This is a slight rehash of one of my previous articles ( http://computer-aid.com.au/blog/2006/04/28/connecting-a-dsl-modem-to-a-4-port-router/ ).

A customer had an optus cable internet modem (connected to his pc via usb). He had purchased a wireless router (d-link) and asked me to set it up (as well as a wireless pcmcia card for his laptop & a second computer).

At first glance, I think: hey, easy… its not a telstra cable internet, and I once had optus cable internet & a router connected at home, so maybe 1 hours work).

Well, I'm now so used to adsl, that I forget to do some reasonable checks:

1. When I change anything, refresh the network settings (or even restart all the network devices)… Not doing this can cause confusion & excursions down 'one-way streets' (ie: what the h*ll is going on here?).
2. If something doesn't make sense, keep investigating, until you understand it.

When the PC is connected directly to the modem, the ipconfig /all command shows the optus gateway & dns servers. I setup the modem & router to use the same IP subnet (192.168.100.xxx). Figuring out the modem IP address was tricky… as it doesn't show up in ipconfig! I had to go to the motorola website to find out!

I disconnect the usb between the modem & the PC, then connect the modem to the router WAN port via ethernet, then PC to router (ethernet)… but it doesn't work well (here is where I should have restarted the modem/router/PC)…

I figure the modem isn't made to work on the wan port, So I'll use the "messy workaround" (ie avoid using the WAN port). It works, but ipconfig still shows the optus gateway & dns servers. This is where alarm bells should have rung, and I should have followed point 2 above.

I now setup the laptop wireless card. It connects to the router, but no internet connection. I also try the second PC, & it also cannot connect to the internet.

I'm really puzzled… 3 PCs connected to the router, but only 1 can see the internet… why???

Eventually, I try 3 wan port configurations (dhcp enabled on only the modem, and dhcp only enabled on the router, and dhcp enabled on both). After doing a correct restart every time, I find that the ipconfig settings look normal (ie gateway & dns settings correspond to the the router address) when only the router is setup for dhcp.

I then restart the laptop: hey, it works! The other PC also works now.

Although the cable modem is slightly different from an adsl modem, the overall setup is still very similar.