determining subnet mask using ip but not using ifconfig in debian 9.0 (stretch)

[Dan Hitt <dan.hitt@gmail.com>]

What is the preferred way to determine the subnet mask of your box
using /bin/ip on debian 9.0 (stretch)?

In the old days, this sort of information would come out of ifconfig,
but it looks like debian really wants to get away from ifconfig, and i
think use ip as a sort of replacement.

So i would like to do the same thing, using ip or whatever the debian
people think is the best practice.

I tried to search for this information using google, but not very skillfully.

TIA for any info.

dan