What are the differences between Public URL: Internet, Custom and External when configuring Alternate Access Mappings in SharePoint 2019?
These zones, and their labels, are just labels. There is no functional difference between the five
Zone is a label representing a Public URL, the zone is used to ‘connect’ an Internal URL to a Public URL. The zone names has no relation what so ever with the four Internet Explorer security zones (Internet, Local Intranet, Trusted sites and Restricted sites) and could just as easily been named 1,2,3,4 and 5. A zone can also represent an authentication provider. Zones: Default, Intranet, Internet, Custom, Extranet
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The answer above is correct but let me explain it to you in a much simpler way .
The public URL is basically the URL that a user sees when he gets a hit to the site.
For example - If your default URL is
and you have made changes in the Alternate access mappings to point the URL
as a public URL then no matter even if a user hits the former URL he will always be routed to the latter .
That is why the name public URL because it's the URL the end user will always see.
Now the Zones are just placeholders for linking the Internal URL to Public URL that's it.
It doesn't matter which Zone you choose as long as the right configurations are being made.