Although your laptop will automatically bring your ethernet and wifi interfaces up and down for you, there are times when you will need to do that manually.
Debian provides two commands for this purpose:
ifup INTERFACE
ifdown INTERFACE
Your ethernet interface is typically referred to as eth0 and your wifi interface is typically referred to as wlan0.
So, to bring down your ethernet interface, you can type:
ifdown eth0
And, to bring it backup, type:
ifup eth0
