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Networking: Connecting Systems

Networking is how systems talk to each other. IP addresses. Ports. Protocols.

Here's the thing: Understanding networking is essential. Servers communicate. Services listen. Everything connects.

The Basic Concept​

IP addresses = Where systems are Ports = Which service Protocols = How they talk

Think of it like mail: IP is the address. Port is the apartment number. Protocol is the language.

Network Interfaces​

View Interfaces​

ip addr show                      # All interfaces (modern)
ifconfig                          # All interfaces (legacy)

Common interfaces:

  • eth0, eth1 = Ethernet
  • wlan0 = Wireless
  • lo = Loopback (localhost)
  • docker0 = Docker bridge

My take: Use ip addr. It's modern. ifconfig is legacy.

Interface States​

ip link show

States: UP = active, DOWN = inactive.

My take: Interfaces need to be UP to work. Check if they're UP.

IP Addresses​

Understanding IPs​

IPv4 format: 192.168.1.100/24

  • 192.168.1.100 = IP address
  • /24 = Subnet mask

Private ranges:

  • 10.0.0.0/8 = Large networks
  • 172.16.0.0/12 = Medium networks
  • 192.168.0.0/16 = Small networks (home/office)

My take: Private IPs are internal. Public IPs are internet. Most servers use private IPs.

View IP Configuration​

ip addr show                      # All interfaces
ip addr show eth0                 # Specific interface
hostname -I                       # All IPs

My take: ip addr shows IPs. Use it to see network configuration.

Ports​

Understanding Ports​

Ports identify services:

  • 22 = SSH
  • 80 = HTTP
  • 443 = HTTPS
  • 3306 = MySQL
  • 5432 = PostgreSQL

My take: Ports are like apartment numbers. Each service has a port.

View Listening Ports​

ss -tlnp                          # Listening ports (modern)
netstat -tlnp                      # Listening ports (legacy)

My take: Use ss. It's modern. netstat is legacy.

Real example:

$ ss -tlnp
State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
LISTEN  0       128     0.0.0.0:22          0.0.0.0:*
LISTEN  0       128     0.0.0.0:80          0.0.0.0:*

Port 22 (SSH) and port 80 (HTTP) are listening.

Testing Connectivity​

ping: Test Connection​

ping hostname
ping 8.8.8.8                      # Google DNS
ping -c 4 hostname                 # 4 packets

What it does: Tests if host is reachable.

My take: ping is basic connectivity test. Use it to check if things are reachable.

Test Ports​

telnet hostname 22                 # Test port 22
nc -zv hostname 22                 # Test port 22 (netcat)

My take: Test if ports are open. Useful for troubleshooting.

Common Network Commands​

hostname​

hostname                           # Show hostname
hostname -I                        # Show IPs
sudo hostnamectl set-hostname newname  # Change hostname

route​

ip route show                      # Show routing table

My take: Routing table shows how traffic is routed. Usually you don't need to change it.

Common Patterns​

Check Network​

ip addr show
ping 8.8.8.8
ss -tlnp

Troubleshoot Connection​

ping hostname                      # Can you reach it?
ss -tlnp | grep 22                 # Is SSH listening?

Common Mistakes (I've Made These)​

  1. Not checking if interface is UP: Interfaces need to be UP. Check with ip link.

  2. Wrong IP address: Make sure you're using the right IP. Check with ip addr.

  3. Port not listening: Services need to listen on ports. Check with ss -tlnp.

  4. Firewall blocking: Firewalls can block ports. Check firewall rules.

  5. Wrong network: Make sure you're on the right network. Check routing.

Real-World Examples​

Check Network Configuration​

ip addr show
ip route show
hostname -I

Test Connectivity​

ping 8.8.8.8
ping google.com

Check Listening Ports​

ss -tlnp | grep nginx

What's Next?​

Now that you understand networking basics, let's talk about SSH for remote access.

Personal note: When I started, networking was confusing. IPs. Ports. Protocols. Then I learned the basics. Now it makes sense. Networking is just connections. Learn the basics. The rest follows.