Skip the Binary Arithmetic
You’re designing a VPC in AWS and need to carve out subnets for your public tier, private tier, and database tier. You know you want a /16 for the VPC, but how many hosts does a /20 subnet give you? What’s the address range for 10.0.48.0/20? Where does it overlap with 10.0.32.0/20?
Subnetting by hand means converting to binary, counting bits, and doing modular arithmetic. It’s the kind of thing you learned for CCNA and never wanted to do again. Type your CIDR notation and get the complete breakdown instantly.
What You Get
Enter 10.0.0.0/16 and see:
- Network address: 10.0.0.0
- Broadcast address: 10.0.255.255
- Subnet mask: 255.255.0.0
- Wildcard mask: 0.0.255.255
- First usable host: 10.0.0.1
- Last usable host: 10.0.255.254
- Total addresses: 65,536
- Usable hosts: 65,534
The wildcard mask is the inverse of the subnet mask — Cisco ACLs and some routing configurations use it instead of the standard subnet mask.
Cloud Networking Decisions
AWS VPC design. You’ve got a /16 VPC (65,534 usable hosts). You want 4 subnets across 2 availability zones. Use /20 subnets (4,094 hosts each) for room to grow, or /24 subnets (254 hosts) if you want more, smaller segments.
Security group rules. You need to allow traffic from a specific subnet. Calculate the range to make sure your CIDR block covers exactly the addresses you intend — not more, not less.
GCP or Azure subnet planning. Same math, different cloud. Enter your proposed CIDR ranges and verify they don’t overlap before creating them.
Subnet Size Quick Reference
| Prefix | Usable Hosts | Common Use |
|---|---|---|
| /30 | 2 | Point-to-point links |
| /28 | 14 | Small office |
| /24 | 254 | Standard LAN segment |
| /20 | 4,094 | Large subnet or cloud tier |
| /16 | 65,534 | VPC or campus network |
Pick the smallest prefix that fits your host count. Oversizing wastes address space. Undersizing means you’ll need to renumber later.
CCNA Study Tool
If you’re prepping for a networking cert, this is your subnetting drill partner. Enter CIDR notations, check the answers mentally, then verify. The binary subnet mask display helps you understand what’s happening under the hood.
For IPv4-to-IPv6 address conversion, the IPv4 to IPv6 Converter handles the math. The IP Address Lookup provides geolocation and ISP info for any IP.
All calculations run in your browser. No network data leaves the page.