Dynamic Domain Name Service (DDNS) is a service to associates a domain name to a dynamic IP address assigned by ISP that changes over time.
Users who subscribed internet services with dynamic IP addresses could use DDNS to provide a path to their resources, such as web services, Emil services, or file sharing services with a easily memorize domain name to access with.

There are several DDNS service providers in the market, such as dyndns, no-ip, duckdns, Freedns, Hurricane Electric, and many others not being mentioned here. One of the providers that still continually providing free DDNS service and I m currently using is no-ip.com.
Although I have never try all of them, I know dyndns and no-ip are using a similar way to run the updates, as I have try dens from both providers on Linux and screenos and they run flawlessly with some minor configuration changes. 
DDNS service can be run on some appliances that has DDNS caperbility, or using the providers designed program on windows, Linux, another operating systems. 
Applying an account is simple. The procedures pretty straight forward. The basic requirement is to have en email amount for registration and verification. For myself, I prefer to go for no-ip DDNS service since it has been in the market for some more that ten years, especially it still provides free service, and each subscriber gets to have up to 3 free DDNS domains. 
Every free DDNS service providers are providing similar services. But they are also providing some additional services that requires subscription fees, such as top level domain with bind with dynamic ip addresses, redirection service,more DDNS domains to apply for,  and etc. If the enhanced features or extra service don’t get your interest, then simply go for the free service is an option too. 
Later on i ll post the procdures of setting up this service at Juniper ScreenOS and Ubuntu.