109.1 Fundamentals of Internet Protocols

Introduction

This lesson is about the basic language computers use to talk over networks.

The main question is:

What do IP addresses, subnets, ports, and protocols actually mean, and how do they fit together in normal Linux networking?

What you should be able to do after this lesson:

• Understand the difference between IP addresses, hostnames, ports, and protocols.
• Tell TCP, UDP, and ICMP apart.
• Recognize IPv4 and IPv6 basics.
• Understand private vs public address space.
• Read simple CIDR notation such as /24.
• Recognize common service ports used in LPIC-1.

Big Idea: Networking Works in Layers

Beginners often mix everything together:

• the address
• the name
• the service
• the application

Keep this picture in your head instead:

IP address identifies a host -> port identifies a service -> protocol decides how traffic behaves

That one line solves a surprising number of beginner questions.

Hostnames and IP Addresses

A hostname is a human-friendly name. An IP address is the numeric address used for routing traffic.

Example:

• hostname: server1.example.com
• IPv4 address: 192.0.2.10
• IPv6 address: 2001:db8::10

Linux often turns a hostname into an IP address through:

• /etc/hosts
• DNS

So the hostname is not the route itself. It is the name that gets resolved into an address.

IPv4 Basics

IPv4 uses 32-bit addresses. You often see them as four decimal numbers:

192.168.1.20

Private IPv4 ranges

These are common inside home and office networks:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

These are not normally routed directly across the public internet.

Loopback

127.0.0.1

This means "the local machine itself".

IPv6 Basics

IPv6 uses 128-bit addresses and looks different:

2001:db8::1

At LPIC-1 level, the main idea is:

• IPv6 is the newer protocol
• it provides a much larger address space
• Linux tools often show both IPv4 and IPv6 information

Useful examples:

• loopback: ::1
• link-local addresses often start with fe80::

Subnets and CIDR

CIDR notation shows how much of an address is the network part.

Example:

192.168.1.20/24

Simple meaning:

• the first 24 bits describe the network
• the remaining bits describe the host part

For beginners, the most useful common values are:

• /24 for many small IPv4 networks
• /16
• /8

You should also recognize the dotted-decimal form of a mask. For example:

/24 = 255.255.255.0

You do not need advanced subnet math yet. You do need to understand that the prefix length changes the network boundary.

Ports

An IP address identifies the host. A port identifies the service on that host.

Examples:

• SSH on port 22
• HTTP on port 80
• HTTPS on port 443

That means:

• same host
• multiple services
• each on a different port

TCP, UDP, and ICMP

TCP

Use TCP when reliable ordered delivery matters.

Common examples:

• SSH
• HTTP
• HTTPS
• IMAP

UDP

Use UDP when lightweight delivery matters more than connection management.

Common examples:

• DNS queries
• NTP
• some streaming or voice traffic

ICMP

ICMP is used for network control and diagnostics.

Common example:

ping

So when you use ping, you are not testing TCP or UDP ports. You are testing ICMP reachability.

Common Ports to Recognize

You do not need to become a walking port table. But LPIC-1 expects awareness of well-known services.

Important examples:

• 20, 21 FTP
• 22 SSH
• 23 Telnet
• 25 SMTP
• 53 DNS
• 80 HTTP
• 110 POP3
• 123 NTP
• 139 NetBIOS
• 143 IMAP
• 161, 162 SNMP
• 389 LDAP
• 443 HTTPS
• 465 SMTPS
• 514 syslog
• 636 LDAPS
• 993 IMAPS
• 995 POP3S

Routing

If a packet is not for the local subnet, Linux sends it toward a gateway using the routing table.

That is why a network problem can come from:

• missing address
• wrong subnet
• wrong gateway
• DNS problem
• service not listening

You will use this mental model more in the next lessons.

Practice Step by Step

1) Inspect your addresses

1. Run: ip -br address
2. Look for:
   • IPv4 addresses
   • IPv6 addresses
   • loopback lo

2) Look at common services

1. Open the services database: grep -E '^(ssh|http|https|domain|ntp)[[:space:]]' /etc/services
2. Notice the port and protocol pairs.

3) Think through one address

1. Take an example such as: 192.168.1.20/24
2. Explain it in plain words:
   • host address = 192.168.1.20
   • network prefix = /24
3. That is enough beginner-level interpretation for this exam.

Cheat Sheet

ip -br address
ip route
grep -E '^(ssh|http|https|domain|ntp)[[:space:]]' /etc/services
ping 127.0.0.1
ping ::1