VPC Subnet Planner
Design a VPC's address space and catch overlaps, out-of-range subnets, and IP exhaustion live, before a single create-subnet call fails on you.
From your mentor
Real network design starts on paper, not in the console. Lay out your subnets here, watch the overlaps and the usable-IP counts update instantly, and you'll never fumble a CIDR plan in the real account.
Lay out your VPC
Set the VPC CIDR, then add subnets. Each one is validated live, in-range, legal size, no overlaps, with usable IPs and total capacity shown as you type.
In plain English
/24 looks like 256 addresses but AWS reserves 5 in every subnet, so you really get 251, and a tiny /28 leaves just 11. Plan small subnets with that −5 in mind, or you’ll run out of room sooner than the maths suggests.What it checks (and why each matters)
The planner enforces the same constraints AWS does when you call create-subnet:
The four rules
In range, a subnet must sit inside the VPC’s CIDR. Legal size, subnets are/16 to /28. No overlaps, two subnets can’t share addresses (the most common real failure). Capacity, all subnets together can’t exceed the VPC, and each loses 5 IPs to AWS.Leave room to grow
Don’t pack the VPC to 100%. A VPC’s primary CIDR can’t be shrunk later, so reserve headroom for new tiers and AZs, most teams start at a roomy/16 and carve /24 subnets.Next up
Go deeper → Networking Foundations
The full lesson with the interactive CIDR explorer, route tables, security groups vs NACLs, and the whole VPC picture.