Network Basics¶

1. Basic Network Concepts¶

IP Address¶

IPv4: 192.168.1.100
      └─┬─┘ └─┬─┘
     Network  Host

IPv6: 2001:0db8:85a3:0000:0000:8a2e:0370:7334

Private IP Ranges¶

Class	Range	Usage
A	10.0.0.0/8	Large networks
B	172.16.0.0/12	Medium networks
C	192.168.0.0/16	Home/small networks

Subnet Mask¶

IP:      192.168.1.100
Subnet:  255.255.255.0   (/24)
         └──Network──┘ └Host┘

CIDR notation: 192.168.1.0/24
→ 256 addresses (192.168.1.0 ~ 192.168.1.255)

Common Ports¶

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

2. Network Configuration Check¶

ip addr - IP Address Check¶

# All interfaces
ip addr
ip a

# Specific interface
ip addr show eth0

Output:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP
    link/ether 00:11:22:33:44:55 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.100/24 brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::1/64 scope link
       valid_lft forever preferred_lft forever

ip link - Interface Status¶

# List interfaces
ip link

# Enable interface
sudo ip link set eth0 up

# Disable interface
sudo ip link set eth0 down

ip route - Routing Table¶

# Check routing
ip route
ip r

Output:

default via 192.168.1.1 dev eth0 proto dhcp metric 100
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100

ifconfig (Legacy)¶

# May need installation
sudo apt install net-tools    # Ubuntu
sudo dnf install net-tools    # CentOS

# Usage
ifconfig
ifconfig eth0

hostname - Hostname¶

# Check hostname
hostname
hostname -I    # IP addresses only

# Change hostname (temporary)
sudo hostname new-hostname

# Change hostname (permanent)
sudo hostnamectl set-hostname new-hostname

3. Connection Testing¶

ping - Connection Check¶

# Basic ping
ping google.com

# Specify count
ping -c 4 google.com

# Specify interval
ping -i 0.5 192.168.1.1

traceroute - Route Tracing¶

# Install
sudo apt install traceroute    # Ubuntu
sudo dnf install traceroute    # CentOS

# Usage
traceroute google.com
traceroute -n 8.8.8.8    # No name resolution

mtr - Combined Tool¶

# Install
sudo apt install mtr    # Ubuntu
sudo dnf install mtr    # CentOS

# Usage (ping + traceroute)
mtr google.com
mtr -r -c 10 google.com    # Report mode

4. DNS Lookup¶

nslookup¶

# Basic lookup
nslookup google.com

# Specific DNS server
nslookup google.com 8.8.8.8

# Specify record type
nslookup -type=MX google.com

dig¶

# Basic lookup
dig google.com

# Short output
dig +short google.com

# Specific record
dig google.com MX
dig google.com TXT

# Specific DNS server
dig @8.8.8.8 google.com

# Reverse lookup
dig -x 8.8.8.8

host¶

# Simple lookup
host google.com
host -t MX google.com

/etc/hosts¶

Local DNS configuration.

cat /etc/hosts

127.0.0.1   localhost
192.168.1.50   myserver.local myserver

/etc/resolv.conf¶

DNS server configuration.

cat /etc/resolv.conf

nameserver 8.8.8.8
nameserver 8.8.4.4
search example.com

5. Port Check¶

ss - Socket Statistics¶

# Check open ports
ss -tuln

# TCP connections
ss -t

# Listening ports
ss -l

# Include process information
ss -tulnp

# Specific port
ss -tuln | grep :80

Options: | Option | Description | |--------|-------------| | -t | TCP | | -u | UDP | | -l | LISTEN state only | | -n | Numeric display | | -p | Process information |

netstat (Legacy)¶

# Open ports
netstat -tuln

# All connections
netstat -an

# Include process
sudo netstat -tulnp

lsof - Open Files/Ports¶

# Process using specific port
sudo lsof -i :80

# Network connections of specific process
sudo lsof -i -a -p 1234

# All network connections
sudo lsof -i

6. SSH - Remote Access¶

Basic Connection¶

# Basic connection
ssh user@hostname
ssh user@192.168.1.100

# Specify port
ssh -p 2222 user@hostname

# Verbose output
ssh -v user@hostname

SSH Key Authentication¶

# 1. Generate key
ssh-keygen -t rsa -b 4096

# Or ed25519 (recommended)
ssh-keygen -t ed25519

# 2. Copy public key
ssh-copy-id user@hostname

# 3. Connect with key
ssh user@hostname

SSH Key Files¶

~/.ssh/
├── id_rsa           # Private key (never share)
├── id_rsa.pub       # Public key (register on server)
├── authorized_keys  # List of allowed public keys
├── known_hosts      # List of connected servers
└── config           # SSH configuration

SSH config Configuration¶

# ~/.ssh/config
Host myserver
    HostName 192.168.1.100
    User ubuntu
    Port 22
    IdentityFile ~/.ssh/id_rsa

Host production
    HostName prod.example.com
    User deploy
    Port 2222

Usage:

ssh myserver
ssh production

7. File Transfer¶

scp - File Copy¶

# Local → Remote
scp file.txt user@host:/path/to/destination/

# Remote → Local
scp user@host:/path/to/file.txt ./

# Copy directory
scp -r directory/ user@host:/path/to/

# Specify port
scp -P 2222 file.txt user@host:/path/

rsync - Synchronization¶

# Basic sync
rsync -av source/ destination/

# Remote sync
rsync -av local_dir/ user@host:/remote_dir/
rsync -av user@host:/remote_dir/ local_dir/

# Sync deletions
rsync -av --delete source/ destination/

# Show progress
rsync -av --progress source/ destination/

# Compressed transfer
rsync -avz source/ user@host:/destination/

# Specify SSH port
rsync -av -e "ssh -p 2222" source/ user@host:/dest/

sftp - Interactive Transfer¶

sftp user@hostname

# sftp commands
sftp> ls              # Remote list
sftp> lls             # Local list
sftp> cd /path        # Remote change directory
sftp> lcd /path       # Local change directory
sftp> get file.txt    # Download
sftp> put file.txt    # Upload
sftp> quit            # Exit

8. Network Configuration (Permanent)¶

Ubuntu/Debian (Netplan)¶

# Configuration file
sudo vi /etc/netplan/01-netcfg.yaml

network:
  version: 2
  ethernets:
    eth0:
      dhcp4: no
      addresses:
        - 192.168.1.100/24
      gateway4: 192.168.1.1
      nameservers:
        addresses:
          - 8.8.8.8
          - 8.8.4.4

# Apply
sudo netplan apply

CentOS/RHEL (NetworkManager)¶

# Configuration file
sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0

TYPE=Ethernet
BOOTPROTO=static
NAME=eth0
DEVICE=eth0
ONBOOT=yes
IPADDR=192.168.1.100
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4

# Apply
sudo systemctl restart NetworkManager
# Or
sudo nmcli connection reload
sudo nmcli connection up eth0

nmcli (NetworkManager CLI)¶

# List connections
nmcli connection show

# Device status
nmcli device status

# Set static IP
sudo nmcli connection modify eth0 ipv4.addresses 192.168.1.100/24
sudo nmcli connection modify eth0 ipv4.gateway 192.168.1.1
sudo nmcli connection modify eth0 ipv4.dns "8.8.8.8 8.8.4.4"
sudo nmcli connection modify eth0 ipv4.method manual
sudo nmcli connection up eth0

# Change to DHCP
sudo nmcli connection modify eth0 ipv4.method auto
sudo nmcli connection up eth0

9. Practice Exercises¶

Exercise 1: Check Network Information¶

# Check IP address
ip addr

# Routing table
ip route

# DNS configuration
cat /etc/resolv.conf

# Hostname
hostname -I

Exercise 2: Connection Testing¶

# Ping test
ping -c 4 google.com

# DNS lookup
dig +short google.com
nslookup google.com

# Route trace
traceroute -n google.com

Exercise 3: Port Check¶

# Check open ports
ss -tuln

# Process using specific port
sudo lsof -i :22

# Test port from outside
nc -zv localhost 22

Exercise 4: SSH Key Setup¶

# Generate key
ssh-keygen -t ed25519 -C "your_email@example.com"

# Check key
ls -la ~/.ssh/

# View public key content
cat ~/.ssh/id_ed25519.pub

Exercise 5: File Transfer¶

# Test local file copy
mkdir -p ~/test_sync/source ~/test_sync/dest
echo "test content" > ~/test_sync/source/test.txt

# rsync sync
rsync -av ~/test_sync/source/ ~/test_sync/dest/

# Check result
ls -la ~/test_sync/dest/

Next Steps¶

Let's learn about system monitoring in 11_System_Monitoring.md!