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ADSL/Cable modem to WAN or Ethernet port?

Discussion in 'Cisco/Linksys Wireless Routers' started by casioguy, Sep 6, 2006.

  1. casioguy

    casioguy LI Guru Member

    I have cable at home and ADSL in the office. Both are connected via ethernet cable to Linksys WRT54G v5.0 routers. Question : when I connect the modem to the WAN port of the router, I get no internet connection to the computers linked to the wireless network. However, as soon as I plug the modem ethernet cable to one of the ethernet ports of the router (as opposed to the WAN port), I get internet connection right away. This is true for both the ADSL and cable modems.

    Logically, I should connect to the WAN port, right? For the ADSL modem, DHCP is enabled, so I disabled DHCP on the router.

    I would apprecaite any feedback on this issue.

  2. HennieM

    HennieM Network Guru Member

    You probably have a router-modem, not just a dumb modem for both cable and ADSL. The modem already does NAT, and probably has a web interface of its own through which it can be configured.

    If you want your WRT to manage your internet connection, you must configure the modem in "bridge mode" or something similarly termed, i.e. the modem must become dumb. In this scenario, you will connect the modem to the WRT's WAN port.
  3. DocLarge

    DocLarge Super Moderator Staff Member Member

    I couldn't have said it better myself, HenniM....

    Casioguy, what type of adsl router/modems do you have?

  4. casioguy

    casioguy LI Guru Member

    My modem types

    For ADSL, I have a Dlink DSL500T. In this case, you are right in that it is a modem-router. So, should I disable DHCP on the modem and enable it on the Linksys? I am not too sure about the setting for bridge mode though as I don't such an option when I logged into the modem via my browser.

    As for the cable modem, I am using a Motorola, and since I am now in the office, I am unable to verify the model number. However, what is strange is that I am able to get an Airport Extreme Base Station to work with the cable modem, while plugged into the WAN port of the Airport unit. With the Linksys, it was a no-no.
  5. Guyfromhe

    Guyfromhe Network Guru Member

    the linksys probably isn't working because you have DHCP disabled on it.
  6. HennieM

    HennieM Network Guru Member

    The thing is not "how to get it to work", but rather "what do you want to do?", followed by "configure the devices to achieve what you want to do".

    I'll mention a few scenarios, with example subnets, and there can be many more...

    (i) --inet-- NAT_modem/router -- WRT_LAN_PORT-- wired+wireless - ALL your devices are on the same subnet, and the WRT just funtions as an AP. Either your modem or your WRT can be the DHCP server. The modem has an internet side IP address, and a homenet IP address (like

    (ii) --inet-- dumb_modem/router --PPPoE-- NAT_WRT_WAN_PORT-- wired+wireless - The WRT does NAT, must also be the DHCP server, and the WRT functions as a GATEWAY. The modem has NO IP address (as it only does ADSL/cable modulation - it does not speak IP), and the WRT has an internet IP address, and a homenet IP address (like

    If you want to get fancy:
    (iii) --inet-- NAT_modem/router -- WRT_WAN_PORT-- wired+wireless - The (intelligent) modem/router has an internet IP address, AND a homenet2 IP address. The WRT has a homenet2 IP address, AND a homenet1 IP address. Your other devices are on the homenet1 subnet, and the WRT funtions as a ROUTER. The modem/router supplies DHCP for homenet2, and the WRT must supply DHCP for homenet1.

    I can't think of a reason why somebody would want (iii)??, but your Airport probably had a setup similar to (iii), which is why it worked through its WAN port.

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