127.0.0.1 is not specific to an app. It represents the whole machine.  A particular port is used to specify which app within the machine.

127.0.0.1 is not some arbitrary thing that SM came up with. It is the standard loopback address for all IP stacks since the dawn of time. There is no other option for the loop back address. You can't change it. It's not a configurable option.  It's part of the standard.

You can configure (via hosts file) any number of aliases for it, such as the alias "localhost". it is this alias that is problematic. The standard doesn't require that the word "localhost" means 127.0.0.1 - that is just a convention.

The IP V6 standard for the loopback address is ::1, and this is somtimes also aliased to the name localhost. When the hosts file says localhost is both 127.0.0.1 and ::1, then it doesn't work at all. This is what Microsoft did wrong in the default hosts file for Vista.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)