I don't know if I completely understand you, but Skype indeed has an
connection
with a central server, but when someone tries to call you, he must be
able to
connect to you. In the Skype option menu you can specify a specific
port for the incoming
connection, but you can also specify to use port 80 or port 443 as
alternative. What
method is behind this?

Erik

If it doesn't accept incoming connections, then why do its incoming  
connections "have to be specifically enabled"?  :)

To the original poster: an application that has a listening TCP socket  
does indeed require that the router _somehow_ be configured to forward  
connection requests to that socket.  The two most common techniques  
involve manually configuring the router or using the "universal  
plug-and-play" protocol (by which a network application can obtain  
specific information from the router and/or configure the router to do  
specific forwarding).

Many routers support "port triggering", by which the router watches  
outbound traffic and if it notes a network client using some particular  
port (either locally or, more commonly, in the remote address), it  
automatically enables forwarding to that client temporarily on some other  
specified port or ports (which may include the original outbound port).  
Specifics on this vary from router to router.

You may also want to Google "nat hole punching".  It's more reliable when  
used with UDP than TCP (different techniques are used with each, and doing  
it using TCP requires lower-level access than sockets normally give you),  
and in either case it's not 100% reliable as it depends on undocumented,  
arbitrary behavior on the part of the router.  But depending on how  
important it is to solve the problem, it's something you might consider.

Note that if the router has literally "blocked" the ports, then the answer  
is "you don't".  Typically, the ports are only "blocked" in that the  
router doesn't know who to forward the traffic to.  This is addressable as  
described above.  But if someone's actually configured the router to not  
allow traffic on those ports to pass, then the only thing that will allow  
traffic on those ports through is to reconfigure the router so that it's  
no longer blocking traffic on those ports.

Finally note that a "router" is not the same as a "firewall".  Sometimes  
the two functions are combined into a single device, but a firewall's job  
is specifically to block traffic.  Either it's blocking traffic on  
specific ports or it's not.  If it's not, you have nothing to do, and if  
it is, nothing you can do short of changing the firewall configuration is  
going to unblock the ports.  Obviously, changing the firewall  
configuration is not something that would be done automatically by a  
software client without any user intervention.  Otherwise it wouldn't be  
much of a firewall.  :)

Pete