port 1720 will be sent to the external network that fortigate is connected to and routed to andy 39 s machine at public ip address 64 58 79 231 andy 39 s machine is listening on port 1720 for call setup requests it picks up the setup request from lisa removes the header and looks into the message body to pick out the source ip and source port from there it understands that the lisa can be reached at source ip address 205 10 10 10 which is the external interface address of fortigate and at port 10000 it also understands that lisa is called on her in netmeeting by her alias lisa kelly lebeis com it then sends a tcp call setup reply back to lisa as illustrated in fig5 g the reply gets routed through the public network and arrives at the fortigate external interface steps 6 7 8 the fortigate unit and gatekeeper implement intelligent h 323 call setup when the call setup reply arrives at the fortigate unit 39 s external interface several things happen 1 the fortigate unit parses all incoming packets and recognizes the h 225 0 call setup reply packet by looking at the header of the tcp packet u2014 all source port 1720 packets are by design h 225 0 call setup packets 2 the fortigate implements u201c port forwarding u201d u2014 in this case it has been configured to route all packets with a source port 1720 to the gatekeeper 3 the gatekeeper on receiving the packet understands that the packet is for call setup and is a reply from the host of the netmeeting session the gatekeeper parses the message and understands that the call participant 39 s alias is lisa kelly lebeis com it.