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IP Addresses & Ports

How the network finds the right machine, and the right program on it.

⏱10 min read📚DevOps Fundamentals

To deliver a message, the network needs two things: which machine (the IP address) and which program on that machine (the port). Get these two ideas and a huge amount of networking suddenly makes sense.

🎯 Learning Objectives

By the end of this lesson you will:

Understand IPv4 addresses and the idea of private vs public IPs
Know what a port is and why it matters
Recognise common ports used in DevOps
Understand the difference between TCP and UDP

IP Addresses — the Machine’s Location

An IP address is the address of a device on a network, like a street address for a house.

The most common form, IPv4, looks like 192.168.1.10 — four numbers (0–255) separated by dots.

Private vs Public

Type	Reachable from	Example ranges
Private	Only inside your local network	`10.x.x.x`, `172.16–31.x.x`, `192.168.x.x`
Public	Anywhere on the internet	Anything not in the private ranges

💡 Why this matters in the cloud

Inside a cloud VPC, servers usually talk to each other over private IPs. Only a load balancer or gateway gets a public IP. This is the foundation of a secure network design.

Ports — the Program’s Door

One machine runs many programs at once — a web server, a database, an SSH service. A port (a number from 0–65535) tells the network which program should receive the message.

An address plus a port is called a socket: 192.168.1.10:443.

Common ports to memorise

Port	Service
22	SSH
80	HTTP
443	HTTPS
53	DNS
3306	MySQL
5432	PostgreSQL
6379	Redis

⚠ Firewalls block ports

If your app “won’t connect,” a closed port in a firewall or cloud security group is one of the most common causes. Always check: is the port open, and is the service actually listening on it?

TCP vs UDP

Both live at the Transport layer, but they make different promises:

	TCP	UDP
Reliable?	Yes — confirms delivery, resends lost data	No — fire and forget
Ordered?	Yes	No
Speed	Slightly slower (more checks)	Faster, lighter
Used for	Web, SSH, databases	DNS, video calls, gaming, metrics

💡 Rule of thumb

Use TCP when every byte must arrive correctly (a web page, a bank transaction). Use UDP when speed matters more than perfection (a live video frame that’s already out of date if it’s late).

See It Yourself

Check which ports are listening on your machine:

bash

ss -tulnp        # show listening TCP/UDP ports (Linux)
curl -v http://localhost:80   # connect to a program on port 80

bash — 80×24

student@devops:~$ss -tuln | grep LISTEN

tcp   LISTEN  0  128  0.0.0.0:22    0.0.0.0:*
tcp   LISTEN  0  128  0.0.0.0:80    0.0.0.0:*
tcp   LISTEN  0  128  127.0.0.1:5432  0.0.0.0:*

Here SSH (22) and a web server (80) accept connections from anywhere, while PostgreSQL (5432) only listens on 127.0.0.1 — localhost only. That’s a deliberate security choice.

🧪 Hands-on Lab

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Map the Ports on Your Machine

List all listening ports
Identify which service each open port belongs to
Note which ones are bound to 127.0.0.1 (local only) vs 0.0.0.0 (all interfaces)

🧠 Knowledge Check

Knowledge Check

What does a port number identify?

The physical location of the serverWhich program on a machine should receive the messageThe speed of the connectionThe user's password

Knowledge Check

Which protocol would you choose for a live video call where speed matters more than perfect delivery?

TCPUDPSSHHTTP

💼 Interview Preparation

Interview Q&A

A user reports they can't reach your service. How do you approach it?

Summary

You now understand IP addresses (which machine), ports (which program), private vs public addressing, and the TCP/UDP trade-off. Next, we’ll see how human-friendly names like google.com turn into IP addresses using DNS.

Up Next

DNS — How Names Become Addresses

You've mastered this lesson. Continue your journey to becoming a DevOps Engineer.

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