ADSL traffic problem via Planet modem - port 80 http blocked

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?
mbhhAsked:
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AutoSpongeConnect With a Mentor Commented:
If the ADSL router was hung, it may have had enough processor power/memory to respond to ICMP requests on port 110 but not able to respond to your HTML requests on port 80.  It's not that the specific port was blocked but that the ADSL was clocking processes and needed to be cleared.
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mbhhAuthor Commented:
AutoSponge - thanks but I don't think that explains why TCP port 110 was working ok from some addresses and ICMP working ok all round. Also I wonder what is ment by ICMP port 110 - port numbers only apply to protocols 6 and 17, not to protocol 1.
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TheKenmanConnect With a Mentor Commented:
Modems and routers are just like pc's - they do weird things at times, especially if it's been awhile since they were reset. Each device runs like a mini server with it's own CPU, RAM, ROM, WAN/LAN interfaces....and ARP tables, IP routing tables...if it has a web interface, then it runs a some light webserver or CGI implementation.

Could be the DHCP lease expired, and somehow the 1 pc obtained the new DHCP info instead of the router....or there was a slight power surge and the IP routing was corrupted...
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