Every AWS resource that communicates over a network does so within a Virtual Private Cloud (VPC). While services like EC2, RDS, and Load Balancers receive most of the attention, the VPC quietly defines how these services connect, isolate, and secure traffic.
This blog introduces Amazon VPC from first principles, explaining what it is, why it exists, and how it forms the foundation of all networking in AWS. This sets the stage for deeper exploration in the following parts of this series.
Why AWS Needs Virtual Private Clouds
Before cloud networking, applications were deployed in physical data centers where networks were tightly controlled through routers, firewalls, and VLANs. Moving workloads to a shared cloud infrastructure required a way to preserve isolation while allowing flexibility.
Amazon VPC solves this problem by giving each customer a logically isolated network within AWS. It allows organizations to define their own IP address space, routing behavior, and security controls while benefiting from AWS’s global infrastructure.
What Is an Amazon VPC?
An Amazon VPC is a logically isolated virtual network where you launch AWS resources. It closely resembles a traditional on-premises network but operates entirely within AWS.
Within a VPC, you control:
- IP address ranges (CIDR blocks)
- Subnet segmentation
- Routing between components
- Network-level security
- Connectivity to the internet and on-premises environments
Every AWS account has a default VPC, but most production systems rely on custom VPC designs.
CIDR Blocks and IP Addressing
When creating a VPC, you define a CIDR block that represents the IP address range for the network.
Key considerations:
- VPC CIDR ranges cannot be changed after creation
- CIDR ranges must not overlap with connected networks
- Proper sizing prevents future expansion issues
A well-planned CIDR strategy is critical, especially when hybrid connectivity or multi-VPC architectures are involved.
Subnets: Dividing the Network
Subnets divide a VPC into smaller segments and are always tied to a single Availability Zone.
Key properties:
- Each subnet belongs to exactly one AZ
- Subnets enable fault isolation
- Resources are launched into subnets, not directly into VPCs
Subnets are commonly categorized as public or private based on routing behavior, not inherent properties.
Public vs Private Subnets
A subnet is considered public or private depending on its route table configuration.
Public subnet
- Has a route to an Internet Gateway
- Can host internet-facing resources such as load balancers
Private subnet
- No direct internet route
- Used for internal services such as application servers and databases
Most secure architectures expose only what is necessary and keep core workloads in private subnets.
Route Tables: Traffic Control
Route tables determine how traffic flows within and outside the VPC.
Each subnet is associated with a route table that defines:
- Local traffic routing
- Internet-bound traffic
- Connectivity to other networks
Routing rules are evaluated in a deterministic manner, making network behavior predictable and auditable.
Internet Gateways and External Access
An Internet Gateway enables communication between a VPC and the public internet.
Key points:
- Only one Internet Gateway can be attached to a VPC
- It supports both inbound and outbound traffic
- Public subnets must route traffic to the IGW explicitly
Without an Internet Gateway, VPC resources remain fully isolated from the internet.
The Default VPC: Convenience vs Control
AWS provides a default VPC in every region, preconfigured with public subnets and internet access. While useful for experimentation, default VPCs are rarely used in production.
Reasons include:
- Limited control over IP addressing
- Less predictable security posture
- Poor alignment with enterprise standards
Custom VPCs offer far greater flexibility and governance.
Shared Responsibility in VPC Networking
AWS manages the underlying physical network infrastructure. Customers are responsible for configuring:
- Subnets and routing
- Security groups and network ACLs
- Connectivity options
- Traffic exposure
Misconfiguration, rather than infrastructure failure, is the most common cause of network-related incidents.
Conclusion
Amazon VPC defines the network boundary for every AWS workload. Understanding how VPCs, subnets, routing, and gateways work is essential for designing secure, scalable architectures. While VPC concepts may seem abstract initially, they directly influence availability, security, and performance.
In Part 2, we will explore VPC security controls, including security groups, network ACLs, and traffic flow patterns, with practical guidance for production environments.
Top comments (0)