Does a TCP Server connect to a specific device?

[j***@gmail.com]

Hi There,

I have the following problem:

I have two connections on my PC, a normal Ethernet port and a GPRS
module (setup as a normal dail-up modem). I want to connect to
another PC with also a GPRS module as a modem.

After dailing into the ISP network, I can ping each PC from the
other. However, when running a small TCP client and server, the
programs fail to connect. When I plug in the Ethernet cable, setting
up a small network, the client and server communicate perfectly when I
use the IP adresses of the "Ethernet network". Does Winsock connect
only to a specific network device (which I doubt) or if I run a
server, does it look at all the IP addresses, not only those from a
specific device (like the Ethernet port).

What confuses me is that with the above setup, a UDP server and client
works fine with both the Ethernet and GPRS network.

Your help and suggestions will be greatly appreciated
Jaco

[Peter Duniho]

Post by j***@gmail.com
[...]
After dailing into the ISP network, I can ping each PC from the
other. However, when running a small TCP client and server, the
programs fail to connect. When I plug in the Ethernet cable, setting
up a small network, the client and server communicate perfectly when I
use the IP adresses of the "Ethernet network". [...]
What confuses me is that with the above setup, a UDP server and client
works fine with both the Ethernet and GPRS network.

There are no fundamental problems with what you're trying to do. It
should work. The fact that the PCs do succesfully communicate by ICMP
(ping) and UDP make it clear that there is a valid point-to-point route
between the two PCs.

So, you should probably be looking at various specifics about the
configuration. Perhaps some sort of firewall or NAT router that for some
reason is letting other traffic through but not TCP. Or maybe port
numbering issues. Double-check everything involved, and you will likely
eventually find the problem.

If I had to guess, I'd suspect something about the way the ISP handles the
GPRS network connection. But I can't say for sure.

Pete

[j***@gmail.com]

Thank you,
Jaco