Day 11 - AWS VPC

When you start learning AWS, services like EC2, S3, Lambda, and RDS feel exciting.

But behind almost every real AWS architecture, there is one silent foundation:

Amazon VPC — Virtual Private Cloud

If EC2 is your server, RDS is your database, and ALB is your traffic manager, then VPC is the private network where all these resources live.

In this blog, we will understand:

• What is VPC


• CIDR and subnetting basics


• Public and private subnets


• Route tables


• Internet Gateway


• Security Groups


• VPC Peering


• Step-by-step VPC Peering example

🔗 Resources

• ** Support the Journey on GitHub:
  
  If you're following along, consider starring and forking the repo:**
  
  




• CIDR Calculater:
  
  

  
  
  
  
  
  
  
  

  
  
  
  Internet Gateway
  

  
  
  

  An Internet Gateway allows communication between your VPC and the internet.

  
  
  

  For a subnet to become public, two things are required:
  
  

  
  
  
  
  

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  1. Internet Gateway attached to VPC
  
  2. Route table route:
  
     0.0.0.0/0 → Internet Gateway
  
  

  
  
  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Security Groups
  

  
  
  

  A Security Group acts like a virtual firewall for AWS resources.

  
  
  

  AWS explains that a security group controls traffic allowed to reach an instance, and only traffic allowed by security group rules can reach that resource.

  
  
  

  Security Groups are attached to resources like:

  
  
  
  
  
  • EC2
  
  
  • RDS
  
  
  • Load Balancer
  
  
  • Lambda inside VPC
  
  
  
  
  

  Example Security Group rule:
  
  

  
  
  
  
  

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  Inbound Rules:
  
  
  
  Type        Port     Source
  
  SSH         22       Your IP
  
  HTTP        80       0.0.0.0/0
  
  HTTPS       443      0.0.0.0/0
  
  

  
  
  
  
  
  
  
  

  Important point:

  
  
  

  Security Groups are stateful.

  
  
  

  That means if inbound traffic is allowed, response traffic is automatically allowed.

  
  
  
  
  
  

  
  
  
  VPC Peering
  

  
  
  

  VPC Peering allows two VPCs to communicate privately using private IP addresses.

  
  
  

  Example:
  
  

  
  
  
  
  

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  VPC-A: 10.0.0.0/16
  
  VPC-B: 192.168.0.0/16
  
  

  
  
  
  
  
  
  
  

  After VPC Peering:
  
  

  
  
  
  
  

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  EC2 in VPC-A can communicate with EC2 in VPC-B privately.
  
  

  
  
  
  
  
  
  
  

  Use cases:

  
  
  
  
  
  • Connect two application VPCs
  
  
  • Connect shared services VPC with app VPC
  
  
  • Connect dev VPC with monitoring VPC
  
  
  • Connect VPCs across accounts
  
  
  • Connect VPCs across regions
  
  
  
  
  

  Important: VPC Peering does not support overlapping CIDR blocks. AWS states that you cannot create a VPC peering connection if the VPCs have matching or overlapping IPv4 or IPv6 CIDR blocks.

  
  
  

  

  
  
  
  
  
  

  
  
  
  Step-by-Step Example: Create VPC Peering
  

  
  
  

  Let’s say we have two VPCs:
  
  

  
  
  
  
  

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  VPC-A: 10.0.0.0/16
  
  VPC-B: 192.168.0.0/16
  
  

  
  
  
  
  
  
  
  

  Goal:
  
  

  
  
  
  
  

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  EC2 instance in VPC-A should communicate with EC2 instance in VPC-B.
  
  

  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Step 1: Create VPC-A
  

  
  
  

  Go to:
  
  

  
  
  
  
  

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  AWS Console → VPC → Create VPC
  
  

  
  
  
  
  
  
  
  

  Create:
  
  

  
  
  
  
  

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  Name: VPC-A
  
  CIDR: 10.0.0.0/16
  
  

  
  
  
  
  
  
  
  

  Create subnet:
  
  

  
  
  
  
  

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  Name: VPC-A-Private-Subnet
  
  CIDR: 10.0.1.0/24
  
  

  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Step 2: Create VPC-B
  

  
  
  

  Create second VPC:
  
  

  
  
  
  
  

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  Name: VPC-B
  
  CIDR: 192.168.0.0/16
  
  

  
  
  
  
  
  
  
  

  Create subnet:
  
  

  
  
  
  
  

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  Name: VPC-B-Private-Subnet
  
  CIDR: 192.168.1.0/24
  
  

  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Step 3: Launch EC2 Instances
  

  
  
  

  Launch one EC2 instance in each VPC.
  
  

  
  
  
  
  

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  EC2-A → VPC-A → 10.0.1.0/24 subnet
  
  EC2-B → VPC-B → 192.168.1.0/24 subnet
  
  

  
  
  
  
  
  
  
  

  Make sure both instances have private IPs.

  
  
  

  Example:
  
  

  
  
  
  
  

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  EC2-A Private IP: 10.0.1.10
  
  EC2-B Private IP: 192.168.1.10
  
  

  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Step 4: Create VPC Peering Connection
  

  
  
  

  Go to:
  
  

  
  
  
  
  

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  VPC Console → Peering Connections → Create Peering Connection
  
  

  
  
  
  
  
  
  
  

  Fill details:
  
  

  
  
  
  
  

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  Name: VPC-A-to-VPC-B
  
  Requester VPC: VPC-A
  
  Accepter VPC: VPC-B
  
  

  
  
  
  
  
  
  
  

  Click:
  
  

  
  
  
  
  

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  Create Peering Connection
  
  

  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Step 5: Accept Peering Request
  

  
  
  

  Go to:
  
  

  
  
  
  
  

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  VPC → Peering Connections
  
  

  
  
  
  
  
  
  
  

  Select the request.

  
  
  

  Click:
  
  

  
  
  
  
  

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  Actions → Accept Request
  
  

  
  
  
  
  
  
  
  

  Now the peering connection status should become:
  
  

  
  
  
  
  

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  Active
  
  

  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Step 6: Update Route Table of VPC-A
  

  
  
  

  Go to VPC-A route table.

  
  
  

  Add route:
  
  

  
  
  
  
  

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  Destination: 192.168.0.0/16
  
  Target: VPC Peering Connection
  
  

  
  
  
  
  
  
  
  

  AWS requires route tables on both sides to be updated so private IPv4 traffic can flow between peered VPCs. The destination should be the peer VPC CIDR and the target should be the VPC peering connection. ([AWS Documentation][6])

  
  
  
  
  
  

  
  
  
  Step 7: Update Route Table of VPC-B
  

  
  
  

  Go to VPC-B route table.

  
  
  

  Add route:
  
  

  
  
  
  
  

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  Destination: 10.0.0.0/16
  
  Target: VPC Peering Connection
  
  

  
  
  
  
  
  
  
  

  Now both VPCs know how to reach each other.

  
  
  
  
  
  

  
  
  
  Step 8: Update Security Groups
  

  
  
  

  For EC2-A security group, allow traffic from VPC-B:
  
  

  
  
  
  
  

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  Type: ICMP / SSH / Custom TCP
  
  Source: 192.168.0.0/16
  
  

  
  
  
  
  
  
  
  

  For EC2-B security group, allow traffic from VPC-A:
  
  

  
  
  
  
  

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  Type: ICMP / SSH / Custom TCP
  
  Source: 10.0.0.0/16
  
  

  
  
  
  
  
  
  
  

  For testing ping:
  
  

  
  
  
  
  

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  Allow ICMP
  
  

  
  
  
  
  
  
  
  

  For testing SSH:
  
  

  
  
  
  
  

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  Allow TCP 22
  
  

  
  
  
  
  
  
  
  
  
  
  

  
  
  
  Step 9: Test Connectivity
  

  
  
  

  Login to EC2-A and ping EC2-B private IP:
  
  

  
  
  
  
  

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  ping 192.168.1.10
  
  

  
  
  
  
  
  
  
  

  Or test SSH:
  
  

  
  
  
  
  

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  ssh [email protected]
  
  

  
  
  
  
  
  
  
  

  If route tables and security groups are correct, communication should work privately.

  
  
  
  
  
  

  
  
  
  Final Thoughts
  

  
  
  

  AWS VPC is one of the most important concepts in cloud networking.

  
  
  

  If you understand VPC properly, then services like EC2, Load Balancer, RDS, EKS, Lambda networking, VPN, Direct Connect, and Transit Gateway become much easier.

  
  
  

  At a high level, remember this:
  
  

  
  
  
  
  

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  VPC = Your private network in AWS
  
  Subnet = Smaller network inside VPC
  
  Route Table = Traffic direction rules
  
  Internet Gateway = Internet access
  
  Security Group = Firewall for resources
  
  VPC Peering = Private connection between two VPCs
  
  

  
  
  
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