What is a Virtual Private Cloud, and how do VPC networking concepts map to traditional on-premises network constructs? Explain subnets, route tables, security groups, and NACLs.

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A Virtual Private Cloud is a logically isolated network within a cloud provider's infrastructure that gives you control over IP addressing, subnetting, routing, and security, much like a private data center network but provisioned through software. Understanding how VPC constructs map to traditional networking concepts helps network engineers design effective cloud architectures.

A VPC is analogous to a network segment or a data center network. When you create a VPC, you assign it a CIDR block, such as 10.100.0.0/16, which defines the IP address space available within that VPC. This is similar to allocating a supernet for a new data center. All resources deployed within the VPC receive IP addresses from this range. You can think of the VPC as the boundary of your network domain within the cloud.

Subnets within a VPC map to VLANs and subnets in traditional networking. Each subnet occupies a subset of the VPC CIDR, such as 10.100.1.0/24, and is associated with a specific availability zone in the cloud region. Public subnets have a route to an internet gateway, making them analogous to DMZ segments. Private subnets have no direct internet route, similar to internal server VLANs. The separation into public and private subnets is the primary segmentation mechanism within a VPC.

Route tables in a VPC function like routing tables on traditional routers. Each subnet is associated with a route table that determines where traffic is directed. The default route table handles traffic within the VPC, with a local route for the VPC CIDR block. Additional routes can direct traffic to internet gateways, NAT gateways, VPN gateways, VPC peering connections, or transit gateways. Unlike traditional networks where a physical router sits between subnets, the VPC router is implicit and handled by the cloud infrastructure. You configure routes but do not manage the router itself.

Security groups are stateful firewalls applied to individual instances or network interfaces. They are analogous to host-based firewalls but managed at the cloud infrastructure level. Security groups operate on an allow-only model: you define rules that permit specific inbound and outbound traffic, and everything else is denied by default. Because they are stateful, allowing inbound traffic on a port automatically allows the return traffic without an explicit outbound rule. Security groups are attached to instances, not subnets, so different instances in the same subnet can have different security rules. This provides the granularity of microsegmentation without additional infrastructure.

Network Access Control Lists, or NACLs, are stateless firewalls applied at the subnet level. They are analogous to router ACLs that filter traffic entering and leaving a network segment. NACLs process rules in numbered order and include both allow and deny rules. Because they are stateless, you must explicitly configure both inbound and outbound rules including ephemeral port ranges for return traffic. NACLs provide a secondary layer of defense that applies to all instances in a subnet regardless of their security group configurations.

The typical cloud security model uses both constructs in layers. NACLs provide broad subnet-level controls, such as blocking all traffic from known malicious IP ranges. Security groups provide fine-grained instance-level controls, such as allowing only the web servers to receive traffic on port 443 and only from the load balancer. This layered approach mirrors the defense-in-depth principles used in traditional network design.