Hey
dant,
This isnt actually anything to do with soapui, but i'll take a stab at answering.
Have you tried using tracert? Its a command line app that resolves domain names into public ip addresses, so this might give you your remote ip/ip range you need to update your firewall to accept content from that URL.
HOWEVER, i dont know much about this stuff, but i believe it could be waaaaay more complicated than just grab the ip and add it to your firewall. The file could actually be hosted on a different server and there could be a proxy, reverse proxy, ftp servers, etc. that repoints (FWDS) download requests to that other server and other scenarios i know even less about.
Also, just cos youve opened a hole in your firewall, you might also have to open up the ftp port on your firewall as without an enabled ftp port on firewall, the download (assuming it uses ftp to download files) wouldnt work.
The default ftp port is 21 if my memory's correct, but thats just the default and quite often admins change from the defaults for security reasons.
Have you investigated alternative methods of obtaining the files from the repo you need?
Ta
Rich