This was an extremely useful post and solved my problem. Thank you - I was
becoming obsessed with this:-)
I have been attempting to stream to an HP4150 running RealOne with no
success.
This is solved by port mapping. The attached section from Draytek help
explains this:

When running NAT (Network Address Translation) the Vigor router takes the
single public IP address, allocated by your ISP and automatically passes
data between it and the local PCs on your private local network. However,
with only one address visible to the outside world, external users cannot
address specific local PCs inside your network.

Therefore, an 'unsolicited' TCP/IP packet to your Internet IP address
would arrive at your router but then the router wouldn?t know who,
locally, the packet is intended for.

What is the difference between Port Mapping, Open Ports and DMZ ?
In the previous section, we discussed port mapping to allow internal
network devices (e.g. servers) to be accessible from the Internet. The
Vigor router actually supports three variants of port mapping methods, as
follows :
Port Redirection - The packet is forwarded onto a specific local PC if the
port number matches that defined. You can also translate the port to
another port locally.
Open Ports - As Port Redirection (above) but allows you to define a range
of ports.
DMZ Host - This opens up a single PC completely. All incoming packets will
be forwarded onto the PC with the local IP address you set. The only
exceptions are packets received in response to outgoing requests from
other local PCs or incoming packets which match rules in the other two
methods.
When using combinations of these three systems, there is a priority
structure; i.e. if a rule in one method co-incides with a rule in another
method, then there is strict precidence, so that the result should be
predictable. The precidence is as follows :
Port Redirection   >   Open Ports   >   DMZ

For example, if an incoming packet's port number matches a rule in both
'Port Redirection' and 'Open Ports' then the packet will be forwarded onto
the local address defined in 'Port Redirection'.