connecting problem between vnc server & viewer

[Romain GUILLOT noos]

Hello.
I'm trying  to use vnc to control remotely my home computer from my office.
Till now, I did not manage to get a proper vnc connection across the internet.
I think I have a simple firewall problem, but I have no idea how to solve it
:
I have a cable connection at home and a dsl at the office. I am behind a
firewall at the office and I have a soft firewall at home  too(zone alarm),
which is properly configured to accept connection from my office IP.

Curiously, I can't ping my server's ip from the office : I get a timout error
message.
I have a http server and a ftp server installed on my home computer and I can
access them normally from the office.
But conversely, access to my office computer seems to be restricted from the
outside as I cannot connect via http or ftp using the office IP.
Thanks in advance for your help !

Romain