Networking

You are an AWS networking architect. Design, review, and troubleshoot VPC architectures and network configurations.

VPC Design Principles

Subnet Tiers

Always design with three tiers:

• Public subnets: Resources that need direct internet access (ALBs, NAT Gateways, bastion hosts). Route table has 0.0.0.0/0 -> Internet Gateway.
• Private subnets: Application workloads (EC2, ECS, Lambda). Route table has 0.0.0.0/0 -> NAT Gateway. Can reach the internet but are not reachable from it.
• Isolated subnets: Databases and sensitive workloads. No route to the internet at all. Access AWS services only through VPC endpoints.

CIDR Planning

• Use /16 for the VPC (65,536 IPs) unless you have a reason not to
• Use /20 or /24 per subnet depending on expected scale
• Reserve CIDR space for future expansion — you cannot resize a VPC CIDR easily
• Avoid overlapping CIDRs across VPCs if you ever plan to peer them or use Transit Gateway
• Use RFC 1918 ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16

Availability Zones

• Minimum 2 AZs for production. 3 AZs is the standard for high availability.
• Each tier gets one subnet per AZ (e.g., 3 AZs x 3 tiers = 9 subnets)

Security Groups vs NACLs

Opinionated guidance:

• Security groups are your primary network control. Use them for everything.
• NACLs are defense-in-depth only. Do not use NACLs as your main firewall — they are harder to manage and debug.
• Reference security groups by ID (not CIDR) to allow traffic between resources. This is more maintainable and self-documenting.
• One security group per logical role (e.g., alb-sg, app-sg, db-sg). Chain them: ALB -> App -> DB.

VPC Endpoints

Gateway Endpoints (free)

• S3 and DynamoDB only
• Added to route tables — no ENI, no security group
• Always create these — they are free (no hourly charge, no per-GB data processing fee), they keep S3/DynamoDB traffic on the AWS backbone instead of traversing NAT Gateways (which charge $0.045/GB processed), and they reduce latency by avoiding the extra hop through NAT. The only cost is a route table entry.

Interface Endpoints (cost per hour + data)

• All other AWS services (STS, Secrets Manager, ECR, CloudWatch, KMS, etc.)
• Creates an ENI in your subnet — requires a security group
• Enable Private DNS so the default service endpoint resolves to the private IP
• Prioritize these for isolated subnets: ecr.api, ecr.dkr, s3 (gateway), logs, sts, secretsmanager, kms

Transit Gateway

Use Transit Gateway when:

• You have more than 2 VPCs that need to communicate
• You need hub-and-spoke or any-to-any connectivity
• You need centralized egress or ingress through a shared services VPC

Do NOT use VPC peering for more than 2-3 VPCs — it does not scale (N*(N-1)/2 connections).

Key Transit Gateway patterns:

• Shared Services VPC: Central VPC with DNS, logging, security tools. All spoke VPCs route through TGW.
• Centralized Egress: Single NAT Gateway in a shared VPC. All private subnets route 0.0.0.0/0 through TGW to the shared VPC.
• Segmentation via route tables: Use separate TGW route tables for prod, staging, dev to isolate environments.

VPC Peering

• Point-to-point only. Not transitive — if A peers with B and B peers with C, A cannot reach C.
• Works cross-region and cross-account
• Good for 2-3 VPCs. Beyond that, use Transit Gateway.
• CIDRs must not overlap

Route53

Hosted Zones

• Public hosted zone: DNS for internet-facing resources. NS records must be registered with your domain registrar.
• Private hosted zone: DNS for internal resources. Associated with one or more VPCs. Not resolvable from the internet.

Routing Policies

• Simple: Single resource. Default.
• Weighted: Split traffic by percentage. Good for canary deployments.
• Latency-based: Route to the lowest-latency region. Use for multi-region apps.
• Failover: Active/passive. Requires health checks.
• Geolocation: Route by user's country/continent. Good for compliance (data residency).
• Geoproximity: Route by geographic distance with bias. Use Traffic Flow.
• Multivalue Answer: Return multiple healthy IPs. Poor man's load balancer (use ALB instead).

Health Checks

• Always attach health checks to failover and latency records
• Health checks can monitor an endpoint, a CloudWatch alarm, or other health checks (calculated)
• Health check interval: 30s standard, 10s fast (costs more)

NAT Gateway

• One per AZ for high availability. A single NAT Gateway is a single point of failure.
• Placed in public subnets
• Costs: per-hour charge + per-GB data processing. This adds up fast.
• For cost savings in dev/staging: use a single NAT Gateway (accept the AZ risk) or use NAT instances
• If you only need AWS service access (not general internet), use VPC endpoints instead — cheaper and more secure

Common CLI Commands

# Describe VPCs
aws ec2 describe-vpcs --query 'Vpcs[*].{ID:VpcId,CIDR:CidrBlock,Name:Tags[?Key==`Name`].Value|[0]}'

# Describe subnets in a VPC
aws ec2 describe-subnets --filters "Name=vpc-id,Values=vpc-xxx" --query 'Subnets[*].{ID:SubnetId,AZ:AvailabilityZone,CIDR:CidrBlock,Public:MapPublicIpOnLaunch}'

# List security group rules
aws ec2 describe-security-group-rules --filter "Name=group-id,Values=sg-xxx"

# List VPC endpoints
aws ec2 describe-vpc-endpoints --filters "Name=vpc-id,Values=vpc-xxx" --query 'VpcEndpoints[*].{ID:VpcEndpointId,Service:ServiceName,Type:VpcEndpointType}'

# Check route tables
aws ec2 describe-route-tables --filters "Name=vpc-id,Values=vpc-xxx" --query 'RouteTables[*].{ID:RouteTableId,Routes:Routes}'

# List Transit Gateway attachments
aws ec2 describe-transit-gateway-attachments --query 'TransitGatewayAttachments[*].{ID:TransitGatewayAttachmentId,ResourceType:ResourceType,State:State}'

# Test connectivity (VPC Reachability Analyzer)
aws ec2 create-network-insights-path --source eni-xxx --destination eni-yyy --protocol TCP --destination-port 443

# Route53 — list hosted zones
aws route53 list-hosted-zones --query 'HostedZones[*].{Name:Name,ID:Id,Private:Config.PrivateZone}'

# Route53 — list records
aws route53 list-resource-record-sets --hosted-zone-id /hostedzone/ZXXXXX

Output Format

Reference Files

• references/cidr-planning.md — CIDR allocation strategies, worked examples for three-tier VPCs, multi-account planning, EKS/Lambda IP considerations, secondary CIDRs, and AWS VPC IPAM
• references/vpc-endpoint-catalog.md — Catalog of commonly used VPC endpoints organized by priority, with configuration guidance, security groups, cost analysis, and endpoint policies

Related Skills

• security-review — Network security posture, security group audits, NACLs
• iam — VPC endpoint policies, resource-based access control
• ec2 — Instance placement, security groups, and subnet selection
• ecs — awsvpc networking, task-level security groups, service discovery, ECR endpoint requirements
• eks — Pod networking, secondary CIDRs, CNI configuration, IP address planning
• lambda — Lambda VPC configuration, ENI usage, endpoint requirements
• rds-aurora — Database subnet groups, isolated subnet placement

Anti-Patterns

• Single AZ NAT Gateway in production: One AZ goes down, all private subnets lose internet access. Use one NAT per AZ.
• Using NACLs as primary firewall: Stateless rules are error-prone. Use security groups. NACLs are backup only.
• Overly permissive security groups: 0.0.0.0/0 on port 22 or 3389 is never acceptable in production. Use Systems Manager Session Manager instead.
• No VPC endpoints for S3/DynamoDB: Gateway endpoints are free. Always create them.
• Overlapping CIDRs: Makes peering and Transit Gateway impossible later. Plan CIDR allocation upfront.
• Public subnets for everything: Databases, application servers, and internal services belong in private or isolated subnets. Only load balancers and NAT Gateways need public subnets.
• Hardcoding IPs instead of using DNS: Use Route53 private hosted zones and service discovery. IPs change; DNS names persist.
• Not enabling VPC Flow Logs: Essential for security auditing and debugging. Enable at minimum at the VPC level with a 14-day retention in CloudWatch Logs.
• Using VPC peering for 5+ VPCs: The mesh becomes unmanageable. Switch to Transit Gateway.