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Cloud (AWS) Level 1, Task 4: Creating a VPC Subnet

Today's task was the logical next step in building my AWS network foundation. After learning what a VPC is, my objective was to create a Subnet inside that VPC. This is like taking the plot of land I've fenced off (the VPC) and dividing it into smaller, usable sections (the subnets) where I can actually build my resources.

This was a great exercise because it required me to first find the default VPC and then create a new subnet within its IP address range. I've documented both the fast, scriptable AWS CLI method and the visual AWS Console (UI) method, along with a detailed explanation of the concepts I learned. This is my first-person guide to that process.

Table of Contents

The Task

My objective was to create a new AWS EC2 Subnet. The requirements were: 1. The subnet's name tag had to be datacenter-subnet. 2. It had to be created inside the default VPC. 3. The operation had to be performed in the us-east-1 region.

Solution 1: The AWS CLI Method (Automation)

This method is fast and scriptable. It's a two-step process: first, find the ID of the default VPC, and second, create the subnet within it.

Phase 1: Find the Default VPC ID

  1. I logged into the aws-client host.
  2. I used the aws ec2 describe-vpcs command with a filter to find the default VPC. This command returns a lot of data, so I used the --query flag to extract only the VPC ID. 
    aws ec2 describe-vpcs --filters "Name=isDefault,Values=true" --query "Vpcs[0].VpcId" --output text
    This command returned the VPC ID, which I copied (e.g., vpc-12345678).

Phase 2: Create the Subnet

Now that I had the VpcId, I could create the subnet. I also had to choose a valid CIDR block for my subnet that was within the VPC's main range (e.g., 172.31.0.0/16). A good, standard choice for a new subnet is 172.31.80.0/24. 

aws ec2 create-subnet \
    --vpc-id vpc-12345678 \
    --cidr-block 172.31.80.0/24 \
    --availability-zone us-east-1a \
    --tag-specifications 'ResourceType=subnet,Tags=[{Key=Name,Value=datacenter-subnet}]' \
    --region us-east-1
(Note: I had to specify an Availability Zone, as a subnet exists in only one AZ.)

Phase 3: Verification

The command itself outputs a JSON block describing the newly created subnet. I could also have run aws ec2 describe-subnets --filters "Name=tag:Name,Values=datacenter-subnet" to find it.

Solution 2: The AWS Console Method (UI)

This method uses the graphical web interface and is very intuitive.

  1. Login to AWS Console: I used the provided URL, username (kk_labs_user_904617), and password to log in.
  2. Navigate to VPC: In the main console, I ensured my region was set to us-east-1 (North Virginia). I then used the search bar to find and navigate to the VPC service.
  3. Navigate to Subnets: In the VPC dashboard's left-hand navigation pane, I clicked on Subnets.
  4. Create Subnet: I clicked the "Create subnet" button in the top right.
  5. Fill in Details:
    • VPC ID: I clicked the dropdown and selected the Default VPC. The console helpfully showed its name and CIDR block (e.g., 172.31.0.0/16).
    • Subnet name: I entered datacenter-subnet.
    • Availability Zone: I selected an AZ from the dropdown, like us-east-1a.
    • IPv4 CIDR block: I entered a valid, non-overlapping subnet range, such as 172.31.80.0/24.
  6. Create: I scrolled down and clicked the final "Create subnet" button. The subnet appeared in my dashboard.

Why Did I Do This? (The "What & Why")

- VPC (Virtual Private Cloud): This is my private, isolated network in the AWS cloud. It's the "plot of land." - Subnet (Subnetwork): This is the most important concept of this task. A VPC is a large IP address range, but it's not usable on its own. It must be divided into smaller sections called subnets. A subnet is a "room" or "fenced-in area" within my VPC where I can actually place my resources, like EC2 instances. - Why Subdivide? 1. Availability: A subnet exists in a single Availability Zone (AZ), which is a physically separate datacenter. To build a highly-available application, I would create multiple subnets in different AZs (e.g., subnet-a in us-east-1a and subnet-b in us-east-1b) and place my servers in both. 2. Security: This is the most common reason. I can create different subnets for different security purposes. I can have public subnets for my web servers that are open to the internet, and private subnets for my databases that are completely locked down. - Default VPC: Every AWS account comes with a "default VPC" in each region. Its purpose is to make it easy for beginners to launch resources without having to manually configure a network from scratch. It's pre-configured with public subnets and an internet gateway.

Deep Dive: Public vs. Private Subnets

This task had me create a subnet, but what makes a subnet "public" or "private"? I learned it's not a setting on the subnet itself. - A subnet is Public if its associated Route Table has a route to an Internet Gateway (0.0.0.0/0 -> igw-...). - A subnet is Private if its Route Table does not have a route to an Internet Gateway.

By default, the default VPC has a main route table that sends traffic to an Internet Gateway, so any new subnet I create in it (like datacenter-subnet) is automatically a public subnet.

Common Pitfalls

- Forgetting the VPC ID: The vpc-id is a required parameter for the CLI command. I had to look it up first. - Invalid CIDR Block: The most common error is choosing a CIDR block that is not a valid subset of the VPC's main CIDR. For example, if the VPC is 172.31.0.0/16, my subnet CIDR must start with 172.31.x.x. - Overlapping CIDR Blocks: If a subnet with the range 172.31.80.0/24 already existed, my command to create another one with the same range would fail. Each subnet in a VPC must have a unique IP range.

Exploring the Commands and UI I Used

AWS CLI Method

  • aws ec2 describe-vpcs --filters "Name=isDefault,Values=true": The command I used to find the ID of the default VPC.
  • aws ec2 create-subnet --vpc-id ... --cidr-block ... --availability-zone ...: The primary command for this task. It provisions a new network segment within a VPC.
  • aws ec2 describe-subnets: A useful verification command to list all subnets and check their state.

AWS Console (UI) Method

  • VPC Dashboard > Subnets: The main navigation path to the subnet management page.
  • "Create subnet" button: The primary UI element that launches the creation wizard, where I filled in the form with all the required specifications.