Nagios communication - NRPE doubt

[akp geek]

Hi all
                  I have a question on how NRPE works. I have installed
Nagios on one server1 and Nrpe on server2. they are on the same subnet.
the way I execute the commands on the server2
is  Server1:nrpe!commandonServer2

It works fine now. both servers are on the same subnet. But if I would like
to monitor a server outside the firewall, Is there particular port that
needs to be opened or how does that communication work?

Regards
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