Subneting question

[CCNA Newbie]

I have been studying for the CCNA exam. Right now I"m studying
subnetting so I thought I might investigate the network where I work.

This is what I found.

10.31.1.125 with a subnet mask of 255.255.255.0

Other computers have IP addresses like 10.31.1.211 with a subnet mask
of 255.255.0.0

According to what I have learned 10.31.1.125 is not a valid host
address for a subnet mask of 255.255.255.0
Am I reading the book wrong? I can ping 10.31.1.211 from 10.31.1.125.
There is no router between them. HOw is this working..? Is windows
ignoring the third octet of the subnet mask for 10.31.1.125?

Newbie

[Ted the Cisco Instructor]

Ok looks like a mis configuration on a desktop at the subnet config.
with a 255.255.255.0 subnet vaild hosts would be 10.31.1.1 -
10.31.1.254
with a 255.255.0.0 subnet vaild hosts would be 10.31.0.1 -
10.31.255.254
Notice these rages overlap. that is probably why your PC are
communicating.