How to add a default route with ip route

Adding a default route with ip route tells the Linux kernel where to send traffic when no more specific route matches the destination. This route usually decides whether a host can reach networks beyond its directly attached subnets.

A normal default route lives in the main routing table and points to a gateway that is reachable through the selected interface. Advanced cases can place the route in another table for policy routing or use onlink for a special nexthop, but the standard form is simply default via <gateway> dev <interface>.

Default-route changes require elevated privileges and take effect immediately in the running routing table. They are not persistent by themselves, and ip route add default is for creating a route that does not already exist in that table, so changing an existing default path should be done deliberately instead of stacking commands blindly.

Steps to add a default route with ip route:

Show the current default route before making changes so you know whether the main table already has one.
```
$ ip route show default
default via 172.17.0.1 dev eth0
```
Add the default route through the chosen gateway and interface.
```
$ sudo ip route add default via 192.0.2.1 dev eth0
```
Append table 100 only when policy-routing rules are meant to use a custom table instead of the main table.

If this command returns RTNETLINK answers: File exists, the table already has a default route. Remove the old route first or use ip route replace default via 192.0.2.1 dev eth0 when the goal is to change the existing default path.
Verify that the default route now points to the expected gateway.
```
$ ip route show default
default via 192.0.2.1 dev eth0
```
Persist the same default route in the system's normal network configuration if it should survive interface restart or reboot.

Commands run with ip route usually change only the live kernel routing table and are lost after reboot unless the distro's network settings are updated as well.