Think about how mail gets to your house. Your home has a unique street address that tells the postal service exactly where to deliver letters and packages. Computers on a network, including the internet, have a similar system: the Internet Protocol address, or IP address.

The most common version you'll see is IPv4 (Internet Protocol version 4). It looks something like this: 172.217.14.227. Each of the four numbers, separated by dots, is called an octet and can range from 0 to 255. This system gives us about 4.3 billion unique addresses. It sounds like a lot, but with all the computers, phones, servers, and smart devices connecting to the internet, we started running out.

A single IP address is like one house. But what if you want to describe a whole neighborhood? You wouldn't list every single house number. Instead, you'd refer to a block or a street. In networking, we do this with Classless Inter-Domain Routing (CIDR) notation.

CIDR notation looks like an IP address followed by a slash and a number, like 10.0.0.0/16. This notation defines a range, or block, of IP addresses. The number after the slash is the prefix, and it tells you how many of the starting bits of the address are fixed for the network portion. The remaining bits are flexible and can be assigned to individual devices (hosts).

A smaller CIDR number (like /16) means a larger network with more available IP addresses. A larger CIDR number (like /24) means a smaller network with fewer addresses. Cloud providers like AWS, GCP, and Azure use CIDR blocks to define the size of your private cloud network, known as a Virtual Private Cloud (VPC). You might start with a large block like 10.0.0.0/16 for your whole organization.

Once you have your big block of addresses, you don't want all your computers to be in one giant, disorganized group. You need to divide that block into smaller, manageable networks called subnets. Think of your main CIDR block as an office building. A subnet is like renting out a single floor. You can then put walls up to create individual, secure offices within that floor.

Subnets come in two main flavors: public and private.

A public subnet is for resources that need to be directly reachable from the internet. This is where you would place a web server, for example. It’s the public-facing part of your application.

A private subnet is for resources that should not be exposed to the internet. This is a secure zone for sensitive components like databases or backend services that only need to talk to other parts of your application, not the outside world. This separation is a fundamental security practice in the cloud.

So how does a request from the internet find its way to your web server in the public subnet? And how does your web server talk to the database in the private subnet?

This is managed by Route Tables. Each subnet has a route table associated with it. A route table is a simple set of rules that tells network traffic where to go. Think of it as a GPS for your data packets. A typical route table has two columns: the destination address range (where the traffic is trying to go) and the target (where to send it).

For a public subnet to be truly public, its route table needs a path to the internet. This is where an Internet Gateway comes in. An Internet Gateway is a component you attach to your VPC that allows communication between your resources and the internet.

The route table for a public subnet will have a rule that says, "For any traffic destined for the internet (represented as 0.0.0.0/0), send it to the Internet Gateway." The private subnet's route table will not have this rule, effectively cutting it off from the internet. It will only have a rule for local traffic within the VPC, allowing it to communicate with the public subnet.

Understanding these building blocks—IP addresses, CIDR blocks, subnets, route tables, and gateways—is the first step to designing secure and scalable infrastructure in the cloud. They are the fundamental tools you'll use to control how your applications communicate with each other and the wider world.