This automatic assignment of IP addresses is accomplished by aDHCPserver.
A DHCP-server-assigned IP address is calleddynamicbecause it will often be different on future connections to the data pipe.
Where Are Dynamic IP Addresses Used?

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What Are the Advantages of Dynamic IP Addresses?
Another advantage of using dynamic IP addresses is that it’s easier to implement than static IP addresses.
What Are the Disadvantages of Dynamic IP Addresses?
This is a major reason most people don’t set up such a service out of their house.
This is why users might opt for adynamic DNSservice.