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How it works...

Again, we're using the ip suite to interact with the various network elements of our machine. Frequently, these commands can be used for troubleshooting and testing of environmental changes, prior to your making those changes permanent.

In the adding or removing of IP addresses to interfaces, you can create a situation where an interface has multiple IP addresses for use with different applications.

Before the days of SNI, it wasn't uncommon to see multiple IP addresses in use on web servers so that each HTTPS site could have its own IP assignment.

In the shutting down or bringing up of network interfaces, we're first telling our system to administratively down an interface, rather than physically unplugging an Ethernet cable. If you down an interface, then check your /var/log/messages file—you should see something like the following:

Aug 12 12:38:09 centos1 NetworkManager[566]: <info>  [1534077489.1507] device (eth1): state change: activated -> unavailable (reason 'carrier-changed', sys-iface-state: 'managed')
Aug 12 12:38:09 centos1 dbus[545]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service'
Aug 12 12:38:09 centos1 systemd: Starting Network Manager Script Dispatcher Service...
Aug 12 12:38:09 centos1 dbus[545]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Aug 12 12:38:09 centos1 systemd: Started Network Manager Script Dispatcher Service.
Aug 12 12:38:09 centos1 nm-dispatcher: req:1 'down' [eth1]: new request (3 scripts)
Aug 12 12:38:09 centos1 nm-dispatcher: req:1 'down' [eth1]: start running ordered scripts...

Here, we can see the network manager running through a series of steps, upon its realization that the device's state has changed.

Network manager dispatcher is the service that controls what happens when an interfaces goes down or comes up. Take a look in /etc/NetworkManager/dispatcher.d/ if you're interested in learning more.

Lastly, when we added a route to our routing table, we were telling the kernel's underlying routing subsystem to send packets for the network 172.16.0.0/12 through our eth1 interface, with a gateway address of 192.168.33.11.

I have frequently found myself modifying the routing table of devices, though most of that was done for cluster setups in phone PBX environments. It may not be something you do day-to-day, but it's worth knowing!

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