Disable Wired Ethernet at Boot

[Patrick Bartek <patrick.bartek@gmail.com>]

Hi! All,

It has become necessary to move this box to a different room where a
wired ethernet connection is impractical.  So, I'm going wireless with
it.  USB.  No problem with that, but . . .  What's the easiest way to
prevent the wired ethernet (built-in on motherboard) from starting up?
I was thinking of just commenting its stanza out
of /etc/network/interfaces. Any other more practical (or proper) way
that's 100% easily reversable?

FYI: This system is an atypical install of Stretch.  Started as
terminal only, converted to sysvinit, then added what I wanted. No
desktop was ever installed. Only X and a window manager -- Openbox.
Boots to terminal where login occurs, then startx, if desired. No
display/login manager, no network manager of any kind. Only udev
running. No other systemd stuff is.

Everything has changed since I last did something like this 10 years
ago. So, I just want to check to be sure.

Thanks.

B