tnx - it helped, but I'm not home free.
Network-->Router Mode showed "Off (Bridge Mode)". Airport Utility-->Help said to change mode to "DHCP and NAT". When I did that, and clicked (+) below Port Settings, THEN I got the screen you sent. Tnx.
The IP address was automatically filled in for the Mac. It's apparently the within-LAN address: 10.0.1.201 rather than the external IP address: xxx.yyy.1.7 shown in the internet-->IPv4 Address pane of the Airport Server interface. Not knowing any better, I left the 10.0.1.201 as it was.
When I clicked on Update, I got a diagnostic that an error had occurred, and that if I clicked "Forget", the device would be removed from the network. So I did that, and the router avatar disappeared from the picture. That was disconcerting, since the Airport Extreme box (and the LAN it contains/supports) is my gateway to the Internet. By unplugging the router for 2 minutes and re-powering, I got operation back - in Off (Bridge Mode) again. So near, and yet so far... What enhancement to my tactics-set do you suggest next? BTW, today I acceded to router firmware-version update, from 7.6.1 to 7.6.4. That at least seems to have gone well.
It seems that your wall-clock time lags mine (Dallas TX) by 9 hours (or precedes it by 15 :-) ). I infer that you operate from eastern Australia. I visited Sydney once, and Canberra, and the radio astronomy facility at Parkes. Got to see a billabong pointed out to me. Fond memories all.
I'm trying not to be a Gulaw - really I am...