Dynamic IP and login to Admin via local when ip changes.

Hi there, I’m doing a wordpress website for my assignment, i’ve installed it on my windows xp machine using Xampp. I’ve also port forwarded to that machine so that i can access over the internet.
But the thing is that if i don’t enter the public ip (121.etc) i wont be able to see the page with its theme over the internet.

So what i did was put the public ip, that 121.72.etc.etc and i can see the theme and it works normal over the net. BUT i have Dynamic IP and my ip could change therefore leaving me no access (unless i go change it from the mysql tables). What should i do to fix this?

Any help will be appreciated, as this is my assignment i don’t want to purchase anything too costly.

Thanks