04 Try it yourself Interactive
One fixed scene: an FTP client, a NAT router in front of it, and a server at 203.0.113.10. Both modes start identically, with the client opening the control connection to port 21 and logging in, then they diverge on one question. Both connections carry an explicit arrow for the direction they were opened in — right for a client dialling out, left for a server dialling in, dotted while nothing has been dialled yet — because direction is the entire teaching payload here. Switch the scene to a client on a public address and run active mode again: it works, which is the honest reason active mode was fine for a decade and then quietly stopped being. Nothing plays on its own — Next, Back and Reset, or the arrow keys.