Now, DNS is largely used for this purpose.

Um, that does not really make sense.

-mail.microsoft.com -ftp.microsoft.com -irc.microsoft.com -print.microsoft.com

These are all domain names and DNS servers are used to resolve these to different IPs. Always has done that job and still does it today. But microsoft.com is a single domain name and DNS can only resolve it to a single address (or set of addresses). So cannot be used to tell different services apart.

DNS does not know anything about http or https or any other protocal like that. It just resolves the domain name to an IP/set of IPs. The only thing it can do is point www.microsoft.com and microsoft.com to the same location so that you can use either to get to the webservers.

In unit testing, a “unit” does not have to be the smallest possible section of code. It can be a while class or module or even set of related classes/modules. Testing individual functions in isolation leads to brittle tests that easily break during refactoring. Testing overall system behaviour results in more robust tests that require fewer changes during refactoring which gives you more confidence then you have not introduced a regression.