If you have encountered this, you'll know that this port annoyance stops you from being able to use battlenet for gaming. After about 4-6hours of internet understandings from google and too many searches...

I also did log into my router on browser and create a allowed port rule for 6112-6119
Via: >> Log into your router and complete steps via [https *colonslashslash* portforward *dot* com/diablo/] "How To Forward Ports" section & [https *colonslashslash* portforward *dot* com/router.htm] for the type of router it is.
The found the solution to it, on Windows 8, is :

A. to download/install Diablo1 (from GOG)
B. Open firewall advanced new inbound rule;
C. Choose Port > TCP port > 6112-6119 > name it Diablo TCP
D. Then AGAIN Choose Port > UDP port > 6112-6119 > name it Diablo UDP
E. Then load the game, get the 6112 error message, log into battlenet, notice only chat is available.
F. Alt Tab,
G. Find the Newly Created Diablo inbound rules > edit them both to include private, public and domain checkboxes.
H. Go back in-game, log out of battlenet to main menu, re-log into battlenet.

Verified that this keeps battlenet available.

I have included step C & D but I do not know necessary they are, someone should try it without those steps and give feedback here.

Kougun's post below is valid information too.

The following guide is the best I have come across. It is updated and current for 2019.

https://d1legit.com/2019/03/31/diablo-1-hosting-on-battle-net-port-forwarding-guide/

Try with updated post / try Kougun's thread above.