Interpret VPC configurations (for example, subnets, route tables, network ACLs, security groups).

VPC, or Virtual Private Cloud, is a cornerstone of AWS infrastructure that provides a logically isolated section of the AWS cloud where you can launch AWS resources in a virtual network that you define. Understanding VPC configurations is key to managing AWS environments effectively and is essential knowledge for the AWS Certified SysOps Administrator – Associate exam.

Subnets

A subnet is a range of IP addresses in your VPC. Subnets allow you to partition your VPC’s IP address range into separate segments that can be used to optimize resource allocation, improve security, and ensure that network traffic flows efficiently.

Each VPC is subdivided into subnets that are located within one Availability Zone. For instance, if your VPC has an IP address range of 10.0.0.0/16, you could create a subnet with a range of 10.0.1.0/24.

Public vs Private Subnets

• Public Subnet: It uses a route table that directs traffic to the VPC’s internet gateway. Instances in a public subnet can be directly accessed from the internet if they have a public IP or Elastic IP address.
• Private Subnet: It uses a route table that doesn’t have a route to the internet gateway. Instances here cannot directly access the internet; they might access the internet via a NAT gateway or NAT instance located in a public subnet.

Route Tables

Route tables contain rules, called routes, that determine where network traffic from your subnet or gateway is directed. Each VPC has a main route table by default, and you can create additional custom route tables. Each subnet must be associated with a route table, which controls the routing for the subnet.

Example of Route Table Entries:

In the table above, the first route sends all traffic destined for the local VPC to the “local” target, ensuring that instances within the same VPC can communicate with each other. The second route directs all other traffic (0.0.0.0/0) to the internet gateway (igw-abc123), allowing instances with public IPs to access the internet.

Network ACLs

A Network Access Control List (ACL) acts as a firewall for controlling traffic at the subnet level. By default, each VPC comes with a default network ACL that allows all inbound and outbound traffic. You can create custom Network ACLs with specific inbound and outbound rules.

Example Network ACL Rules:

Network ACLs are stateless; responses to allowed inbound traffic are subject to the rules for outbound traffic (and vice versa).

Security Groups

Security groups act as a virtual firewall for your instance to control inbound and outbound traffic. When you launch an instance, you associate it with one or more security groups. You can add rules to each security group that allow traffic to or from its associated instances.

Unlike Network ACLs, security groups are stateful – if you send a request from your instance, the response traffic for that request is allowed to flow in regardless of inbound security group rules.

Example Security Group Rules:

Comparing Network ACLs and Security Groups

Here’s a comparative look at some key differences:

To effectively interpret VPC configurations, it is critical to understand the use-cases and implications of subnets, route tables, network ACLs, and security groups, and how they interact to facilitate secure and efficient network architecture within AWS. Practitioners should ensure they apply the principle of least privilege, meaning every resource should only have the access that it absolutely needs to function.