Gentlemen,
I feel like a proper fool. Sure I had problems, but I should have looked in the right place, i.e. the switch/router (DI-624). Since it also routes wireless and I am located in a city center, I, some time ago, thought I should be clever and lock out anyone who was able to catch the wireless signal.
Consequently, I implemented MAC filters, so that only those MAC addresses which were explicitly listed would be able to access the Internet. I did not know that it also included reaching the router part of the device, only the switch part would be accessible. Now, to set this on the DI-624 you have to go to the 'Advanced' tab and there select 'Filters'. On the page which then appears, on the third line, it has two check circles, the first labeled 'IP Filters' and the second one 'MAC Filters'. The first one is checked by default. Not until you move the check to the second option do you see the MAC filters.
Needless to say, the MAC address of my problem computer did not appear in the list of MAC addresses allowed. I entered the proper detail and, bingo, it all works!
I greatly appreciate all the help I got, especially from Rick Stevens but also Jeff Vian! Thanks a million. The list works, bad language or not - which by the way does not offend me, it rather shows some qualities (good or bad?) of the people who use such words. It may be wise to remember that you catch more flies with honey than vinegar.
Regards, Mats
