will this iptables rule work?

Will this work to forward netmeeting port to an internal machine (so they videoconference, cause right now the outsider can see the internalmachine(can get video) but we cant see the outside, probably the firewall is cutting as out... but I cant quite understand our firewall :-( ): iptables -t nat -p all --protocol tcp --source-port 1718 --destination-port 1718 -s 0/0 -d 192.168.0.4 This would append it to the rules(or tables, how do you say that in iptables jargon, I am an ipchains fan myself :-) ) right? Or would it erase my existing chains(tables)? Thanks! Raul P.D.: I cant send in my actual rules from iptables -nvL if you want.