Enabling and disabling Ethernet in ubuntu 15.10

This tutorial is for Ubuntu Unity (the default version/download). If you have another flavor (MATE, KDE, GNOME, etc.), comment on this answer and I will update it.

1. Click the gear and wrench icon in the launcher to open System Settings. If you can't find this icon, or it isn't there, just search for it in the Dash (~Start Menu).
2. Once Settings opens, double click the Network tile.
3. Once there, select the Wired or Ethernet option in the panel on the left.
4. Toward the top right of the window, there will be a switch that says On. Flip it off.

That should disable the adapter. Just turn the switch back on to reset it.

Terminal Method (advanced, but works for all flavors)

1. Open a terminal by pressing Ctrl+Alt+T.
2. In the terminal, type sudo ip link set down eth0.
3. Enter your password when prompted and hit Enter (NOTE: you will not see anything being entered. This is a security feature, and your password is indeed being entered.)
4. Now, enable the Ethernet adapter by running sudo ip link set up eth0.

If you need any clarification, or have a different version of Ubuntu than Unity, please leave a comment and I'll do my best to help :-).