More on port forwarding(ssh, netcat and amule!)

[houkensjtu <houkensjtu@gmail.com>]

Hi debianer!

I post a question about port forwarding yesterday and got quick reply, big thanks!
Now I still have sth. not clear and it can be described as:

I have a laptop in my home, which is connected to my router. Yesterday, I succeeded in open a ssh port(22) on router, and start ssh server on my laptop. Now I can access my laptop from office by ssh USER@my_home_external_ip.

Also, scan port by using netcat from office:
nc -vz my_home_external_ip 22
gives me:
Connection to my_home_external_ip port [tcp/ssh] succeeded!

Today, I tried to open another port on my router, let me call it 1234. And I set amule on my laptop to use port 1234 for both tcp and udp. Fortunatelly, I got a High ID as expected.

And then I got confused.
I tried to scan port 1234 from my office:
nc -vz my_home_external_ip 1234
it says:
nc: connect to my_home_external_ip port 1234 (tcp) failed: Connection refused

However, after I remote access my laptop, launch amule on it, netcat now can:
Connection to my_home_external_ip 1234 port [tcp/*] succeeded!

It seems that, not only on the router, but also I should open a specific port on my laptop, otherwise netcat will not be able to connect from outside my home.

I wonder why this happens and what is the mechanism behind it.
Is it possible to open a port in debian, without launch certain software so I can netcat from outside to my home laptop?

Thanks!