AWS VPC CIDR & Subnet Calculator
AWS VPC Subnet Calculator
AWS always reserves five IP addresses per subnet: network, .1 router, .2 DNS, .3 future, and broadcast. This calculator shows usable ranges and reserved addresses instantly.
Why AWS Reserved IPs Matter
Every AWS subnet blocks five IP addresses. This reduces usable capacity, especially on small subnets like /28 or /29. The calculator below highlights those addresses so you can plan realistic host counts.
Use this tool to estimate usable IPs, compare subnet sizes, and validate VPC splitting before creating your AWS VPC configuration.
Example Use Cases
- Split a /16 into /20 subnets for regional isolation.
- Compare /24 vs /26 usable IPs for private workloads.
- Plan public subnets with enough space for NAT and load balancers.
Input
Output
VPC splits into /18 subnets (total 4).
4 private / 0 public.
Availability zones
Private subnet 1
10.0.0.0
us-east-1a
First usable
10.0.0.4
Last usable
10.0.63.254
Private subnet 2
10.0.64.0
us-east-1b
First usable
10.0.64.4
Last usable
10.0.127.254
Private subnet 3
10.0.128.0
us-east-1c
First usable
10.0.128.4
Last usable
10.0.191.254
Private subnet 4
10.0.192.0
us-east-1a
First usable
10.0.192.4
Last usable
10.0.255.254
AWS Subnet Calculator FAQ
Why does AWS reserve 5 IPs per subnet?
AWS uses them for network, routing, DNS, future growth, and broadcast. They are not usable by your instances.
What is the smallest usable AWS subnet?
AWS allows /28 as the smallest subnet, which yields 16 IPs total and 11 usable after reservations.
Does public/private selection change the math?
No. The label is for organization only. AWS reservation rules are the same for all subnets.