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I guess you have to know the network.
As a web engineer, you must also be well versed in networking. Creating a web service without knowing the network is like a midwife not being able to babysit a child.
I had looked up IP addresses, web servers, URLs, DNS, etc. and knew what they were, but networking is very complicated and that alone does not explain it at all.
So I did some additional research on the network.
Summary of Survey Results
trivia
MAC stands for Media Access Control.
MAC is called a physical address because it is an address attached to hardware, while IP is called a logical address because it is not hardware.
How to look up a MAC address
For Macintosh, you can see it in the settings screen.
The MAC address of pocket WiFi is here. 
How to check various addresses
How to look up an ARP address
arp -a
How to check netstat
netstat -rn
Reference Site
A clear diagram of the router can be found here. A view of the mapping between IP and MAC addresses. Structure of ARP protocol packets. Sample routing table.
Reference Books
The Nikkei network series is well known. This series has helped me a lot. Other recommended books are Why Object-Oriented Systems and Why Phones Work.
＼電子工作歴9年の私のイチ押し！ELEGOOの電子工作キット／
