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What is the period you sometimes see at the end of a domain name?

The root node of DNS has a null label

The DNS tree is restricted to 127 levels of depth so you could

. is used to mark a domain as absolute eg;

Behind the scenes, a full domain name would be<root/null>

Some websites, or perhaps more accurately the load balancers and proxies in front of them, don’t acknowledge the existence of such a thing.

One high profile example is Amazon. If you visit, you’ll see a blank page with the title x. Note the period on the end of the URL to see this issue in effect.