FTP gives unknown host

I am setting up ftp through dial up networking. I use a network modem from an nt workstation with sp6a. I establish the connection and open a command prompt to ftp, when I am logged in to our network and do this it gives an “unknown host” error. when my co-worker logs in (same access level) it works fine. He is the only one it will work for and all the settings are the same. any Ideas why his works and no one elses does?? I have checked the host files and other system files but they should all be the same for who ever is logged in. I am stumped…