AWS Networking Explained for Backend Engineers
AWS networking is one of the most critical yet misunderstood areas for backend engineers. Poor networking design leads to downtime, security issues, high costs, and painful debugging.
This article explains AWS networking from a backend engineer’s perspective, focusing on why components exist, how they interact, and how they are used in real systems.
Virtual Private Cloud (VPC)
A VPC is a logically isolated network in AWS where your resources live.
Think of it as your own private data center inside AWS.
Key Properties
- CIDR range (e.g.
10.0.0.0/16)
- Region-scoped
- Fully isolated by default
Subnets
A subnet is a subdivision of a VPC CIDR.
Important Rules
- A subnet belongs to one Availability Zone
- Cannot span multiple AZs
Types of Subnets
Public Subnet
- Route to Internet Gateway
- Used for Load Balancers, Bastion Hosts, NAT Gateways
Private Subnet
- No direct internet access
- Used for backend services, databases, EKS nodes
Backend services should almost always run in private subnets.
Internet Gateway (IGW)
An Internet Gateway enables communication between a VPC and the internet.
Key Points
- One IGW per VPC
- Required for public subnets
- Fully managed and highly available
Route Tables
Route tables control how traffic is routed within a VPC.
Each subnet is associated with one route table.
Example Routes
10.0.0.0/16 → local 0.0.0.0/0 → igw-xxxxxxxx
Routing misconfiguration is the most common cause of network timeouts.
NAT Gateway
A NAT Gateway allows private subnet resources to access the internet securely.
Why NAT Is Needed
- OS updates
- External API calls
- OAuth and identity providers
Best practice: one NAT Gateway per AZ for production workloads.
Security Groups
Security Groups are stateful virtual firewalls.
Characteristics
- Allow rules only
- Applied at resource level
- Return traffic automatically allowed
Common pattern:
- ALB → Internet
- Backend → ALB only
- Database → Backend only
Network ACLs (NACL)
NACLs are stateless firewalls applied at subnet level.
Characteristics
- Allow and deny rules
- Evaluated in order
- Used mainly for coarse-grained control
Elastic Load Balancer (ELB)
Load balancers distribute traffic across backend services.
Types
Application Load Balancer (ALB)
- Layer 7 (HTTP/HTTPS)
- Path and host-based routing
- Ideal for APIs and microservices
Network Load Balancer (NLB)
- Layer 4 (TCP/UDP)
- High performance and low latency
VPC Endpoints
VPC Endpoints allow private access to AWS services without internet or NAT.
Types
- Gateway Endpoints: S3, DynamoDB
- Interface Endpoints: Most AWS services
Benefits include better security and reduced NAT costs.
High Availability with AZs
Regions contain multiple Availability Zones.
Best Practice Architecture
- Public and private subnets per AZ
- NAT Gateway per AZ
- Load balancer spanning AZs
This design avoids single points of failure.
Real-World Backend Architecture Example
A production SaaS setup typically includes:
- One VPC
- Multiple AZs
- Public subnets for ALB and NAT
- Private subnets for backend services
- Database subnets with restricted access
- Strict Security Group rules
- VPC Endpoints for AWS services
Common Mistakes
- Running backend services in public subnets
- Using a single NAT Gateway for multi-AZ production
- Overly permissive Security Groups
- Ignoring routing while debugging
- Not using VPC Endpoints
Conclusion
AWS networking is foundational knowledge for backend engineers.
Understanding VPCs, subnets, routing, NAT, and security enables you to build secure, scalable, and cost-efficient systems.
Once networking is clear, everything else in AWS becomes easier.
Happy building scalable backend systems 🚀