Skip to main content

Networking Basics

Switching, routing, and gateways (one idea at a time)​

John now has multiple Linux systems.
They all work individually, but real systems are useless unless they can talk to each other.

This page explains how communication works, step by step, without skipping logic.

Switch (how two systems meet)​

Two systems talk to each other by being part of the same network.

A switch creates that network between systems.

John connects:

  • System A
  • System B

to the same switch.

At this point, the switch only knows how to pass packets, not who is who.

Network interface (how a system connects)​

To connect to a switch, each system needs a network interface.

John checks available interfaces:

ip link

He sees:

eth0

This interface represents the physical or virtual connection to the switch.

No interface means no network connectivity.

IP address (identity on the network)​

A switch alone is not enough. Each system needs an IP address.

John chooses a network:

192.168.1.0/24

He assigns IPs.

System A​

ip addr add 192.168.1.10/24 dev eth0

System B​

ip addr add 192.168.1.11/24 dev eth0

Now:

  • Both systems are on the same network
  • Each has a unique identity
  • They can talk to each other

Route (how Linux decides where to send packets)​

Linux does not guess. It uses a routing table.

John checks it:

route

This shows the kernel IP routing table.

Every outgoing packet is matched against this table to decide:

  • Where to send it
  • Through which interface

If no route matches, the packet is dropped.

Router (how networks talk to networks)​

Now John has two networks:

192.168.1.0/24
192.168.2.0/24

A router connects networks.

A router is simply a system that:

  • Has an IP in each network
  • Can forward packets between them

Example router IPs:

  • 192.168.1.11
  • 192.168.2.11

Gateway (how systems find the router)​

The router is just another device. How does a system know it should use it?

That device becomes the gateway.

A gateway is:

  • The next hop for another network
  • The “door” out of the current network

John adds a route:

ip route add 192.168.2.0/24 via 192.168.1.11

Meaning:

“To reach 192.168.2.0, send packets to 192.168.1.11”

This must be added on every system that needs access to the other network.

Default gateway (door to everything else)​

The internet has too many networks to define routes for each.

So systems use a default gateway.

Default means:

  • “If no route matches, send traffic here”

John adds:

ip route add default via 192.168.1.11

Now:

  • Known networks use specific routes
  • All other traffic goes to the gateway

This is how systems reach the internet.

Linux as a router (packet forwarding)​

John now wants his Linux system to act as a router.

By default, Linux does not forward packets between interfaces.

He checks:

cat /proc/sys/net/ipv4/ip_forward

Output:

0

This means forwarding is disabled.

Enabling forwarding (making Linux a router)​

John enables forwarding temporarily:

echo 1 > /proc/sys/net/ipv4/ip_forward

To make it permanent, he updates:

/etc/sysctl.conf

Adds:

net.ipv4.ip_forward = 1

Applies it:

sysctl -p

Now Linux forwards packets between networks.

What John understands now​

  • Switch connects systems in the same network
  • Interfaces connect systems to the switch
  • IP addresses identify systems
  • Routes decide where packets go
  • Gateways point to routers
  • Default gateway handles unknown networks
  • Linux can be a router, but only if forwarding is enabled

Networking works because everything is explicit.

Quick cheat sheet​

# Show interfaces
ip link

# Assign IP
ip addr add <ip>/<mask> dev <iface>

# Show routes
route

# Add route
ip route add <network> via <gateway>

# Add default gateway
ip route add default via <gateway>

# Enable forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward