Ubuntu Release Portal houses all the ISO images of Ubuntu distributions (and it's sisters). Till now you could access the portal via HTTP, FTP and RSYNC at a universal address only i.e. http://releases.ubuntu.com. But now Ubuntu has made a small change to the way you access the Ubuntu Release Portal via FTP and RSYNC.

For FTP access to the Ubuntu Release Portal you now have to head over to: ftp.releases.ubuntu.com

For Rsync access to the same: rsync.releases.ubuntu.com

This is just a recommendation that Ubuntu has released for the time being via the Ubuntu Fridge as the old URLs to FTP and RSYNC still seem to be valid and working. But there is a possibility that Ubuntu will discontinue the old URLs and switch to the new URLs as mentioned above.