Where in NSX-T 3.0 would I go to allow certain port numbers to be accessible in urls?
For example, lets say https://abc-def.ghijklm:15300/xyz/here
I'm assuming you are hosting a website within a VM let's call it WEBVM on port 15300. You have to just allow traffic on DFW for that VM as DST on port 15300 or on Gateway firewall.
