Linux Routing Basics (ip route)

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Routing is the process by which a Linux kernel determines where to send network packets. Whether a packet is destined for a local server or a remote website, the Routing Table acts as the system's internal map. In modern Linux, the ip route utility is the primary tool for inspecting and manipulating this map.

graph TD
    User([Linux Server]) --> RT{Routing Table}
    RT -- "Match found" --> GW[Specific Gateway/Interface]
    RT -- "No match" --> Default[Default Gateway]
    Default --> Ext((External Internet))

    classDef client   fill:#dbeafe,stroke:#93c5fd,color:#1e3a5f
    classDef gateway  fill:#ede9fe,stroke:#a78bfa,color:#3b1f6e
    classDef route    fill:#fef9c3,stroke:#fbbf24,color:#78350f
    classDef external fill:#f1f5f9,stroke:#94a3b8,color:#1e293b

    class User client
    class RT gateway
    class GW external
    class Default route
    class Ext external

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1. Viewing the Routing Table

To see the current map of network paths, use the show object of the ip route command.

ip route show

Common Output Fields:

• default: The catch-all route for any traffic not matching other entries.
• via: The IP address of the gateway (router).
• dev: The network interface (e.g., eth0, wlan0).
• proto kernel: Indicates the route was created automatically by the kernel.
• metric: The priority of the route (lower values have higher priority).

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2. Managing the Default Gateway

The default gateway is the most critical entry in your routing table. Without it, your server cannot communicate with any host outside its immediate local subnet.

Adding a Default Gateway: If your server loses its gateway, you can manually restore it:

sudo ip route add default via 10.0.0.1 dev eth0

Tip

Always verify that the gateway IP (via) is reachable within your local subnet before adding it, or the route will remain inactive.

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3. Creating Static Routes

Static routes are manual entries used to direct traffic for specific IPs or subnets through non-standard paths.

Host Route (Single IP): Direct traffic for a specific server through a dedicated interface:

sudo ip route add 1.1.1.1 dev eth0

Network Route (Subnet): Route an entire subnet (using CIDR notation) through a specific gateway:

sudo ip route add 192.168.100.0/24 via 10.0.0.1

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4. Deleting Routes

To avoid routing conflicts or clean up temporary paths, you can remove entries using the del command.

sudo ip route del 192.168.100.0/24

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5. Persistence and Runtime State

Commands executed with ip route are applied directly to the kernel's runtime memory.

Important

These changes are temporary. If the system reboots, all manually added routes will be lost. To make them permanent, you must add them to your distribution's network configuration files (e.g., /etc/netplan/*.yaml on Ubuntu).

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🧠 Quick Quiz - Routing Basics

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Which keyword in the routing table represents the path for all traffic that doesn't match a specific subnet?

gateway

any

default

static

The 'default' keyword acts as the catch-all destination for external traffic.

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What is the effect of running 'sudo ip route add 1.1.1.1 dev eth0'?

It creates a host-specific route for a single IP address.

It sets the default gateway for the entire system.

It deletes the routing entry for 1.1.1.1.

It enables the eth0 interface.

Host routes are used for specific IP destinations, overriding more general network routes.

#

True or False: Routes added via 'ip route' will persist after a system reboot.

True

False

Changes made with 'ip route' are applied to runtime memory and are temporary unless added to configuration files.

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📝 Want More Practice?

To strengthen your understanding and prepare for interviews, try the full 20-question practice quiz based on this chapter:

👉 Start Networking Quiz (20 Questions)
(If there is no exact matching directory, default to the top-level topic quiz page URL).

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