Network consists of 3 XP workstations connected to the net via a switch and then to a Planet ADSL modem Conexant chipset.
A weird problem arose today where port 80 internet traffic was blocked from all PCs, but ping and DNS worked ok. Port 110 (pop mail fetch) worked from one workstation but not another. Port 80 LAN traffic also seemed to be blocked thus preventing access to browser management of the adsl router.
The problem was wrongly assumed to be either Microsoft XP firewall or a recent install of Eset NOD32 antivirus but ...
... problem finally solved by power-down reset of the ADSL router.
Was the problem the router or the adsl link status?
If the problem occurs again, how to further diagnose the problem?
Why did port 110 work from one workstation and not another?
Why was port 80 specifically blocked?