Routing Tables & Gateways

Imagine your computer needs to send data to another device on a different network. How does it know which path to take?

That's where the routing table comes in! It's like a map for your operating system, containing rules that tell it where to forward network traffic.

• Each rule specifies a destination network or host.
• It also points to the 'next hop' or interface to use.

In Linux, the primary command to view the routing table is ip route show (or just ip route). Let's see what it looks like:

When you run ip route show, you'll see entries that typically include:

• Destination: The network or host traffic is headed to.
• via: The gateway (next hop router) to send traffic through.
• dev: The local network interface to use (e.g., eth0, wlan0).
• src: The source IP address your system will use for outgoing packets.
• metric: A preference value; lower is generally preferred.

One crucial entry in the routing table is the default route. It's often shown as default via <IP> dev <interface>.

This is your network's 'gateway of last resort'. If no other specific route matches the destination IP, traffic is sent to this default gateway. This is usually your router that connects you to the internet.

You can manually add routes to the routing table. This is useful for directing traffic to specific networks that aren't reachable via the default route, or for creating isolated network segments.

Use ip route add to add a temporary static route. Remember, sudo is often required.

After adding a route, it's good practice to verify that it's present in the routing table. Just run ip route show again and look for your newly added entry.

Just as you can add routes, you can also remove them. This is useful for correcting mistakes or cleaning up temporary configurations.

Use ip route del with the exact same parameters you used to add the route.

A gateway is a network node that serves as an access point to another network. It's essentially a router that forwards packets between different networks.

When your computer sends data to a destination outside its immediate local network, it sends the data to its configured gateway, which then handles the forwarding.

Routes added with the ip route command are temporary. This means they will be lost if your network service restarts or if the system reboots.

For permanent routes, you need to configure them in system-specific network configuration files. The exact location depends on your Linux distribution (e.g., files under /etc/netplan/, /etc/network/interfaces, or in network scripts).

You just learned how to inspect and manipulate routing tables. Let's test your understanding!

Great job! You've learned the basics of Linux routing tables and gateways.

• Routing tables guide network traffic.
• Use ip route show to inspect them.
• The default route is for unknown destinations.
• You can temporarily add and delete routes with ip route add and ip route del.
• Gateways are next-hop devices for reaching other networks.

Understanding routing is key to troubleshooting network connectivity!