Sooooo…
This is my situation. I am trying to “steal” internet access from my father in law (to be… .:)) He has cable, dynamic IP, through a Thomson TCM420 cable modem, that has only 1 RJ45. Id like to connect myself too, so I thought in using my old adsl router as a router to connect to the cable modem, so we 2 can share the access.
So what Id like to do is this:
Internet -> cable -> modem -> router -> 2 PCs
If I connect the modem to the router (through RJ45, I am not using the adsl connection of the router) and a PC to the router, I can see the cable modem and connect to its admin page, but cant connect to the internet (looks like I cant find a DNS).
The address of the modem is 192.168.100.1, and the router has 10.0.0.2. If I set my PC with static IP 192.168.100.2, I can see the modem but not the router. If I set my PC as dhcp client, I can see the router but not the modem.
Anybody can help me with this?
Thanks in advance 
The 192.168.100.1 address of the modem is the IP address on the LAN (Local Area Network), you’ve then got the router in a seperate subnet, which is fine but you need to set the subnet mask accordingly otherwise it wont route to the modem.
It would make more sense to put the Router in the 192.168.0.x subnet (give it a 192.168.0.1 IP) and set your subnet mask on the router to 255.255.255.0. Then set the default gateway on the router as 192.168.100.1
Set your IP address on your PC as 192.168.0.2, subnet mask as 255.255.255.0 and default gateway as 192.168.0.1 (the router).
What this should do is mean any requests outside of the 192.168.0.x subnet will go to the router, the router will route these to the cable modem.
The cable modem should have an external (WAN/Internet) IP address as well, which will be assigned by your ISP.
Yup, Keets is right Nepe, I was just about to say all that as well but he beat me to it! Smart Ass.
WB.
Same here yes WB just missed the boat again
Ming
Thanks all yes, even Joe
Still didnt get it working. If I set it up like Keets said (except for the fact that I do not see where to set up a GW for the router), my PC (192.168.0.2) does not see the modem (192.168.100.1) saying that the subnet is not reachable. So I tried setting everything on the same 192.168.100.x subnet. Then I see both modem and router, but still no access to internet. It could be that I need a null cable to connect router and modem (as per modem manual)
So I am trying to fing a null modem and set it up again
Btw, you think I need to disable the DHCP server on the router, and let that job to the modem?
Thanks again.
Btw, you think I need to disable the DHCP server on the router, and let that job to the modem?
No your Modem doesn’t handle DHCP for you, you still need to put a router in between the modem and the PC’s. If your current router is ASDL and you only have 4 network ports in the back of it, then I’m sure what you are doing is using it as a hub rather than a router. As a hub you will not be able to go out to the internet, you’ll be able to see everything your side of the modem as it will all be physically connected.
My advice would be to pick up a cheap standard router.
Sounds sensible, thx Brigs. I will need to look for a router, if the null modem cable doesnt work
Uffffff, at least I will learn something about networks
With the null cable between modem and router I can see the internet, finally
Problem is, only 1 computer can access it 
The service provider only allows 1 PC connection to internet, the modem checks the MAC and only allows the first card to access internet. What I need is that my router give different internal IPS, even if the modem only gives 1 IP…is it possible? Right now this PC has an external IP 217.x.x.x , the second can’t get one
How can I get diffrent internal IPs from the router, while having noly 1 external IP? The router DHCP server is active.
THX
In router settings, change mac-address to use the router mac-address.
Then the isp will be happy.
Thx Hans, that is something I will try if I find where to change the MAC address of the router (something I didnt know it could be done…isnt the MAC unique for each network card??)
If not, I will buy a new router, non-adsl, with NAT.
I just talked with the provider. It seems that is in fact possible to connect a router to the cable modem to give service to different PCs, all of them using the same external IP
So the problem now must be in my adsl router. As Brigs said, looks like I am using it as hub, not as a router. My only possibility now seems to be buying a normal router.
Thank you all. A beer is waiting for the first one coming to my house 
Nepe, I think that Hans is suggesting to put the Router’s MAC address in the Modem configuration: thus the router would be the unique WAN client of the modem; the router will be the DHCP server of all the PCs, given their two IPs.
This is what I meant:
MAC Cloning Enabled:
Some ISP’s may check your computer’s MAC address. Each networking device has it’s own unique MAC address defined by the hardware manufacturer. Some ISP’s record the MAC address of the network adapter in the computer or router used to initially connect to their service. The ISP will then only grant Internet access to requests from a computer or router with this particular MAC address. Your new D-Link router has a different MAC address than the computer or router that initially connected to the ISP. To resolve this problem, the D-Link router has a special feature that allows you to clone (that is, replace the router’s MAC address with) another MAC address.
(From help file for my D-Link DGL-4300)
Thx guys. I do not think that I can do that with my zoom router. Anyway it should not be necessary. I only need 1 external IP, given by my ISP provider, and that my router gives 2 different internal IPs to my PCS.- The problem is that I can not get my router to do that, probably because its an adsl router, and has a ADSL input instead on a WAN one.
So what it is doing now is just acting as a hub, and the modem then give an IP to the first PC connected, and deny access to the rest.
What Nepe has been trying to do is use an ASDL modem/router. It is not possible to use one of these devices as a standalone router. It will only act as a hub. He needs a dedicated router to sit between the cable modem and his network for all PC’s to see the internet. It may be possible that his ISP will allow multiple accounts to access the internet, in which case the asdl router will allow the registered MAC addresses to connect, however only one at a time.
He needs a new router, D-Link, Netgear or Linksys will all be easy to set up with minimum config needed. All are reasonable intuitive to use and use MAC address cloning. Or he can get the MAC address from the router, phone the ISP and give that address to them to use as his account MAC.
Done. Got a Linksys wireless router, everything works now. Thanks all