Computers use IP addresses to locate and talk to each other on the internet in the same way people use phone numbers to locate and talk to one another on the phone network.
When you connect to the internet you are assigned an IP address and this is usually provided dynamically. If you have a dynamic IP address, when you disconnect from the internet, the IP address goes back into the IP address pool so it can be assigned to another user. This means that when you connect again you will probably get a different IP address.
A dynamic IP address is not suitable if you want a computer to be permanently visible on the internet. If you want a computer to be visible on the internet, you should use a Static IP address.
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