Designing a VPC is only the beginning. In real-world systems, networks rarely exist in isolation. Applications need to communicate with other VPCs, access the internet securely, and integrate with on-premises infrastructure.
This final part of the VPC series explores the primary connectivity options available in AWS, how they differ, and when each should be used in production environments.
Outbound Internet Access from Private Subnets
Most production workloads run in private subnets but still require outbound internet access for updates, API calls, or external integrations.
NAT Gateway
A NAT Gateway allows instances in private subnets to initiate outbound internet traffic while remaining unreachable from the internet.
Key characteristics:
- Fully managed and highly available within an AZ
- Scales automatically
- Requires a public subnet and Elastic IP
- No inbound connections allowed
NAT Gateways are the standard solution for outbound internet access in private subnets.
NAT Instance (Legacy Approach)
A NAT instance is an EC2-based alternative to a NAT Gateway.
Limitations include:
- Manual scaling and patching
- Single point of failure unless carefully designed
- Lower throughput
Today, NAT instances are used only in specialized or cost-sensitive scenarios.
VPC Peering: Direct VPC-to-VPC Connectivity
VPC peering enables private connectivity between two VPCs using AWS’s internal network.
Important constraints:
- CIDR ranges must not overlap
- No transitive routing
- One-to-one relationships only
VPC peering is simple and effective for small architectures but becomes difficult to manage at scale.
Transit Gateway: Centralized Network Hub
AWS Transit Gateway acts as a hub that connects multiple VPCs and on-premises networks.
Advantages:
- Simplified routing
- Supports thousands of attachments
- Enables transitive connectivity
- Ideal for multi-account architectures
Transit Gateway is commonly used in enterprise-scale AWS environments to replace complex peering meshes.
Hybrid Connectivity: On-Premises to AWS
Many organizations operate in hybrid environments where workloads span both on-premises data centers and AWS.
Site-to-Site VPN
- Encrypted tunnel over the internet
- Quick to set up
- Suitable for low to moderate traffic
Often used as:
- An initial connectivity solution
- A backup for Direct Connect
AWS Direct Connect
- Dedicated private network connection
- Consistent latency and higher bandwidth
- More expensive but highly reliable
Direct Connect is preferred for latency-sensitive or high-throughput workloads.
Private Connectivity with AWS PrivateLink
PrivateLink allows services to be exposed privately between VPCs without full network connectivity.
Key benefits:
- No need for peering or routing changes
- Reduced attack surface
- Ideal for SaaS-style architectures
PrivateLink is increasingly used in service-oriented and multi-tenant designs.
Multi-Account and Landing Zone Architectures
In mature AWS environments:
- Each workload or team often has its own AWS account
- Networking is centralized
- Shared services are isolated
Typical design includes:
- A shared networking account
- Transit Gateway for connectivity
- Centralized inspection and logging
This approach improves security, governance, and scalability.
Choosing the Right Connectivity Model
| Use Case | Recommended Option |
|---|---|
| Internet access from private subnets | NAT Gateway |
| Simple VPC-to-VPC communication | VPC Peering |
| Large-scale multi-VPC networking | Transit Gateway |
| On-premises integration | VPN / Direct Connect |
| Private service exposure | PrivateLink |
Architectural simplicity should always be prioritized over premature complexity.
Operational Considerations
- Monitor NAT Gateway costs and traffic
- Plan CIDR ranges with future growth in mind
- Avoid overlapping networks
- Centralize logging and monitoring
- Document network architecture clearly
Networking issues are often difficult to debug without proper visibility.
Conclusion
Amazon VPC provides a powerful and flexible networking foundation, but its true strength lies in how well it connects systems together. Whether enabling secure internet access, linking multiple VPCs, or extending networks to on-premises environments, choosing the right connectivity option is crucial for long-term success.
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