Amazon Web Services provides one of the most widely used cloud computing platforms in the world, and at the heart of how resources are organized and secured within that platform lies the concept of networking. When businesses and developers move their applications and infrastructure to AWS, they need a way to control how those resources communicate with the internet, with each other, and with external systems. Subnets are a fundamental building block of that control, and the distinction between public and private subnets is one of the most important concepts anyone working with AWS networking needs to grasp from the very beginning.

A subnet, short for subnetwork, is essentially a segment of a larger network that has been divided to allow for more granular control over traffic flow and resource placement. In AWS, subnets exist within a structure called a Virtual Private Cloud, or VPC, which is a logically isolated section of the AWS cloud where you can launch resources in a network that you define. The way you design your subnets within a VPC has direct implications for the security, performance, and accessibility of everything you build on top of that infrastructure.

What a Virtual Private Cloud Actually Represents

Before diving into the specifics of subnets, it helps to have a clear picture of what a VPC is and why it exists. When you use AWS, you are sharing physical infrastructure with many other customers, but the VPC creates a boundary around your resources that makes them behave as though they exist on a private, dedicated network. You control the IP address range of your VPC, you decide how it is divided into subnets, and you determine what traffic is allowed to flow in and out of it.

A VPC is defined by a range of IP addresses specified using a format called CIDR notation, which stands for Classless Inter-Domain Routing. This range determines the total pool of IP addresses available to all the resources you place within that VPC. When you create subnets, you are carving out smaller portions of that total IP address range and assigning them to specific segments of your network. Each subnet occupies a distinct slice of the VPC’s address space, and resources placed in a subnet receive IP addresses from that subnet’s range.

The Core Difference That Separates Public from Private

The distinction between a public subnet and a private subnet comes down to one essential question: can resources placed in that subnet communicate directly with the internet? A public subnet is one where resources can send and receive traffic directly to and from the internet, assuming they have been assigned a public IP address and the appropriate permissions are in place. A private subnet is one where resources have no direct path to the internet, making them inaccessible from outside the network unless traffic is routed through an intermediary.

This difference is not inherent to the subnet itself based on its name but is determined by its configuration. Specifically, whether a subnet is public or private depends on whether its route table includes a route that directs traffic to an internet gateway. An internet gateway is an AWS component that serves as the connection point between a VPC and the public internet. Subnets whose route tables point to an internet gateway are effectively public, while those whose route tables contain no such route remain private. This configuration-driven distinction is important to understand because it means the classification of a subnet can change if its routing configuration changes.

Internet Gateways and Their Function in Public Subnets

An internet gateway is one of the most important components in any AWS network architecture that requires internet connectivity. It is a horizontally scaled, redundant, and highly available VPC component that allows communication between instances in a VPC and the internet. When you attach an internet gateway to a VPC and add a route in a subnet’s route table that directs traffic destined for external IP addresses to that gateway, the subnet becomes capable of supporting internet-facing resources.

For a resource in a public subnet to actually communicate with the internet, it needs more than just the route to the internet gateway. It also needs a public IP address, either a static one known as an Elastic IP address or a dynamically assigned public IP. Without a public IP, the resource has no identity on the public internet and cannot participate in inbound or outbound internet communication. The combination of a public IP address, a route to the internet gateway, and appropriate security group rules is what enables a resource in a public subnet to be genuinely accessible from the outside world.

Why Private Subnets Exist and What They Protect

The existence of private subnets reflects a fundamental principle of security architecture: resources should only be exposed to as much of the network as they actually need to function. A database server, for example, does not need to be directly accessible from the internet to do its job. It only needs to receive queries from application servers that are processing user requests. By placing the database in a private subnet, you ensure that even if an attacker were somehow able to reach your application layer, they would face a significant additional barrier before reaching the data layer.

Private subnets are used for anything that should not be directly reachable from the internet. This commonly includes database servers, application servers in multi-tier architectures, internal processing systems, machine learning workloads, and any other components that only need to communicate with other resources within the same VPC. The isolation provided by a private subnet is not absolute, because resources within a private subnet can still communicate with other resources inside the VPC, but it removes the most obvious and direct attack vector that internet exposure would otherwise create.

How Resources in Private Subnets Reach the Internet

A common question when people first learn about private subnets is how resources placed there can reach the internet when they need to, for example to download software updates, connect to external APIs, or send data to third-party services. The answer lies in a component called a NAT gateway, which stands for Network Address Translation gateway. A NAT gateway sits in a public subnet and acts as an intermediary through which private subnet resources can initiate outbound connections to the internet without being directly reachable from it.

When a resource in a private subnet sends traffic destined for the internet, that traffic is routed to the NAT gateway in the public subnet. The NAT gateway then forwards the request to the internet using its own public IP address, receives the response, and sends it back to the originating resource in the private subnet. From the perspective of the external destination, the traffic appears to come from the NAT gateway, not from the private resource. This arrangement allows private resources to communicate outbound with the internet while remaining completely inaccessible to inbound traffic originating from outside the VPC.

Route Tables and Their Central Role in Subnet Behavior

Route tables are the configuration mechanism that ultimately determines how traffic flows within and between subnets in a VPC. Every subnet must be associated with a route table, and the entries in that route table tell the network where to send traffic based on its destination IP address. The route table is what makes a subnet public or private, and it is also what enables communication between different subnets within the same VPC.

A typical route table for a public subnet contains at least two entries. One entry handles traffic destined for IP addresses within the VPC itself, directing that traffic locally without any need to exit the network. A second entry handles all other traffic, directing it to the internet gateway. A route table for a private subnet typically contains a local entry for VPC traffic and a separate entry that directs internet-bound traffic to the NAT gateway rather than directly to an internet gateway. The absence of a direct internet gateway route is what keeps the subnet private and protects the resources it contains from direct internet exposure.

Security Groups as the Second Layer of Protection

While subnets and route tables control the broad flow of traffic through a network, security groups provide a more granular level of control at the individual resource level. A security group functions like a virtual firewall that surrounds each resource, controlling what traffic is allowed to reach it and what traffic it is allowed to send. Even if traffic successfully reaches a subnet, it still must pass through the security group rules associated with the specific resource it is trying to reach.

Security groups are stateful, meaning that if an inbound connection is allowed, the response traffic is automatically permitted without needing a separate outbound rule. This makes them easier to configure than traditional stateless firewalls. For resources in public subnets, security groups are critical for limiting which ports and protocols are accessible from the internet. A web server in a public subnet, for instance, might allow inbound traffic on port 80 for standard web traffic and port 443 for encrypted web traffic while blocking everything else. For resources in private subnets, security groups are still important because they control what traffic is permitted from other resources within the VPC.

Network Access Control Lists as an Additional Boundary

In addition to security groups, AWS provides another layer of network security called Network Access Control Lists, commonly abbreviated as NACLs. While security groups operate at the individual resource level, NACLs operate at the subnet level and control traffic flowing in and out of the subnet as a whole. This makes them a complementary tool that can add an additional security boundary around all resources in a subnet regardless of their individual security group configurations.

Unlike security groups, NACLs are stateless, meaning that both inbound and outbound rules must be explicitly configured to permit the corresponding traffic in each direction. NACLs evaluate rules in numerical order and stop processing as soon as they find a rule that matches the incoming or outgoing traffic. This ordered evaluation means that the sequence of rules matters significantly, and poorly ordered rules can lead to unexpected traffic behavior. Most organizations use NACLs as a coarse-grained control mechanism to block specific IP ranges or protocols at the subnet boundary while relying on security groups for more precise resource-level filtering.

Typical Architecture Patterns Using Both Subnet Types

A common and recommended architecture pattern in AWS involves using both public and private subnets together in a layered design that separates different tiers of an application. The classic three-tier web application architecture places a load balancer in the public subnet to receive incoming traffic from the internet. The application servers that process requests sit in a private subnet, receiving traffic only from the load balancer rather than directly from the internet. The database servers that store persistent data sit in an even more restricted private subnet and communicate only with the application servers.

This layered approach is often called a defense-in-depth architecture because it creates multiple layers of security that an attacker would need to penetrate sequentially to reach the most sensitive resources. Each layer has only as much network exposure as it requires to fulfill its function. The load balancer needs to be internet-facing by definition. The application servers do not. The database servers certainly do not. By matching network exposure to actual functional requirements, this architecture minimizes the attack surface of the overall system without sacrificing the functionality that users expect.

Availability Zones and Subnet Placement Across Regions

AWS divides its global infrastructure into regions and, within each region, into multiple availability zones. An availability zone is essentially a separate physical data center with its own power, cooling, and network infrastructure, designed to be isolated from failures affecting other availability zones in the same region. Subnets in AWS are tied to a specific availability zone, meaning that each subnet exists within one and only one availability zone.

Designing for high availability in AWS requires distributing resources across multiple availability zones, which in turn means creating corresponding subnets in each zone. A well-designed architecture might have a public subnet and a private subnet in each of two or three availability zones within a region. If one availability zone experiences an outage, resources in the other zones continue to function, and the application remains available to users. This multi-zone subnet strategy is one of the most important practices for building systems that can withstand infrastructure failures without becoming unavailable.

Cost Considerations Related to Subnet Architecture Choices

While the security and architectural benefits of using public and private subnets are clear, it is also worth considering the cost implications of different subnet design choices. The internet gateway itself is provided at no additional charge, but data that transfers out of a VPC to the internet is subject to data transfer fees based on volume. NAT gateways, which are essential for allowing private subnet resources to access the internet, carry both an hourly charge for the gateway itself and a per-gigabyte charge for the data processed through them.

These costs can add up significantly in architectures where private subnet resources need to transfer large volumes of data outbound to the internet. Organizations should evaluate whether all outbound traffic from private subnets truly needs to go through a NAT gateway or whether some of it can be routed through AWS-native services that do not incur the same charges. AWS PrivateLink, for example, allows resources to communicate with certain AWS services without sending traffic through the public internet or a NAT gateway, which can reduce both costs and security exposure simultaneously. Cost awareness in subnet architecture is not a secondary concern but an important part of designing systems that are both secure and economically sustainable.

Common Mistakes Beginners Make With Subnet Configuration

People who are new to AWS networking frequently make predictable errors when configuring subnets that can compromise either security or functionality. One of the most common mistakes is placing all resources in public subnets for simplicity, reasoning that security groups will provide sufficient protection. While security groups do add meaningful protection, placing sensitive resources in public subnets exposes them to a broader threat surface than necessary and goes against established best practices for cloud security architecture.

Another frequent error involves misconfiguring route tables so that a subnet intended to be private inadvertently has a route to the internet gateway, effectively making it public without the architect realizing it. This kind of misconfiguration is particularly dangerous because it may not immediately cause any obvious problems but silently exposes resources to internet traffic they were never intended to receive. Careful documentation of subnet configurations, regular audits of route tables, and the use of infrastructure-as-code tools that make configurations explicit and reviewable are all effective ways to avoid these and other common networking mistakes in AWS.

Conclusion

Public and private subnets in AWS are not simply technical details for networking specialists to worry about. They are foundational architectural decisions that shape the security, resilience, and cost profile of everything built within a VPC. Every resource placed in a subnet inherits the network characteristics of that subnet, which means that subnet design choices ripple outward to affect the entire system in ways that are often difficult to undo once an architecture is in place.

The core principle to carry away from any study of this topic is that network exposure should always match functional need. Resources that must be reachable from the internet belong in public subnets. Resources that only need to communicate with other internal components belong in private subnets. This simple rule, consistently applied, eliminates the most common and most damaging category of network security mistakes in cloud environments.

The tools that AWS provides to implement this principle, including internet gateways, NAT gateways, route tables, security groups, and network access control lists, work together to give architects fine-grained control over how traffic flows through their infrastructure. Each of these tools addresses a different layer of the networking stack, and understanding how they interact is what allows architects to build systems that are both functionally capable and genuinely secure.

As cloud infrastructure continues to grow in complexity and as the threat landscape facing internet-connected systems becomes more sophisticated, the importance of sound subnet design only increases. Organizations that invest in understanding these foundational concepts early in their cloud journey find themselves better positioned to build systems that can scale, adapt, and withstand the pressures of real-world deployment. For anyone working with AWS in any capacity, public and private subnets are not advanced topics to be deferred until later but essential knowledge that belongs at the very beginning of any serious engagement with cloud networking. Building that understanding now, rather than patching gaps after problems arise, is the approach that consistently produces the most secure and the most maintainable cloud architectures over time.