Web VNC Client Requirements?

[L.M.]

Hello,

Suppose the situation is the following:
----------------------------------------------
- running VNC server on host A (network A),
- running VNC client using an "Internet browser"
  and the "Java Runtime Environment" on host B
  (network B).


Problem:
-----------
HOW and WHY host B or its network (network B)
could prevent host B from connecting to the VNC server on host A?
Which could be the reasons?

  - Could the "Java Runtime Environment" be responsible for that?
  - Could it be because the VNC client on host B uses ports other
    than port 80 to communicate with the VNC server on host A?
    (Does the client also uses ports 5800 and 5900?)
  - In other words, are there any requirements on the client other
    than having an "Internet browser" and a "Java Runtime Environment"
    available?

I am only interested on the client side of the communication
(suppose everything is OK on the server side: no firewall, etc.).

Please help!

Thanks in advance,

--
Leon