Subject Can't connect to Linux FB 2.5 remotely

We have set up a FB 2.5 on a Linux server. We can access it locally from the server, but when we try to access it from any other computer on our local network (from which we can successfully ping the server), we get:

Statement failed, SQLSTATE = 08006
Unable to complete network request to host "servername".

The command we are using to try to access it is like:

isql -u username -p password servername:aliasname

The exact same command works on the server itself, omitting the server name of course.

We are quite sure that there is no active firewall stopping the connections.

Any bright ideas? Thanks.