Local Networking is fine, but Internet access is not only from certain machines

I have a very simple network.  There are 6 devices on an all-wired windows TCP/IP netowrk.  There are 2 windows XP machines, one windows 2003 server, 2 windows 2000 machines and one playstation.  There is a single SMC 8 port router attached to a cable modem.  All machines on the network can see the network shares of the local network and access the shared directories and files.  There is no DHCP running and all machines have their own ip address specified.  There is a switch between the router and three of the machines and the rest are connected directly to the router.  For some reason, the computers behind the switch are having difficulty accessing the internet.  I cannot ping a domain name or external IP.  All the other computers have no trouble accessing the internet.  At times all of the computers have been able to access the internet, but now it has stopped.  Unfortunately due to cable length, I cannot remove the switch to deeterine if ther is a problem with that.  Any suggestions?  I thought since all computers could see eachother that it wasn't a hardware issue, but I have no idea where to turn to for help with anything software related that might be blocking the access.  All computers have current up to date antivirus software and anti bot programs running.
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jkurznerAsked:
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Problem_SolverCommented:
What's the model of the switch?
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jkurznerAuthor Commented:
It's a netgear 5 port.  Unfortunately the access to it is blocked and I can check it out till the wife moves some furniture.
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Problem_SolverCommented:
Difficult to see what might be the problem, can only suggest, you might try knocking the clients attached to this switch to 10mbs half duplex for starters.
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barrymulhollandCommented:
Are the machines plugged into the switch on the same IP address range as the machines plugged straight into the router? (e.g. 192.168.0.x)
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jkurznerAuthor Commented:
RE: IPaddress

Yes, all the machines have specified IP addresses on the 192.168.123.x range.

The 2003 server is 192.168.123.1
2000 clients on 192.168.123.5 and 192.168.123.20
xp clients are 192.168.123.15 and 192.168.123.6
router is 192.168.123.254
playstation is 192.168.123.200

No DHCP server on the network
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Problem_SolverCommented:
Ensure DNS and default gateway are 192.168.123.254 on all clients
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jkurznerAuthor Commented:
DNS and default gateway are set to 192.168.123.254  Another odd thing is that sometimes one of the two machines that can't access the Internet, can.  Today, one of the machines was able to access the Internet without fail.  Then later this evening, nothing.  I'm trying to figure out what if anything has changed, but nothing is consistent except for the frustration.  I'm leaning toward NIC or the hub or switch.  I'm more inclined to blame the NIC.  I'm going to try and find the latest and greatest drivers if any and check back with you guys.
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Problem_SolverCommented:
The times I have encountered this type fault have been when a switch is not permitting more than one mac address, but that doesn't seem right for the setup you have, but worth checking the SMC is functioning OK.

ie I'm thinking the SMC might be blocking more than one mac per port, so try resetting the smc in between trying each PC connected to the switch in turn with the others off. If you have no fault then try 2 devices to the switch, if it then fails the smc is blocking the second mac and we know the fault.

It might be caused buggy firmware and you could try upgrading/downgrading to see if fault resolves. might be caused by any port mapping set up in the smc firewall/router.

you may of course have bad wiring not following 586A/B standards
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jkurznerAuthor Commented:
Some more insights:  First one of the problem machines was fixed last night.  Turned out to be a back door trojan that was taking over the LMHOSTS file and redirecting TCP/IP traffic.  (Damn kids)  In any event I've re-instructed my children on not turning off the virus protection software and from installing programs without my approval.  Therefore, all computers directly connected to the router and all computers directly connected to a hub behind the router are functioning.  However, I have one computer and one Playstation that are connected to a switch behind the hub behind the router that is not functioning with TCP/IP traffic.  File and printer sharing and access to all the other computers shared resources works fine, but no TCP/IP outside of the local network are working.  I can ping all devices on my local network from that problem machine without fail.  One other thing is that the cable coming in to the switch from the hub shows a collision light when plugged into the switch.  The cable seems fine, but I'm going to redo the connectors and see if that solves the problem.  If the cable were a problem, would I be able to see the other machines in the network and access their shared resources?  I did remove and reinstall the TCP/IP protocol, but the problem persists.
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Problem_SolverCommented:
I have seen this sort of problem in a network before where the previous non-tech had installed the wiring using 586A and 586B standards for wiring interchangably for drop cables and between devices causing intermittent connections so check the wiring which you are using is correct, which you are doing. The important address to ping is the gateway. If the collision light is on permanently then traffic will not get through and this could be due to faulty wiring a NIC transmitting packets so much that other traffic cannot get through reliably. You could download something like the 3com network supervisor and this would give a more visual indication of the problem for isolation.

http://techrepublic.com.com/5102-6329-1061903.html
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