Module 0.7: What is Networking?

Complexity: [QUICK] - Absolute beginner

Time to Complete: 25-30 minutes

Prerequisites: Module 0.4: Files and Directories — You should be comfortable running commands and navigating directories.

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What You’ll Be Able to Do

After this module, you will be able to:

• Explain how computers find each other using IP addresses and DNS
• Distinguish between local and public IP addresses and know when each matters
• Use ping, curl, and nslookup to diagnose basic connectivity issues
• Interpret HTTP status codes (200, 404) and explain what they mean

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Why This Module Matters

Kubernetes is a system for running applications across multiple computers that talk to each other over a network. If you don’t understand what a network is, what an IP address means, or what a port does, Kubernetes will feel like magic — and not the good kind.

This module gives you the mental model. By the end, you’ll understand how computers find each other, how they communicate, and you’ll have used real networking commands from your terminal.

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What is a Network?

A network is two or more computers connected together so they can share information.

That’s it. That’s the whole concept.

Your home Wi-Fi? That’s a network. The internet? That’s a network of networks — millions of computers all connected. When you visit google.com, your computer sends a message across the network to Google’s computer, which sends a message back.

Restaurant kitchen analogy: A network is like the intercom system in a large restaurant chain. Each kitchen (computer) has its own intercom (network connection). Any kitchen can call any other kitchen to share recipes, request supplies, or coordinate orders. The intercom system connecting all the kitchens? That’s the network.

Local vs. Internet

Type	What It Is	Example
Local Network (LAN)	Computers near each other, connected directly	Your laptop and printer on home Wi-Fi
Internet (WAN)	Computers anywhere in the world, connected through infrastructure	Your laptop talking to a server in Tokyo

In Kubernetes, you’ll work with both: local networks inside a cluster (where containers talk to each other) and internet connections from the outside world to your applications.

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IP Addresses: How Computers Find Each Other

Every computer on a network has an IP address — a unique number that identifies it, like a street address identifies a building.

192.168.1.42

This is an IP address. It’s four numbers separated by dots, each between 0 and 255.

The Street Address Analogy

Concept	Network	Real World
IP Address	192.168.1.42	123 Main Street
What it does	Identifies one specific computer	Identifies one specific building
Who assigns it	Your router (for local) or your ISP (for internet)	The city planning office

Types of IP Addresses

Private IP addresses — used inside your local network:

192.168.x.x    (most common for home networks)
10.x.x.x       (common in offices and data centers)
172.16-31.x.x  (less common but valid)

These only work within your local network. It’s like apartment numbers — they make sense inside the building, but someone outside wouldn’t know which building you mean.

Public IP addresses — your address on the internet:

Your router has a public IP that the rest of the internet can see. Every device behind your router shares this one public address.

Finding Your IP Address

On macOS:

$ ipconfig getifaddr en0
192.168.1.42

On Linux:

$ hostname -I
192.168.1.42

On any system (your public IP):

$ curl -s ifconfig.me
203.0.113.55

Don’t worry if the output looks different from the examples. The numbers will vary — that’s expected, because every computer gets its own unique address.

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Ports: Many Doors on One Address

Stop and think: Imagine a single physical server hosting three different company websites and an internal email system, all sharing the exact same public IP address. When a customer’s browser sends a request to that IP, what mechanical sorting process ensures the request reaches the correct website instead of the email system?

So every computer has an IP address. But a computer runs many programs at once — a web browser, an email client, a chat app. How does the computer know which program should receive an incoming message?

That’s what ports are for.

A port is a number (from 0 to 65535) that identifies a specific program or service on a computer.

The Apartment Building Analogy

If an IP address is a street address (the building), then a port is the apartment number (which unit inside the building).

IP Address:  192.168.1.42     = 123 Main Street
Port:        80               = Apartment 80

Full address: 192.168.1.42:80 = 123 Main Street, Apt 80

The colon : separates the IP address from the port number.

Common Ports

Port	Service	What It Does
80	HTTP	Regular web traffic (when you visit a website)
443	HTTPS	Secure web traffic (the padlock in your browser)
22	SSH	Secure remote terminal access
53	DNS	Domain name lookups
6443	Kubernetes API	How kubectl talks to your cluster

In Kubernetes, you’ll see ports everywhere. Every service has a port. Every container exposes ports. Understanding this concept now saves you enormous confusion later.

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DNS: The Phone Book of the Internet

When you type google.com in your browser, your computer doesn’t actually know where google.com is. Computers only understand IP addresses (numbers). So how does it work?

Pause and predict: If you disconnect your computer from the internet, you can still reach your home router by typing 192.168.1.1 into your browser, but typing router.local might fail. Based on how computers identify each other, what specific infrastructure piece is missing when you try to use the human-readable name offline?

DNS — the Domain Name System — translates human-friendly names into IP addresses.

You type:    google.com
DNS returns: 142.250.80.46
Your computer connects to: 142.250.80.46

The Phone Book Analogy

Without DNS	With DNS
”Call 142.250.80.46"	"Call Google”
You need to memorize numbers	You just remember names
Like the old days of memorizing phone numbers	Like your phone’s contact list

DNS is why you don’t need to type 142.250.80.46 every time you want to search something. You type google.com and DNS looks up the number for you.

How DNS Works (Simplified)

1. You type: google.com
2. Your computer asks your router: "What's the IP for google.com?"
3. Your router asks a DNS server: "What's the IP for google.com?"
4. The DNS server responds: "142.250.80.46"
5. Your computer connects to 142.250.80.46

This all happens in milliseconds. You never notice it.

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Putting It All Together: Sending a Letter

Let’s use one big analogy to connect everything:

Visiting a website is like sending a letter.

1. You want to send a letter to "Google HQ"
   (You type google.com)

2. You look up the address in the phone book
   (DNS translates google.com → 142.250.80.46)

3. You write the street address on the envelope
   (IP address: 142.250.80.46)

4. You write the apartment/department number
   (Port: 443 for HTTPS)

5. You put the letter in the mailbox
   (Your computer sends the request over the network)

6. Google receives it and sends a reply
   (The web page comes back to your browser)

And just like that, you understand the basics of networking.

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Your First Networking Commands

Let’s get our hands dirty. Open your terminal and try these.

ping — “Are You There?”

ping sends a tiny message to another computer and waits for a reply. It’s like knocking on someone’s door to see if they’re home.

$ ping -c 4 google.com
PING google.com (142.250.80.46): 56 data bytes
64 bytes from 142.250.80.46: icmp_seq=0 ttl=118 time=11.4 ms
64 bytes from 142.250.80.46: icmp_seq=1 ttl=118 time=10.8 ms
64 bytes from 142.250.80.46: icmp_seq=2 ttl=118 time=11.2 ms
64 bytes from 142.250.80.46: icmp_seq=3 ttl=118 time=10.9 ms

• -c 4 means “send 4 pings then stop.” Without this, ping will run forever (on Linux/macOS) until you press Ctrl+C.
• time=11.4 ms means the round trip took 11.4 milliseconds. That’s how fast light (well, electrical signals) can travel to Google’s server and back.

If you see Request timeout, the server either doesn’t exist, is blocking pings, or there’s a network issue.

curl — “Give Me That Web Page”

curl fetches content from a URL. Think of it as a web browser in your terminal — but it shows you the raw text instead of rendering a pretty page.

$ curl -s example.com

You’ll see raw HTML:

<!doctype html>
<html>
<head>
    <title>Example Domain</title>
...

The -s flag means “silent” — it hides the progress bar so you just see the content.

Fetching just the headers (metadata about the response):

$ curl -I example.com
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
...

200 OK means the server received your request and everything is fine. You’ll learn more about HTTP status codes later, but the important ones are:

Code	Meaning	Analogy
200	OK — here’s what you asked for	”Order up! Here’s your food.”
404	Not found — that page doesn’t exist	”Sorry, we don’t have that dish on the menu.”
500	Server error — something broke on their end	”The kitchen is on fire.”

nslookup or dig — “Look Up the Address”

These commands manually ask DNS to translate a name to an IP address:

$ nslookup google.com
Server:    192.168.1.1
Address:   192.168.1.1#53

Non-authoritative answer:
Name:   google.com
Address: 142.250.80.46

This confirms that google.com translates to 142.250.80.46 (your result may differ — Google has many servers around the world).

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Did You Know?

1. A common production issue: DNS caching. Sometimes, even after a server migration is completed and DNS records are updated, your application keeps connecting to the old IP address because it cached the old DNS result. This has caused real, prolonged outages at major scale. For instance, a global e-commerce giant once suffered a 4-hour outage during a migration when their payment gateways aggressively cached outdated DNS records, costing an estimated $3.2 million in lost transactions because the application simply refused to look up the new address.

2. NAT (Network Address Translation) and “It works on my machine”. Your local machine might have an IP like 192.168.1.42, but the internet only sees your router’s public IP. When you run a server on your laptop, it’s bound to your local IP. This is why you can access it locally, but your friend across town gets a “connection refused” error — they cannot route traffic through the public internet directly to your private local IP without NAT rules on your router.

3. DNS was invented because memorizing IP addresses was too hard. Before DNS existed (1983), there was literally a single text file called hosts.txt that listed every computer on the internet and its address. Someone maintained it by hand. As the internet grew, this became impossible, so Paul Mockapetris invented DNS to automate the process.

4. IP address exhaustion is real. The original IP address system (IPv4) only has about 4.3 billion addresses. By the 2010s, we effectively ran out. To solve this, engineers created IPv6, which has so many addresses (340 undecillion) that we could assign one to every atom on the surface of the Earth and still have plenty left over. However, moving the entire internet to IPv6 is taking decades, which is why NAT (mentioned above) is so crucial today to share IPv4 addresses.

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Common Mistakes

Mistake	What Happens	Fix
Forgetting -c with ping	Ping runs forever	Press Ctrl+C to stop it, or always use ping -c 4
Pinging a site that blocks ping	Request timeout — looks like it’s down	The site might be fine — many servers block ping. Use curl instead
Confusing IP address and port	Connecting to the wrong thing	IP = which computer. Port = which service on that computer. 192.168.1.42:80
Thinking your local IP is your public IP	Confusion when sharing with others	192.168.x.x is local only. Use curl ifconfig.me for your public IP
Forgetting http:// with curl	Sometimes curl guesses wrong or errors	Be explicit: curl http://example.com or curl https://example.com
Thinking DNS failure means the site is down	Can’t reach the site	The site might be fine but DNS can’t resolve the name. Try using the IP directly

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Quiz

Question 1: Your friend says their new laptop’s address is 192.168.1.42 and asks you to connect to it from your house. You try, but it fails. Based on the type of address provided, why didn’t this work?

Show Answer

The address 192.168.1.42 is a private, local IP address designated specifically for internal networks. It only identifies computers within the same local network (LAN) and is completely invisible to the outside internet. Since you are at your house and your friend is at theirs, you are on two entirely separate local networks. To connect over the internet, your computer would need to target their router’s public IP address, and their router would need a rule to forward that traffic to their specific laptop.

Question 2: You are setting up a new web application. You can successfully run ping 203.0.113.55 and receive replies, indicating the server is online. However, running curl http://203.0.113.55 simply times out. What layer of the network is likely causing the problem?

Show Answer

The issue is occurring at the port or application layer, not the network routing layer. The successful ping command proves that the IP address is reachable, the server is powered on, and the network path is clear. However, curl specifically attempts to connect to a web service on port 80 (HTTP). The timeout indicates that either a firewall is blocking traffic specifically to port 80, or the web server software has crashed and isn’t listening for connections on that port.

Question 3: A user reports they cannot access your company’s API at api.example.com. You run ping api.example.com and it immediately says cannot resolve api.example.com: Unknown host. However, you know the server’s public IP is 203.0.113.100, and curl http://203.0.113.100 returns a 200 OK. Diagnose the exact point of failure.

Show Answer

The failure is isolated to the DNS resolution system, meaning the ‘phone book’ of the internet is failing to translate the name. Because the curl command works perfectly with the direct IP address and returns a 200 OK, we can definitively prove the server is running, the network path is clear, and the application is healthy. The Unknown host error from ping shows that your computer cannot figure out which IP address belongs to api.example.com. To fix this, the DNS records for the domain need to be investigated and updated to point to 203.0.113.100.

Question 4: A customer complains that your company’s website is down. You run curl -I https://example.com and receive a 500 Internal Server Error response. Is the problem with the customer’s internet connection, DNS, or your company’s server?

Show Answer

The problem is definitively with your company’s server and application infrastructure. A 500-level HTTP status code is generated by the web server itself, which means the customer’s internet connection works, DNS resolved correctly, and the network successfully delivered the request. The server received the request, but the application code encountered a fatal error (like a database connection failure or a syntax error) while trying to process it and build the webpage. You need to check the server’s application logs to find the exact software bug causing the crash.

Question 5: A junior developer runs ping -c 4 api.example.com and it returns Request timeout for every packet. They immediately declare, “The API server is completely down and broken!” You run curl -I https://api.example.com on the same machine and receive HTTP/1.1 200 OK. Why was the junior developer’s conclusion wrong, and what is actually happening?

Show Answer

The junior developer incorrectly assumed that a failed ping definitively means a server is offline or network-unreachable. In reality, ping uses a specific type of diagnostic network traffic (ICMP) that many modern servers and corporate firewalls intentionally drop for security reasons. Meanwhile, they allow normal web traffic (TCP) through on ports 80 and 443. The successful curl command proved that the web server is actively running, network routing is fine, and it is serving content successfully to legitimate web requests. The ping simply failed because the server’s firewall is configured to ignore diagnostic knocks.

Question 6: You are helping a colleague troubleshoot an application that connects to a database. The database is supposed to be running on 10.0.5.50:5432. The application logs show an error: Connection refused to 10.0.5.50 on port 80. Based on this log, what is the most likely cause of the failure?

Show Answer

The application is trying to connect to the database using the wrong port number. While the target IP address 10.0.5.50 is correct, the application is attempting to communicate over port 80, which is the default port for regular unencrypted web traffic. The database service is actually listening on port 5432, leaving port 80 completely unattended. Because there is no service listening on port 80 at that address, the operating system on the database server actively rejects the connection attempt, resulting in the ‘Connection refused’ error. The application’s configuration file must be updated to explicitly specify port 5432.

Question 7: Your team just launched a new marketing site at promo.company.com. When you type this into your browser, it fails to load. You run nslookup promo.company.com and the command returns server can't find promo.company.com: NXDOMAIN. However, the lead engineer says the server is running perfectly at 198.51.100.22. What specific system needs to be updated to fix this issue?

Show Answer

The Domain Name System (DNS) records for the company’s domain need to be updated. The NXDOMAIN (Non-Existent Domain) error from the nslookup command proves that there is no public DNS record linking the human-readable name promo.company.com to any IP address. Even though the server is fully operational at 198.51.100.22, browsers on the internet have no way to discover that routing information because the ‘phone book’ entry is missing. The infrastructure team must log into the DNS provider and create an ‘A record’ that points the new subdomain to the correct public IP address.

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Hands-On Exercise: Exploring the Network

Objective

Use networking commands to explore connections, look up addresses, and fetch web content.

Steps

1. Verify network connectivity to a popular website: Use the terminal tool that sends small messages to a server to see if it responds. Send exactly 4 messages to google.com so the command doesn’t run forever. Note the IP address it resolves to and the response times.

2. Compare latency with another service: Run the exact same command from step 1, but target cloudflare.com instead. Are the response times faster or slower? The difference depends on how far their servers are from you.

3. Find your local IP address: Use the appropriate command to find your local IP:

On macOS:

$ ipconfig getifaddr en0

On Linux:

$ hostname -I

Write down your local IP. It should start with 192.168., 10., or 172..

4. Discover your public IP address: Use curl in silent mode (with the -s flag) to fetch the page at ifconfig.me. This will output the IP address the rest of the internet sees for you. Notice how it differs from your local IP.

5. Manually resolve a domain name: Use the DNS lookup tool you learned about (like nslookup) to find the exact IP address for github.com. Locate the IP address in the resulting output.

6. Fetch a web page:

$ curl -s example.com

You should see the raw HTML of the example.com page.

7. Examine server response headers: Instead of getting the full HTML, use curl to ask example.com just for its metadata headers (using the -I flag). Look for HTTP/1.1 200 OK in the output — that means success!

8. Trigger a specific HTTP error code: Use curl to request only the headers for a path that you know doesn’t exist on example.com (such as example.com/this-page-does-not-exist). Look for the 404 status in the response — that means “not found.”

9. Combine networking with file skills: Fetch the raw HTML of example.com silently again, but this time, redirect the output into a new file called my-first-webpage.html inside a ~/kubedojo-practice/ directory. Then, use a file reading command to output the contents of that new file to the screen to verify it worked.

Success Criteria

You’ve completed this exercise when you can:

• Ping a website and see response times
• Find your local IP address
• Find your public IP address
• Look up a domain name with nslookup
• Fetch a web page with curl
• Identify a 200 (OK) and 404 (Not Found) response
• Save a web page to a file

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You just used a tool that senior engineers use every day. You belong here.

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What’s Next?

You now understand how computers find and talk to each other. IP addresses, ports, DNS, and basic networking commands are in your toolkit.

Next Module: Module 0.8: Servers and SSH — Learn what a server is, where they live, and how to connect to them remotely.