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.

Last verified: April 2026

CIDR Input

Raw Output

Output will appear here...

See It in Action

You're setting up a new multi-region deployment and the network team asks for CIDR allocations that don't overlap with the existing 4 VPCs. You plug in 10.0.0.0/16 and immediately see it provides 65,536 addresses across a range you can carve into /20 subnets per AZ. You allocate 10.1.0.0/16 for the new region and confirm zero overlap with existing ranges using the overlap checker.

What This Tool Does

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.

Technical Details

The calculator converts the CIDR prefix length to a 32-bit subnet mask using bitwise left-shift, then applies bitwise AND between the IP address and mask to derive the network address. The broadcast address is calculated by ORing the IP with the inverted mask. Usable host range is network+1 to broadcast-1. All math uses JavaScript's 32-bit integer operations for accuracy.

Common Use Cases

1Verifying the usable host count for a proposed VPC CIDR block before creating it in the AWS Console or CLI.
2Calculating the network and broadcast addresses to confirm firewall allow-list boundaries.
3Converting between CIDR prefix lengths and dotted-decimal subnet masks for legacy documentation.
4Double-checking that a CIDR block provides enough IP addresses for anticipated workloads before committing infrastructure.

Common 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.

Expert Tips

TIP

When designing VPCs, always start with a /16 even if you think you won't need it. Address space is free, but running out of IPs in a production VPC requires painful migrations.

TIP

Remember AWS reserves 5 IPs per subnet, not 2. A /28 gives you only 11 usable hosts, not 14. This catches people when sizing small subnets for NAT Gateways or bastion hosts.

TIP

If you're using EKS, plan for pod networking. With the VPC CNI plugin, each pod gets its own IP address, so a /24 subnet can only hold about 250 pods, not the thousands you might expect.

Featured In

Related Learning Guides

AWS Networking Deep Dive30 min read
VPC Architecture Patterns28 min read

Was this tool helpful?

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.