AWS, Azure & GCP · Cloud across providers
What are subnets and how do you design them?
Short answer
Subnets partition a VPC CIDR into smaller ranges placed in one availability zone. Public subnets route to an internet gateway, private subnets use NAT or private endpoints, and isolated subnets have no internet route.
A subnet is both an address range and a placement decision. Because it belongs to one AZ, a multi-AZ design needs at least one subnet per tier per zone. Routing is controlled by the route table associated with the subnet.
10.0.0.0/16
10.0.1.0/24 public-a
10.0.2.0/24 private-a
10.0.11.0/24 public-b
10.0.12.0/24 private-bCommon tiers
- Public: load balancers and bastions, routed to an internet gateway
- Private: application and data tiers, routed to NAT for outbound access
- Isolated: databases and internal services with no internet route
- Reserved: spare capacity for growth or future services
Size subnets for peak scale, not current usage. Cloud providers reserve addresses in each subnet, and container or serverless networking can consume many IPs quickly. A /24 is common for small tiers, but large node pools may need /20 or bigger.
Follow-ups you should expect
Interviewers use these to check whether the first answer was understanding or memorisation.
- 1Why does a multi-AZ design need multiple subnets per tier?
- 2How many IP addresses does a cloud provider reserve per subnet?
- 3What routing makes a subnet private rather than public?
- 4How would you plan subnet size for a large Kubernetes cluster?
Say it in the interview
Subnets split a VPC into smaller ranges, and each subnet lives in one availability zone. Public subnets route to an internet gateway, private subnets use NAT or private endpoints, and isolated subnets have no internet path. I design parallel tiers per AZ and size ranges for peak scale, because running out of IPs is disruptive to fix.
Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.
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