cannot get ip automaticly from modem speedtouch

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in my class i have 15 computers now they connect to switch with 24 ports and this switch connect to modem speedtouch this modem configure as dhcp to give ip automaticly to computers when they start now the problem is that all computers dont get ip automaticly so i need to give them manualy so what can be the problem?with the modem or with the switch?
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If when you give the IP manually, they can access the router and have internet access, then it is the DHCP configuration on the router.
If you don't have a connection to the router, even after you configured static IP's, then It most propabley the router problem (not configure, not connected)
Try to connect a machine (or couple of machines) directly to the router and see if all is ok, if so, check your link between the router and switch (change cable, change port,..etc)

There are two main things to check here: One, is your router dhcp service setup to give out the number of IP's you need? Second, are ALL of your pc's setup to recieve a dhcp address automatically? You mentioned you were setting them manually, and I hate to mention this but are you sure they were set to automatically get an IP?

If your router is setup to 'give out' at least 16 IP's, (you said you needed 15) and ALL pc's are setup to 'get' an IP automatically then I would suspect the switch first.  Why? Because the router appears to be doing its job in handing out at least some IP's correctly, but all the traffic has to go through the switch.  If the switch has a problem it could easily be dropping a bit from the Ip address packets and causing a dhcp failure. Only way to prove it is to replace it or bypass it for testing.

One last test could help  you isolate the switch from the router.  Take a cable from one of the pc's that is not getting an automatic IP and plug it directly into your router, if you get an IP then its the switch, if you don't, then there must be something wrong with that pc.  In that case, try winsock fix if its an xp machine.  Or goto the device manager on that pc and unistall the nic card driver and do a search for new hardware and see if the new drivers fix the problem.

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