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In chapter 3.1.1 an IP address is characterized as xxx.xxx.xxx.xxx where 0<=x<=9. This would lead to invalid addresses like 300.999.0.1. Correct would be: x.x.x.x where 0<=x<=255, since the ipv4 address consists of 32 bit separated into 4 blocks of 8 bit each.

In chapter 6.2.1 you talk about the IP and that it is the local address of your computer.
From my understanding the local IP is aliased with localhost.
For Webservers the just tells the user that the server is listening on all addresses the computer is using.
With Firefox I can't even connect to while localhost:4000 or are working fine. It might be Safari or Mac specific that it works for you.