VPC CIDR and subnet calculator
Type your VPC range, pick a subnet size and the number of availability zones. You get the subnet list with the addresses AWS really leaves you. It runs in your browser.
Runs in your browser, nothing is uploadedTry:,,
This range
Subnets
| # | AZ | CIDR | First IP | Last IP | Usable in AWS |
|---|
Check two ranges for overlap
Planning VPC peering or a VPN? Overlapping ranges cannot be connected.
AWS keeps 5 addresses in every subnet (the first four and the last one), so a /24 gives 251 usable IPs, not 254. A VPC can be between /16 and /28. Subnets are handed out to zones in turn (a, b, c), which is a common starting layout, not a rule. Plan extra room: you cannot resize a subnet later.
How to use the AWS subnet calculator
- Type your VPC CIDR, for example 10.0.0.0/16. AWS accepts sizes from /16 (65,536 addresses) to /28 (16 addresses).
- Pick a subnet size and the number of availability zones. Most production setups use 2 or 3 zones.
- Read the table. "Usable in AWS" already removes the 5 addresses AWS reserves in each subnet.
Example: 10.0.0.0/16 across 3 zones
A /16 VPC split into /20 subnets gives 16 subnets of 4,091 usable addresses each. Use three for public subnets and three for private ones, one per zone, and keep the other ten free for later.
Planning a VPN or VPC peering? Paste the other network into the overlap check first. Ranges that overlap cannot be connected, and changing a VPC range later means rebuilding.
Common questions
Why does a /24 subnet give 251 addresses, not 256?
AWS reserves the first four addresses (network, router, DNS and a spare) and the last one in every subnet.
Should I use the default VPC range 172.31.0.0/16?
It works, but every account has it. Pick your own range so you can connect accounts and offices later without overlaps.
Can I resize a subnet later?
No. You can add more CIDR ranges to a VPC, but a subnet keeps its size, so leave room when you plan.
Read next: How to plan VPC CIDR ranges so you never have to rebuild
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