DHCP problems - WRT54G v1.1

Discussion in 'Cisco/Linksys Wireless Routers' started by lilmhz, Oct 24, 2006.

  1. lilmhz

    lilmhz Network Guru Member

    I've had the Linksys WRT54G v 1.1 for a few years now, it worked fine with hyperwrt v 2.0 until a few days ago. My main computer uses .1.100 as a static address and DNS stopped working for that address.

    I tried using the address on other machines and they could not do DNS but could PING the gateway and an external ip.

    Trying to figure out what was wrong, I reset the router to factory defaults and tried to release and renew the IP on the router. It didn't work, so I hooked my laptop up to the cable modem and ran dhcp to it and there wasn't a problem connecting, so I know that the cable modem isn't to blame.

    The computers (wired) on my network get DHCP via the router and can't even get an internal address now, but the wireless can.

    This is the biggest thing I don't understand: When I plug the cable modem into the wan port on the router, the wireless connections get the ip address that the ROUTER is supposed to get (the external one).. My cable modem is NOT a wireless gateway! I have attached some screenshots.

    I don't need any special settings, dhcp is supposed to configure it automatically.

    DHCP wont work for the router connecting to the cable modem, or the computers on the wired network. I think it could just be a fried switch possibly?

    I tried plugging everything in the correct order, doesnt work, the Internet light on the router blinks constantly but the cable modem doesnt do anything.

    Router (Linksys WRT54G 1.1) wont get DHCP from the cable modem(motorola SB 5120), but a computer can get DHCP from the cable modem. Computers on the internal network cannot get DHCP from the router now, and getting DHCP from the cable modem via the router wont work anymore.

    Is my linksys dead?

    Attached Files:

  2. lilmhz

    lilmhz Network Guru Member


    Well, I just came from a testing enviornment and concluded the following:

    The switch (ports 1-4) is inoperable:
    Computers using static ip addresses cannot communicate with each other or the gateway.
    Computers using dhcp cannot retrieve an address via the wired network.

    However, the wireless is pretty good (still working):

    If I plug just the internet(dhcp) into the internet port, it will give the wireless client the credentials of what the router is supposed to get from the Internet port.

    So say if I plug in my cable modem to the internet port, the first wireless client will get the DHCP from the cable modem. Every other client will receive an internal address and will not work.

    Overall, I can still use this as a wireless gateway.

    I love this router and will be buying a new one. Props to cisco for making such a good consumer router!

    Any comments?
  3. rickyschrieber

    rickyschrieber LI Guru Member

    It sounds like your cable company has changed their DNS server.
    Since you have chosen a static IP you need to see what the new DNS IP address is on the router.
    Assuming that the router provides DHCP server to your LAN, it pushes the DNS addresses to its clients or runs a DNS server for benefit of clients.

    Usually cable modems only allow one MAC address to map an IP address.
    That means you have to power cycle the modem if you change what is connected to the interface!!!

    I suggest you factory default settings on your router; set wireless security to WEP or WPA + pre-shared key and go from there. If your server is DHCP client you could make an IP address reservation to force it to a specific address.
    If you are using a static IP on your server get it out of the range of the DHCP server (usually .100-199), set the netmask(, gateway(
    If your server does not run its own DNS then copy the DNS values of the router + router IP address into computers DNS IP table. If your server runs DNS then setup its FORWARDER addresses from what router has, set the router DHCP to broadcast your servers IP as local DNS to the LAN (needed if being used as the DC domain controller)

    Drop me a private message if you need more help.
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