What Are APIs and How Do They Work?

If you've ever logged into a website using your Google account, checked the weather on your phone, or made an online payment, you've already used an API. You just didn't know it at the time. APIs are everywhere in modern technology, and understanding them is a foundational skill for anyone pursuing network automation or the CCNA DevNet certification.

What Is an API?

API stands for Application Programming Interface. At its core, an API is a defined way for two software applications to talk to each other. Think of it like a waiter in a restaurant. You (the customer) don't walk into the kitchen and cook your own food. Instead, you tell the waiter what you want, the waiter takes your order to the kitchen, and the kitchen sends back exactly what you asked for. The waiter is the API: a structured, predictable middleman that handles communication between two separate systems.

In networking terms, this idea translates directly. Your network management tool doesn't need to know everything about how a Cisco router works internally. It just needs to know how to send the right request to the router's API and interpret the response it gets back.

How Do APIs Work?

Most modern APIs follow a model called REST (Representational State Transfer). A REST API communicates over HTTP, the same protocol your web browser uses. When one application wants data or wants to make a change, it sends an HTTP request to the API. The API processes that request and sends back a response, usually formatted in JSON or XML.

There are four common HTTP methods you'll see repeatedly in API work:

• GET: Retrieve information (like reading the current configuration of a device)
• POST: Send new data or create something (like pushing a new configuration)
• PUT: Update or replace existing data
• DELETE: Remove data or a configuration entry

Here's a simple analogy to lock it in. A GET request is like asking "what interfaces are currently configured on this router?" A POST request is like saying "here is a new VLAN, please add it."

A Real-World API Example

Let's look at a practical example using the Cisco DevNet Sandbox. If you were querying a router's interface information via a REST API, the request might look like this:

GET https://sandbox-iosxe-latest-1.cisco.com/restconf/data/ietf-interfaces:interfaces
Headers:
  Content-Type: application/yang-data+json
  Accept: application/yang-data+json

And the response from the router might come back looking like this:

{
  "ietf-interfaces:interfaces": {
    "interface": [
      {
        "name": "GigabitEthernet1",
        "type": "iana-if-type:ethernetCsmacd",
        "enabled": true
      }
    ]
  }
}

That JSON response is structured, readable, and easy to parse with Python or any other scripting language. This is exactly how tools like Ansible, Terraform, and custom Python scripts automate network devices at scale.

Why APIs Matter for Network Engineers

Traditional network management relied on CLI commands typed manually into each device. That approach doesn't scale. APIs make it possible to configure hundreds of devices in seconds, retrieve data programmatically, and integrate your network into broader DevOps pipelines.

For the CCNA DevNet exam (Exam 2.3 specifically), you're expected to understand what APIs are, the difference between REST and other API types, and how HTTP methods map to network operations. You don't need to build APIs from scratch, but you do need to know how to consume them.

Key Takeaways

• API: A defined interface that allows two applications to communicate
• REST API: The most common API type, using HTTP methods over a network
• HTTP methods: GET, POST, PUT, and DELETE map to read, create, update, and delete operations
• JSON: The most common format for API request and response data in networking

What's Next?

Now that you understand what APIs are and how they work at a conceptual level, the next step is exploring the difference between REST APIs and other API styles like SOAP and RPC. We'll also dive deeper into how REST APIs are structured, including endpoints, headers, authentication, and status codes. These concepts build directly on what you've learned here and bring you closer to writing your first real API call in Python.

For structured exam preparation, check out the Cisco Press CCNA DevNet Associate DEVASC 200-901 Official Cert Guide, which covers API fundamentals in detail aligned to the exam blueprint.