c['slavePortnum'] = 10000
The buildmaster will listen on a TCP port of your choosing for connections from buildslaves. It can also use this port for connections from remote Change Sources, status clients, and debug tools. This port should be visible to the outside world, and you'll need to tell your buildslave admins about your choice.
It does not matter which port you pick, as long it is externally visible, however you should probably use something larger than 1024, since most operating systems don't allow non-root processes to bind to low-numbered ports. If your buildmaster is behind a firewall or a NAT box of some sort, you may have to configure your firewall to permit inbound connections to this port.
c['slavePortnum'] is a strports specification string,
defined in the twisted.application.strports module (try
pydoc twisted.application.strports to get documentation on
the format). This means that you can have the buildmaster listen on a
localhost-only port by doing:
c['slavePortnum'] = "tcp:10000:interface=127.0.0.1"
This might be useful if you only run buildslaves on the same machine,
and they are all configured to contact the buildmaster at
localhost:10000.