Networking Fundamentals

The OSI Model

Network communication is organized in layers. The OSI model defines 7:

TCP/IP Model (Simplified)

TCP vs UDP

IP Addressing

IPv4 Format

An IPv4 address is 4 octets (32 bits) in dotted decimal:

192.168.1.100

Each octet ranges from 0-255.

Private vs Public IPs

Subnet Masks and CIDR

A subnet mask separates the network portion from the host portion:

Example: 192.168.1.100/24 means:

• Network: 192.168.1.0
• Hosts: 192.168.1.1 to 192.168.1.254
• Broadcast: 192.168.1.255

Network Configuration with ip

View Interfaces

ip addr show
# or shorter:
ip a

Key information:

• lo — Loopback (127.0.0.1, internal communication)
• eth0 / ens5 — Primary network interface
• inet — IPv4 address
• link/ether — MAC address

View Specific Interface

ip addr show dev eth0
 
# IPv4 only
ip -4 a
 
# IPv6 only
ip -6 a

Add/Remove IP Addresses

# Add an IP
sudo ip addr add 192.168.1.50/24 dev eth0
 
# Remove an IP
sudo ip addr del 192.168.1.50/24 dev eth0

Enable/Disable Interfaces

# Disable
sudo ip link set down eth0
 
# Enable
sudo ip link set up eth0

DNS — Domain Name System

DNS translates domain names to IP addresses:

google.com → 142.250.180.3

How DNS Works

1. Browser asks local DNS resolver
2. Resolver checks cache, then queries root servers
3. Root → TLD server (.com) → Authoritative server
4. IP address returned to browser

DNS Configuration

# View configured DNS servers
cat /etc/resolv.conf
 
# Test DNS resolution
host google.com
dig google.com
nslookup google.com

Local DNS Override (/etc/hosts)

sudo nano /etc/hosts

127.0.0.1   localhost
192.168.1.10  myserver.local
192.168.1.20  database.local

Routing

Routing determines how packets reach their destination across networks.

View Routing Table

ip route
# or
ip r

Output:

default via 172.31.16.1 dev eth0
172.31.16.0/20 dev eth0 proto kernel scope link src 172.31.24.148

• default — Gateway for all external traffic
• 172.31.16.0/20 — Local network (direct access)

Add/Remove Routes

# Add route to a network via gateway
sudo ip route add 10.0.0.0/24 via 172.31.16.1 dev eth0
 
# Add default gateway
sudo ip route add default via 192.168.1.1
 
# Remove a route
sudo ip route del 10.0.0.0/24

Essential Network Commands

ping — Test Connectivity

# Test if host is reachable
ping -c 4 8.8.8.8
 
# Ping a domain
ping -c 4 google.com

traceroute — Trace Path

# Show hops to destination
traceroute google.com

DHCP — Automatic IP Assignment

# Request IP from DHCP server
sudo dhclient eth0
 
# Release DHCP lease
sudo dhclient -r eth0

Hostname Management

# View hostname
hostname
 
# Change hostname (persistent)
sudo hostnamectl set-hostname my-server
 
# Verify
hostnamectl

Ports and Sockets

Services listen on specific ports. A socket = IP + Port + Protocol.

# View listening ports
sudo ss -tuln
 
# Or with netstat
sudo netstat -tuln

Summary

• OSI model has 7 layers; TCP/IP simplifies to 4
• IPv4: 32-bit addresses in dotted decimal (e.g., 192.168.1.1)
• CIDR notation (/24) defines network size
• ip addr manages interfaces; ip route manages routing
• DNS translates names to IPs; configured in /etc/resolv.conf
• ping tests connectivity; traceroute shows the path
• Ports identify services; common: 22 (SSH), 80 (HTTP), 443 (HTTPS)

Next Steps

Next, we'll put networking into practice with firewalls (iptables/UFW), port scanning, and SSH configuration.