provider documentation before deployment.
Private, Browser-Based Calculation
IP addresses, masks, subnet splits, and exported reports are calculated locally in your browser. The calculator does not need to upload the values you enter to a processing server.
Related Developer Tools
Convert address values between binary, decimal, and hexadecimal with the Number Base Converter, format exported infrastructure data with the JSON Formatter, safely prepare query parameters with the URL Encoder, verify file or configuration checksums with the Hash Generator, or schedule network jobs with the Cron Builder.
To see how IP addresses, routers, DNS, packets, and servers fit together, read How Does the Internet Really Work?
For the system that maps domain names to IPv4 and IPv6 destinations, continue with How DNS Works.
To follow the shift from dial-up access to broadband, mobile networks, and an always-connected world, continue with Internet: Then vs Now.
Meet one of the architects behind TCP/IP and internetworking in the Vint Cerf profile.
Authoritative Networking References
Frequently Asked Questions
Planning note: The output follows IPv4 CIDR arithmetic and the special /31 and /32 rules described above. Confirm cloud-provider limits, reserved addresses, routing policies, and overlapping ranges before applying a production network configuration.