What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation expresses an IP address range as a base address followed by a slash and prefix length, such as 10.0.0.0/16. The prefix length indicates how many leading bits are fixed, so /16 means the first 16 bits are the network portion and the remaining 16 bits are available for host addresses, yielding 65,536 total addresses.

How many usable IPs does AWS reserve per subnet?

AWS reserves five IP addresses in every VPC subnet: the network address, the VPC router (.1), the DNS server (.2), a future-use address (.3), and the broadcast address. So a /24 subnet with 256 total addresses has only 251 usable host IPs.

What is the largest and smallest CIDR block allowed in an AWS VPC?

AWS VPCs support CIDR blocks from /16 (65,536 addresses) down to /28 (16 addresses). You can associate multiple CIDR blocks with a single VPC to expand its address space up to five blocks.

VPC CIDR Calculator

NetworkingAWS

Calculate network, broadcast, host range, and mask from CIDR notation.

CIDR Input

Raw Output

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About This Tool

The VPC CIDR Calculator converts CIDR notation into its constituent network details including network address, broadcast address, usable host range, subnet mask, and total host count. Understanding CIDR math is fundamental to designing cloud networks, and this tool eliminates manual binary arithmetic. Paste any CIDR block and instantly see every derived value, making it easy to verify address space before provisioning VPCs, subnets, or firewall rules.

When to Use This Tool

•Verifying the usable host count for a proposed VPC CIDR block before creating it in the AWS Console or CLI.
•Calculating the network and broadcast addresses to confirm firewall allow-list boundaries.
•Converting between CIDR prefix lengths and dotted-decimal subnet masks for legacy documentation.
•Double-checking that a CIDR block provides enough IP addresses for anticipated workloads before committing infrastructure.

Frequently Asked Questions

What is CIDR notation?: CIDR (Classless Inter-Domain Routing) notation expresses an IP address range as a base address followed by a slash and prefix length, such as 10.0.0.0/16. The prefix length indicates how many leading bits are fixed, so /16 means the first 16 bits are the network portion and the remaining 16 bits are available for host addresses, yielding 65,536 total addresses.
How many usable IPs does AWS reserve per subnet?: AWS reserves five IP addresses in every VPC subnet: the network address, the VPC router (.1), the DNS server (.2), a future-use address (.3), and the broadcast address. So a /24 subnet with 256 total addresses has only 251 usable host IPs.
What is the largest and smallest CIDR block allowed in an AWS VPC?: AWS VPCs support CIDR blocks from /16 (65,536 addresses) down to /28 (16 addresses). You can associate multiple CIDR blocks with a single VPC to expand its address space up to five blocks.

Related Learning Guides

AWS Networking Deep Dive30 min read
VPC Architecture Patterns28 min read

Disclaimer: This tool runs entirely in your browser. No data is sent to our servers. Always verify outputs before using them in production. AWS, Azure, and GCP are trademarks of their respective owners.