Subnet & CIDR IP Range Calculator
Calculate network boundaries, wildcard masks, usable host ranges, and AWS/Azure cloud subnet reservations instantly.
1. Network IP & Prefix
2. Binary Bitmask Representation
192.168.1.0/24
256
2^8 Addresses254
Available to VMs255.255.255.0
Dotted Decimal0.0.0.255
ACL / OSPF MaskSubnet Splitter & VLSM Breakdown
Divide into Equal Subnets| Subnet CIDR | Network Range | Usable Hosts | Mask |
|---|
Click "Run AI Network Audit" to generate architectural VPC subnetting blueprints, multi-AZ high availability recommendations, and Kubernetes pod CIDR planning. Your data is 100% private to you—zero server access.
Zero Data Access: Subnet math and network architectures are computed 100% in your local browser. Zero Server Access
Architecting an AWS Multi-AZ VPC: How Proper /20 Subnetting Prevented IP Exhaustion Across 14 Microservices
A real-world engineering breakdown of designing scalable cloud network architectures, avoiding overlapping CIDRs, and accounting for reserved cloud IPs.
The Cost of Inadequate Subnet Planning
When provisioning cloud infrastructure on AWS, Azure, or GCP, resizing an active Virtual Private Cloud (VPC) or subnet is nearly impossible without costly downtime and service re-deployments. Furthermore, cloud providers reserve 5 IP addresses per subnet for internal routing and DNS. Selecting an overly narrow CIDR block (e.g. `/28` with only 11 usable IPs) causes immediate Kubernetes pod eviction and autoscaling failures.
The Scenario: Alex's Kubernetes Migration on AWS
Alex is a Principal DevOps Engineer migrating an e-commerce platform with 14 containerized microservices to Amazon EKS. The initial VPC design allocated small `/26` subnets across 3 Availability Zones:
Total IPs per Subnet: 64
AWS Reserved: -5 IPs
Usable IPs: Only 59 Pods (Exhausted!)
VPC CIDR: 10.0.0.0/16 (65,536 IPs)
Subnet Allocation: /20 (4,096 IPs/AZ)
Multi-AZ across 3 zones.
Usable IPs per Subnet: 4,091 IPs
Autoscaling Headroom: 12,000+ Pods!
Zero downtime during Black Friday peaks.
Standard CIDR vs Cloud Provider Usable IP Comparison
| CIDR Prefix | Subnet Mask | Total Addresses | Standard Usable (-2) | AWS / Azure Usable (-5) |
|---|---|---|---|---|
| /28 | 255.255.255.240 | 16 | 14 | 11 (68.7% efficiency) |
| /24 | 255.255.255.0 | 256 | 254 | 251 |
| /20 | 255.255.240.0 | 4,096 | 4,094 | 4,091 (99.8% efficiency) |
| /16 | 255.255.0.0 | 65,536 | 65,534 | 65,531 |
Best Practice for Enterprise Cloud VPCs
Always start with a /16 CIDR block (e.g. 10.0.0.0/16) for your primary production VPC. Subdivide into /20 subnets across at least 3 Availability Zones for application workloads, and separate private /24 subnets for database clusters and NAT gateways to ensure complete fault isolation.
Frequently Asked Questions (Subnet & CIDR)
How do you calculate the number of usable hosts in a CIDR subnet?
For IPv4, the number of usable hosts is calculated using the formula 2^(32 - prefix) - 2. Two addresses are subtracted because the first address is the Network ID and the last address is the Broadcast address.
How many IP addresses does AWS reserve in a VPC subnet?
AWS reserves 5 IP addresses in every subnet: the Network address (.0), the VPC Router (.1), the Amazon DNS (.2), Future Use (.3), and the Network Broadcast address (.255). For example, a /24 subnet has 251 usable IPs in AWS instead of 254.
What is the difference between Classless Inter-Domain Routing (CIDR) and legacy Classful networking?
Classful networking restricted subnets to fixed boundaries (Class A: /8, Class B: /16, Class C: /24), wasting millions of IP addresses. CIDR allows variable-length subnet masking (VLSM), enabling network architects to allocate subnets of any size from /1 to /32.