Locking my machine causes my network Connection to sleep
If your MacBook is plugged in I would try the following.


Change the Engery Prefs to not spin down the disk or sleep the computer when it's plugged in.
Activate the Screen Saver to turn on at a hot corner.
Set the Screen Saver prefs to require password to log back in.


Now see if activating the Screen Saver causes you to loose your assigned IP.

The alternate method is to manually assign an IP to your MacBook while you're at work.