Network Gurus: Comcast, Ethernet, AirPort Express/Extreme?

Discussion in 'Mac Basics and Help' started by TwinCities Dan, Jun 20, 2009.

  1. TwinCities Dan macrumors 603

    TwinCities Dan

    May 19, 2008
    Double Parked out front of the Courthouse
    I finally convinced a friend to get a Mac and bragged about how easy everything works. He got a new iMac (loves it!) and signed up for Comcast High Speed Internet. Now there is a new MacBook in the house so I explained that all he would need is the AirPort Express in order to share the internet wirelessly for both Mac's. I went to set it up and I screwed everything up, after hours of work I not only couldn't get the Express working, I had to leave him without even ethernet internet. :eek:

    That's really all the info about his setup I have at the moment. So I came home to examine my own setup which is very similar: we both have the same Comcast modem (Arris TM502G) except I have an AEBS and he has a Express.

    So I came home to examine my setup, it has been a while since I set mine up but its been flawless wirelessly (I never used the ethernet before). So, I plugged a ethernet cable in to the AEBS and connected my MBP. Works great. So I want to try to cut out the AEBS and just connect via ethernet from my MBP to the cable modem, that's where I start getting IP address problems.

    When an ethernet cable is connected to the AEBS my IP address is and works fine. When I cut out the AEBS and try to run ethernet directly from my modem to my computer my IP Address changes to and I get an error message saying that "Ethernet has a self-assigned IP address and may not be able to connect" and sure enough, I can't. Network Diagnostics tells me to configure via DHCP and restart the modem, still no luck.

    As I was searching tonight I came upon some discussion about Comcast limiting MAC addresses or something?

    I feel really bad for leaving them with no internet. Anybody have any input? :confused:
  2. LimeiBook86 macrumors 604


    May 4, 2002
    Go Vegan
    I had a similar problem with a cable modem and a router. I basically had to have the router clone the MAC address of the cable modem. After that the router was able to distribute IP addresses just fine. Hope this helps out! :)
  3. JediMeister macrumors 68040

    Oct 9, 2008
    When power cycling the modem, are you also disconnecting the coaxial cable, assuming there is one? Leaving it connected I have found still causes some settings to be retained that a standard power off-power on does not clear.
  4. TwinCities Dan thread starter macrumors 603

    TwinCities Dan

    May 19, 2008
    Double Parked out front of the Courthouse
    Clone MAC addresses? It shouldn't be that hard, should it? :confused:

    Very interesting, no I did not think about disconnecting the co-ax during the power cycling. Good suggestion, thanks JediMaster! Anybody else (please)? :eek:
  5. sdsvtdriver macrumors 65816

    Jul 31, 2008
    Southern California
    cable modems are finicky.

    If you disconnect a device from it that can connect to the internet (router, computer...) and plug a different device into it you will need to reset the modem. By reset, I mean disconnect the power from the wall, wait 30 seconds, and plug it back in. The standby button does not qualify as a reset. You may need to then reboot your machine/router. If it doesn't work, do it again.

    To get your friends to work:
    1. Disconnect everything.
    2. Plug in coax to cable modem. plug ethernet into AEBS. Power up cable modem. When the cable light is done blinking (or just wait 2 minutes for it to complete booting), powercycle it again.
    3. After 2 minutes, power up AEBS.
    4. Power up computer, browse to AEBS and check for WAN IP. If not, reboot AEBS.
    5. If there still is no WAN IP start over, powercycle cable modem. When it boots, power cycle AEBS.

    Sooner or later, it'll go. What has to happen here is:
    1. Cable modem has to boot, sync and request IP from cableco, then complete the boot process.
    2. AEBS has to boot, request IP from the device connected to its Internet port. The cable modem will pass the IP it received from the cableco to the AEBS assuming it doesn't believe it has already passed that IP to another computer. it tracks this by MAC address hence why people have success with the clone MAC address strategy.
    3. AEBS will pass NAT'd IP addresses to any computer that connects to it.

    "Back in the day" the cablecos had the modem set up in such a way that it would only connect one MAC address so they could charge for multiple computers. If you changed computer, you had to call in. I don't believe this policy is still in place today.
  6. Lanbrown macrumors 6502a

    Mar 20, 2003
    Comcast sucks and several years ago, they still had to clear the entry from their DHCP server but they don't know how to do it. You are allowed one IP address, this is why people usually clone the mac address.

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