Currently, YouTube videos can be accessed through two types of links: either the regular URL created by the user interface itself, since you click one video on the following:
https:
Or via the sharing URL created in the user interface by clicking the Share button:
https:
Regarding the second part of your question, which domains are required to access YouTube? Unfortunately, this is a moving target as YouTube changes. At the time of writing (March 2015), this list is as follows:
*.youtube.com *.googlevideo.com *.ytimg.com
In addition to the primary domains above, some secondary domains are also required for ad serving, etc. on most YouTube pages:
apis.google.com *.googleusercontent.com *.gstatic.com
Most of the traffic comes from * .googlevideo.com. Starting in late 2014, YouTube began to gradually enable SSL on the youtube.com and googlevideo.com domains. You should be aware of this if using this information on a filtering or caching device.
Also note that this has been tested through a browser; native clients (iOS, Android, etc.) can work in different ways.
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