How to update changes to external dynamic ip address?

I am running an ftp server to share files with friends, I need to find a way to keep my friends updated about changes to my ip address, maybe they could click on a link on my webpage to get updates ! I don't want to keep updaing the link on my webpage. I use google sites to link my ftp site, if there is anything that can update the link automatically or provide me with permenant link to allow visitors see the changes then this would be an ideal solution.