VPC Sizing and Structure

Ref: https://learn.cantrill.io/courses/1820301/lectures/41301346 and https://learn.cantrill.io/courses/1820301/lectures/41301347

Designing Business Networks and IP Plans

🔧 Deploying a good solid infrastructure platform is about good design just as it is about a good technical implementation!! - Adrian Cantrill

Design of IP plan for a business = Design 1+ NWs within that plan
- In AWS in particular: design a VPC
- Knowledge in VPC/NW sizing and structure is key to identifying current and future problems or performance issues
Calculating IP ranges in advance…:
- …Correctly ✅ → Well-deployed NW structure
- …Incorrectly ❌ → A huge pain to fix later!
  - Will you use 1 or more CIDRs? Decide in advance (difficult and painful to change later)
  - VPCs/NWs should NOT overlap at all

Try to predict the future → What do we have now? Is it worth it to add buffers? Usually yes!
- Number of AWS regions now + adding a few as a buffer is usually a good idea (consider possible business expansion)
- Number of AWS accounts now + spare
- A good cautionary approach: reserve 2+ NWs per region being used & per account
  - In the course scenario (A4L): 4 VPCs per region and per account
- Also good to reserve a spare tier and AZ (check VPC Structure)
❗ Are there NWs we can NOT use or that we will need to interact with? In other words, are there NWs or CIDRs we should NOT overlap with?
- Consider: other VPCs, other cloud NWs, on-premises NWs, partners & vendors…
- If in doubt: assume the worst
- 💡 If we don't know this stuff, ask around in the organization! We need to know! What do we have? How will users access the NW? Etc, etc
Two Valid approaches, which do you choose?
1. Top-down: take the whole available IP range and make it fit needs/requirements
2. Bottom-up: start with the minimum subnet size you need and scale up

🔧 Decide the CIDR of the VPC (IP ranges)
VPC IPv4 size limits (min-max) = /28 to /16 (16 to 65536 IPs)
How many services/resources will fit into the VPC? → each service occupies an IP address