docs: add 100 Days of DevOps challenge notes

100 - days of devops/devops-94.md

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+# Assignment
+
+The Nautilus DevOps team is strategizing the migration of a portion of their infrastructure to the AWS cloud. Recognizing the scale of this undertaking, they have opted to approach the migration in incremental steps rather than as a single massive transition. To achieve this, they have segmented large tasks into smaller, more manageable units. This granular approach enables the team to execute the migration in gradual phases, ensuring smoother implementation and minimizing disruption to ongoing operations. By breaking down the migration into smaller tasks, the Nautilus DevOps team can systematically progress through each stage, allowing for better control, risk mitigation, and optimization of resources throughout the migration process.
+
+Create a VPC named datacenter-vpc in region us-east-1 with any IPv4 CIDR block through terraform.
+
+The Terraform working directory is /home/bob/terraform. Create the main.tf file (do not create a different .tf file) to accomplish this task.
+
+Note: Right-click under the EXPLORER section in VS Code and select Open in Integrated Terminal to launch the terminal.
+
+# Solution
+
+# Terraform VPC — `datacenter-vpc`
+
+Create a VPC in `us-east-1` with any IPv4 CIDR block.
+
+## Create `main.tf` (heredoc → file)
+
+```bash
+cd /home/bob/terraform
+
+cat > main.tf <<'EOF'
+terraform {
+  required_providers {
+    aws = {
+      source  = "hashicorp/aws"
+      version = "~> 6.0"
+    }
+  }
+}
+
+provider "aws" {
+  region = "us-east-1"
+}
+
+resource "aws_vpc" "datacenter_vpc" {
+  cidr_block = "10.0.0.0/16"
+
+  tags = {
+    Name = "datacenter-vpc"
+  }
+}
+EOF
+```
+
+## How to run
+
+```bash
+cd /home/bob/terraform
+terraform init
+terraform apply -auto-approve
+```
+
+## How it works
+
+### The heredoc
+
+`cat > main.tf <<'EOF'` writes the file in one shot. The **quoted** `'EOF'` disables shell expansion,
+so any `$` or backticks in HCL reach the file literally rather than being interpreted by bash — the
+right default whenever writing config files this way.
+
+Note the task says **`main.tf` only** — don't split the provider block into a separate
+`provider.tf` or `versions.tf`.
+
+### The provider block
+
+- **`region = "us-east-1"`** — pins the deployment region as required. A VPC is a regional resource,
+  so this determines where it's created.
+- **`required_providers`** with a version constraint keeps `terraform init` from pulling an
+  unexpected major version.
+
+### The `aws_vpc` resource
+
+One resource does the whole job:
+
+- **`cidr_block = "10.0.0.0/16"`** — the task allows *any* IPv4 block, so this uses a standard
+  RFC 1918 private range. A `/16` gives 65,536 addresses, plenty of room to carve subnets from
+  later. Any valid private CIDR (`172.16.0.0/16`, `192.168.0.0/24`, …) would satisfy the requirement
+  equally; `10.0.0.0/16` is the conventional default.
+
+- **`tags = { Name = "datacenter-vpc" }`** — this is what "named" means for a VPC. AWS VPCs have **no
+  native name field**; the console and any grader read the `Name` tag. Omit the tag and the VPC still
+  exists but shows as unnamed — the most common way to fail this task.
+
+The Terraform resource label (`datacenter_vpc`) is just the internal identifier used for references
+within the config; it's unrelated to the AWS-visible name, which comes from the tag.
+
+## Verify
+
+```bash
+aws ec2 describe-vpcs \
+  --filters Name=tag:Name,Values=datacenter-vpc \
+  --query 'Vpcs[0].{Id:VpcId,Cidr:CidrBlock,Name:Tags[?Key==`Name`]|[0].Value}'
+```
+
+Expected — the VPC ID, `CidrBlock: 10.0.0.0/16`, and `Name: datacenter-vpc`.
+
+You can also confirm from Terraform's own state:
+
+```bash
+terraform state show aws_vpc.datacenter_vpc
+```
+
+> If `describe-vpcs` returns nothing, the `Name` tag is missing or misspelled — the filter matches on
+> that tag, not on the resource label in the config.