Dynamic Domain Name Server (DDNS)
When we use the US Postal Service we need an address to receive and send mail. Similarly on the Internet we need a unique Internet Protocol (IP) address that identifies us. When we rent service from an Internet Service Provider (ISP) they assign us an IP address. Unfortunately the ISP can re-assign that IP address at will, especially when there is a major outage or maintenance. In other words, the assigned IP address is dynamic, not fixed or static. Normally we don’t care, leaving the problem to our ISP.
However, if we wish to access our personal computer, home network, etc. we need to use the current valid address. If it changes intermittently without our knowledge we cannot access our home devices.
Access to Internet resources (e.g. web pages) usually use names (e.g. google.com) rather than IP address like 188.8.131.52. Globally, Domain Name Servers (DNS) maintain a correspondence table of IP addresses to names. This allows us to remember names rather than numbers.
A Dynamic DNS (DDNS) service allows us to update a name with a newly assigned IP address. Typically you need a DDNS client program running on a computer on your local network that periodically checks whether your ISP has re-assigned you a new IP address. When a change is detected, the client program will send a correction to your DDNS provider to update your name/address entry on the DNS table.
Most DDNS services charge an annual fee, but a few provide a free service for a limited number of name/IP address pairs. Typically they offer up to five free pairs which is more than enough for home use.
For example, KH7U.dynu.net connects to a router in Kailua serviced by Spectrum who changes the IP address intermittently.
I hope this this helps demystify DDNS services.