6th computer stops internet working

[Kilrov]

Hey, tried searching but couldnt find any help regarding an issue we have with our network!

We have 5 laptops/PC's running happily off a demon connection, netgear router and switch but whenever we try connecting the final person in our house's laptop it stops everyones working!
It looks to me like a problem with his laptop - are there any settings that could stop it working on the network? (We have tried re-booting the switch and router, his windows firewall is off, network connection set to never dial, virus checked the harddrive). We can see him on our network but it stops the internet working for everyone, and as soon as he unplugs his network cable everyones works again - very odd! Also we have tried it with his and just 4 others connected but it still wouldnt work - all this appears to rule out a router or switch problem.

Any help appreciated, I know this isnt very clear,
Mike

[TheAnimus]

hub or a switch?

[Cronus]

What IP addresses are being used? Is it possible that his IP is the same as the router IP?

[Kilrov]

Hardware is a Netgear DG632 (ADSL modem router) and a Netgear FS108 switch,

i'll check the IP addresses - but correct me if im wrong, we can see his laptop on the network and have pinged it successfully from the router.

Thanks guys!

[TheAnimus]

also you should get a warning on the laptop because of address conflicts.

it could be a malfunctioning NIC, but on a switch the traffic shouldn't grind to a halt.

you using DHCP?

You might want to try networking just that laptop to another computer via a crossover with manually set IPs.

[Cronus]

Actually there would be a warning if there was a IP conflict

EDIT: Beaten to it

[TheAnimus]

it is possible to get a MAC address conflic, for a prank i once wrote a simple program which set the MAC address of all of the PCs to the same. These don't generate conflicts, they send networking gear nuts, even good managed gear.

now there are some viruses that do this, but i didn't think many were in the wild (due to the nature of the effect of the virus, it dosen't prophegate well!) I wrote one of these programs for my own network security (a little trick for people trying to join a honypot wireless network).