Exploring how traffic moves inside a VPC can make AWS networking feel much more approachable. Route tables and Internet Gateways (IGWs) quietly influence how your subnets function across your network. In this article, we’ll walk through them step by step so the concepts stay clear and grounded.
The Role of Route Tables in a VPC
Route tables are essentially instruction sets that guide traffic within your VPC. Each subnet connects to exactly one route table, which determines what that subnet can reach. The default local route (for example, 10.0.0.0/16 local) lets all subnets talk to each other without extra configuration. Anything beyond that depends on the routes you add.
Think of routes like road signs: “If traffic wants to go to X, send it to Y.”
For instance, a 0.0.0.0/0 route pointing to an IGW makes a subnet public. Pointing the same destination to a NAT Gateway makes it private with controlled outbound access. The logic is simple, but the design impact is significant.
Route tables also become more interesting in multi-VPC environments. You might add a route to another VPC via peering or a route to an on-premises network through a VPN. Without clear routes, traffic may fail to reach its destination or take inefficient paths.
Public Subnets and Internet Gateways
An Internet Gateway provides a path between your VPC and the internet. It doesn’t automatically expose your resources; the route table determines traffic flow, and instances need public IPs or Elastic IPs. Without those, the IGW sits idle.
Only subnets explicitly configured with routes to the IGW and instances with public IPs become public. Everything else remains internal by default.
Example: Your web server subnet is public, allowing external traffic. Meanwhile, your backend API or database subnet remains private and unreachable from the internet.
This separation ensures intentional design and helps prevent accidental exposure of internal systems. It also means you can confidently design hybrid architectures without worrying about default internet access.
Private Subnets and Controlled Outbound Traffic
Private subnets typically route outbound traffic through a NAT Gateway or sometimes VPC endpoints. Their route tables may have a 0.0.0.0/0 → NAT route, allowing them to reach the internet without being reachable from outside.
Subnets can also remain fully isolated. If the route table only contains the local VPC route, they cannot leave the VPC. This is common for internal services, like database layers or batch processing workloads, where internet access isn’t needed.
You can even mix private subnets: some with NAT access for updates or API calls, and some fully isolated for sensitive workloads. This flexibility is one of the reasons AWS networking scales well for different application needs.
How Traffic Flows End-to-End
Understanding route tables in isolation is useful, but seeing the end-to-end traffic flow makes networking clearer. For example:
- A client requests data from a public web server in your VPC.
- The server’s subnet route table directs traffic through the IGW.
- Responses travel back via the IGW to the client.
For a private subnet:
- An internal application needs to call an external API.
- Traffic goes to the NAT Gateway.
- The NAT translates the private IP and sends the request out.
- Responses return via the NAT, preserving internal IP addresses.
Visualizing these flows—either in a diagram or with mental models—makes it easier to predict how changes in routes affect the network.
Route Table Patterns That Keep Things Predictable
As VPCs grow, predictable routing becomes essential. Some patterns to follow:
- Separate route tables for public and private subnets.
- Avoid mixing IGW and NAT routes unnecessarily.
- Keep routes minimal and descriptive.
- Consider how peering, Transit Gateway, or multi-account architectures will interact with your tables.
Another useful approach is documenting all route table associations in a simple table or spreadsheet. When you add subnets or connect new accounts, this reference helps avoid misconfigurations.
These habits reduce mistakes and make debugging easier as the network grows.
How IGWs Fit Into Larger Architectures
In hybrid setups, shared services, or multi-account designs, IGWs maintain their role as the gateway to the internet. The route table determines which traffic exits through the IGW versus staying internal through Direct Connect or VPNs. AWS doesn’t forward traffic between these paths automatically, so each destination needs a clear route.
Large architectures benefit from keeping IGW routes simple. Complex configurations can introduce unexpected routing loops or unintended internet exposure. Even small changes—like adding an IGW route to a newly created subnet—can have cascading effects if not planned.
Monitoring and Troubleshooting Route Tables and IGWs
Managing route tables and Internet Gateways is not just about creating the correct routes, it is also about making sure everything works as intended. VPC Flow Logs are a useful tool to monitor traffic within the VPC and verify whether packets reach their destinations or are blocked along the way.
It is important to regularly check route table associations. Make sure each subnet is connected to the correct route table and that routes to the IGW or NAT Gateway are properly configured. Even small mistakes, such as a missing default route or an unassociated subnet, can cause instances to lose internet connectivity.
Practical tests using ping or curl from instances in public and private subnets help confirm that traffic flows as expected. Keeping documentation of route tables, routes, and subnet associations in a spreadsheet or diagram helps teams understand the network, reduce errors when adding new subnets, and identify problems more quickly. These practices make VPC management more predictable and easier to scale as the infrastructure grows.
Operational Considerations
Routing decisions impact both operations and cost:
- NAT Gateways incur charges, so overuse in private subnets can increase costs.
- Public subnets reduce NAT reliance but require careful security configurations.
- Troubleshooting connectivity issues often begins by checking route tables and subnet associations.
Clear labeling of route tables and consistent patterns make ongoing management simpler. Over time, this makes scaling, auditing, and monitoring much more manageable.
Conclusion
Route tables and Internet Gateways define how your VPC connects to the outside world and how your subnets behave internally. Once you understand how a single default route can shift a subnet’s role, AWS networking starts to feel much more manageable. And once that idea becomes clear, it’s easier to reason about how everything in your VPC fits together.
Top comments (2)
Really clear explanation of how route tables and IGWs work together—especially the emphasis on default routes and subnet behavior. I'd love to see a follow-up on designing VPC architectures with Transit Gateway and VPC peering, including common routing pitfalls and how to avoid asymmetric routing.
Thanks for the feedback! Glad the explanations were helpful. Im planning to explore some of the broader routing patterns and challenges in VPC architectures in a follow-up.