Wireless router and base station help

Question

Hello fellas -- I'll really, really appreciate any help I can get with this, so thanks in advance for any advice given. Here we go...
I purchased a MacBook (2.16 GHz Intel Core 2 Duo, 160GB hard drive, 2GB RAM) about two months ago. It runs Mac OS X Tiger (10.4.9). Previously, I had a Sony Vaio desktop in my downstairs bedroom, and an HP Pavilion desktop in my mom's office upstairs. I have Comcast "high-speed" cable internet service. Downstairs (where the Vaio was) I have a surfboard cable modem with three cords coming out of the back: 1.) a power cord (obviously); 2.) a cable cord that runs into the wall for internet service; and 3.) an ethernet-looking network cable. This ethernet/network cable would have normally run right out the back of the surfboard cable modem and into the network port on the back of my Sony Vaio, but in order to provide internet service for my mom's HP upstairs, the network cable went from the back of the surboard modem and into a D-Link wireless router that we bought. Another network cable then went from the D-Link wireless router into the network port on the back of my Vaio. This provided both my Vaio and my mom's HP with good, steady cable internet access.
When I bought this new MacBook, it came with an AirPort Extreme card (no base station yet) that allowed me pick up on the wireless signal from the D-Link wireless router and this provided me with internet service. To connect to the internet, I would connect to the wireless network that my computer called "default." By this point, I had shut down and unplugged my Vaio, so it's out of the picture now -- the MacBook has replaced it. However, I left the surfboard cable moden and the D-Link wireless router plugged in, and this way I was able to connect to the default network wirelessly with my MacBook, and my mom was still provided with wireless service from the D-Link router on her HP upstairs. This worked great for awhile, until eventually this default network started getting rediculously slow and started disconnecting itself every 30 to 45 seconds -- literally. For this reason, I went out and bought an AirPort Extreme Base Station at my local Apple Store.
First, to make sure it worked, I unplugged the network cable (the one coming from the back of the surfboard cable modem) from the back of the D-Link wireless router and I plugged it directly into the base station I just bought. In other words, I took the D-Link router out of the picture and plugged the base station directly into my surfboard cable modem. This worked great, and it's how I'm connected to the internet as we speak -- it's providing me with steady, fast service. Instead of connecting to the default wireless network that was slowing me down before, I now connect to the base station's network, called "Apple Network 2b1d46." There's only one problem, though -- it involved taking the D-Link wireless router out of the picture, so now my mom can't get on the internet with her HP. The solution seemed very simple -- instead of plugging the surfboard cable modem directly into the base station, I would plug the surfboard modem into the D-Link wireless router, and then plug that into the base station. I saw no reason why this wouldn't work, and I still don't, but for whatever reason, it didn't.
This is where I need help. When I try to plug it in through the router like that, I simply can't get on the internet. Everything else works perfectly -- the surfboard cable modem's lights are properly lit, as are the D-Link router's lights. The Apple base station's status light is a solid green like it's supposed to be, and strangest of all -- when I open Internet Connect to check my AirPort status, everything is fine there as well. It says "Connected to Apple Network 2b1d46," which is the new base station network that I should be connected to, and under that it says "Connected to the internet via Ethernet," yet when I try to open Safari, it says "You are not connected to the internet," which is in direct contradiction to what Internet Connect is simultaneously telling me (that I am connected to the internet).
I've tried alot of things to trouble-shoot this. I've tried FireFox and Opera, and they can't bring up any web pages either, so I know the problem isn't just with Safari -- it's that my computer just can't connect to the internet or bring up web pages, even though Internet Connect and everything else says that it's connected. I've also run the Network Diagnostics an uncountable amount of times, and all it can tell me, in all its wisdom, is that my internet connection appears to be working fine and that I must have spelled "www.apple.com" (my home page) incorrectly (it says the same thing about any other web page I try to open). I've ran the network assistant to create a new location, and it also gets the the end of the wizard and tells me that it can't connect to the internet. Finally, I've also ran the AirPort Utility multiple times and set up the base station in every possible way, but still nothing works.
On that note, I do have one question. When you run the AirPort Utility and choose "Assist Me", at one point during the wizard it asks, "How do you connect to the internet?" Right now, remember, I have the D-Link wireless router taken out of the picture (the base station is plugged directly into the surfboard cable modem). For the answer to this part of the wizard, I chose the option that says, "I use a DSL or cable modem with a static IP address or DHCP." Obviously, given that I'm on the internet as we speak, this is working fine. However, if I bring the D-Link wireless router back into the picture and plug the base station into it (the router), and then plug the router into the surfboard modem, should I still have the same option checked (the one thats says I use a DSL or cable modem with a static IP address or DHCP), or by connecting through the router, should I change my answer to one of the other two options. The second option says, "I use a DSL or cable modem using PPP over Ethernet (PPPoE)." The third option says, "I connect to my local area network (LAN)." I've tried all three numerous times, and none of them work -- I still have the same problem getting on the internet once I connect through the D-Link router, but I thought I could at least find out which one is the right option so I could stop having to keep trying all three of them. Can anyone tell me which of those three options I should pick when connecting through the D-Link wireless router (cable modem with static IP address or DHCP, cable modem with PPPoE, or local area network).
Sorry, I know that was a long one, but if anyone can answer the previous question for me and/or give me some general advice regarding my problem with getting the internet to work once I hook up the base station through the router instead of directly into the cable modem, I would appreciate it more than you can imagine.
Thank you very much for your time and help.
-- Andrew

Answer 1

I am doing something similar. I have the WAN port on my Linksys 4-port wired router connected to my DSL modem. My G5 and G4 computers are connected directly to the Linksys LAN ports. My older Airport Extreme (flying saucer) is connected to another LAN port on the Linksys. I had to setup the Airrport as a bridge in this configuration. I did this by using the Ethernet LAN port on the Airport instead of the WAN port. In the Airport Setup Utility I said to connect using Ethernet. This seems to work for me.
I also use manual IP addresses for everything but you will probably want to use DHCP.
Cheers,
-= Ken =-