Networking Concepts#

Prerequisite: This lesson covers the networking theory you need before configuring Linux networking. If you are already comfortable with IP addresses, subnets, DNS, and DHCP, you can skim this and move to the next lesson.

Concepts#

IP Addresses#

Every device on a network has an IP address — a numeric identifier that allows other devices to find and communicate with it.

IPv4#

The most common format. Four numbers (0-255) separated by dots:

192.168.1.100
10.0.0.5
172.16.0.1

There are approximately 4.3 billion possible IPv4 addresses (2^32). They are running out, which is why IPv6 exists.

Private vs Public Addresses#

Your home router has a public IP from your ISP and assigns private IPs to devices on your local network. This is called NAT (Network Address Translation).

Subnet Masks and CIDR#

A subnet mask defines which part of an IP address identifies the network and which part identifies the device.

IP:          192.168.1.100
Subnet mask: 255.255.255.0
             ├── Network ──┤ Host │
             192.168.1      .100

CIDR notation is a shorthand: /24 means the first 24 bits are the network part.

In 192.168.1.0/24:

• Network address: 192.168.1.0 (the subnet itself)
• Usable range: 192.168.1.1 to 192.168.1.254
• Broadcast: 192.168.1.255 (sent to all hosts on the subnet)
• Default gateway: Usually 192.168.1.1 (the router)

Gateway and Routing#

The default gateway is the router that forwards traffic to other networks (including the internet). When your machine needs to reach an IP outside its subnet, it sends the packet to the gateway.

Your PC (192.168.1.100) → Gateway (192.168.1.1) → Internet

DNS — Domain Name System#

DNS translates human-readable domain names to IP addresses:

google.com → 142.250.80.46

The resolution process:

1. You type google.com in your browser.
2. Your system checks /etc/hosts (local static mappings).
3. If not found, it asks the DNS resolver configured in /etc/resolv.conf.
4. The resolver queries DNS servers (recursive resolution) and returns the IP.
5. Your browser connects to that IP.

Common DNS servers:

DHCP — Dynamic Host Configuration Protocol#

DHCP automatically assigns network settings to devices. When your machine connects to a network:

1. It broadcasts a DHCP Discover (“I need an IP address”).
2. The DHCP server (usually the router) responds with a DHCP Offer (IP, subnet mask, gateway, DNS servers).
3. Your machine sends a DHCP Request (“I accept this offer”).
4. The server sends a DHCP Acknowledge (“Confirmed, it’s yours”).

The assigned IP has a lease time — after it expires, the client must renew it.

Most home and office networks use DHCP. Servers often use static IPs (manually configured) for stability.

TCP vs UDP#

Data travels over the network using protocols:

Ports#

A port is a number (1-65535) that identifies a specific service on a machine. An IP address gets you to the machine; a port gets you to the service.

192.168.1.100:22    → SSH on that machine
192.168.1.100:80    → HTTP (web server) on that machine
192.168.1.100:443   → HTTPS on that machine

Common ports:

The Network Stack Summary#

Application     (HTTP, SSH, DNS)
    ↓
Transport       (TCP or UDP + port)
    ↓
Network         (IP + routing)
    ↓
Link            (Ethernet, Wi-Fi + MAC address)
    ↓
Physical        (cables, radio waves)

Lab#

Exercise 1: Check Your Network Configuration#

# Your IP address
ip addr show

# Your default gateway
ip route show default

# Your DNS configuration
cat /etc/resolv.conf

# Your hostname
hostname

Exercise 2: Examine /etc/hosts#

cat /etc/hosts
# You should see at least:
# 127.0.0.1  localhost
# This means "localhost" resolves to 127.0.0.1 (your own machine)

Exercise 3: DNS Resolution#

# Resolve a domain name
host google.com

# More detailed
dig google.com +short

# Or nslookup
nslookup google.com

# Check which DNS server you are using
cat /etc/resolv.conf | grep nameserver

Exercise 4: View Listening Ports#

# See which ports are open on your system
sudo ss -tlnp
# t = TCP, l = listening, n = numeric (don't resolve names), p = show process

# Common entries you might see:
# :22   → sshd
# :53   → systemd-resolved (DNS)
# :631  → cupsd (printing)

Exercise 5: Check Connectivity#

# Ping your gateway (check local network)
ip route | grep default    # find your gateway IP
ping -c 3 $(ip route | grep default | awk '{print $3}')

# Ping an external server
ping -c 3 8.8.8.8

# Ping a domain name (tests DNS too)
ping -c 3 google.com

Review#

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