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I'm trying to help a friend remotely with an Internet connectivity problem. His internet went down this morning and the cable company had him switch to a Motorola Surfboard SB6141 modem from an older model Surfboard. It seems to be struggling to acquire a DHCP lease. On either of his Macs, when hooked up directly, it typically seems to take trying to force the computer to acquire a DHCP lease several times before it finally works. Mysteriously, however, when we hook the SB6141 up to a router even multiple attempts to renew the lease never leads to the router acquiring the lease (and this was true with his original Linksys WRT54G, a new Linksys E1200 and a new Linksys E2500). The routers consistently report an IP address of 0.0.0.0 (i.e., they haven't received any DHCP information).

Since the latter two routers were brand new, I don't think the router could be at fault and, normally, I've found that this sort of setup is a straightforward one that doesn't require any odd settings. Is there anything peculiar about the SB6141 that might be interfering with a normal router configuration? I'm wondering if the fact that the DHCP process seems less than smooth when directly connected to the computer indicates that there is something amiss with Charter's DHCP server.

UPDATE: To clarify, the routers are routers with all of the settings set to the OEM's factory settings. Normally, in such a case, the router would seemingly just start working with a cable modem and pick up a DHCP lease, which is what is puzzling here.

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I encountered this same issue. The SB6141 only seems to give out one DHCP address when connected to the Internet (Comcast). First I connected my laptop PC directly to the modem, and it had Internet access fine. But then I disconnected the laptop and plugged in a Linksys 1900AC router. No DHCP address for the router, and the modem still had the MAC address of the laptop associated with the DHCP reservation. Resolved by powering down the cable modem and the router, and ensuring the cable modem and router were connected via ethernet. Then powered up the cable modem, then powered up the router. Now the cable modem sees the router and vice versa and have internet access through the router. Did not have to change address spaces

  • This worked to fix my issue – Ted Jun 14 '15 at 3:40
  • Yep.. subsequent connections to the modem did not vend a new DHCP lease. Had to plug modem directly in to router and reboot the modem. Pretty lame. – Chaos Mar 27 '16 at 2:34
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The cable modem will have an IP assigned by the cable company's network. The router should assign itself 191.168.1.1 or something similar to that. Some cable providers require that the cable modem's mac address be whitelisted, otherwise, no external IP for you. Are you sure that this is not what is going on here?

EDIT: After re-reading your problem, are you sure your friend plugged the modem in the right port?

  • Thanks, @Matt. The router is assigning its internal IP OK, just not receiving an external IP from the provider. The modem should be whitelisted, as he was assisted by a Charter tech in setting it up (before he called me) and it does work when hooked directly to a computer. I don't think there is any kind of MAC whitelisting for the computers themselves -- we tried two different computers and they worked (other than the caveats, above). Unless Charter is blacklisting router MACs. As to hooking things up, I'm a tad worried about that, but I tried to confirm it every way I could think of. – Timothy R. Butler Dec 31 '13 at 1:13
  • If all else fails, do the push pin reset of the router. If there's something in his config that he doesn't like, that will take care of it.. He'll have to re-configure his wifi settings though. – Matt Dec 31 '13 at 1:24
  • Thanks, @Matt -- I really appreciate it. I tried that. Actually, so far all of what we've been doing has been with a wired connection to a router with all fresh, default settings (including an unconfigured wireless network -- I was waiting to mess with that until it actually got a DHCP lease). This just seems like a very odd issue. – Timothy R. Butler Dec 31 '13 at 1:34
  • The "191.168.1.1" IP address that you mention is the IP address of your local area network (LAN), which may be a different IP address than the one "seen" by the cable or DSL modem. You can change the LAN side IP address to anything you want, and the router provides network address translation (NAT) between the wide area network (WAN, Internet or modem) side and the LAN. The default IP address of the modem is usually in the range of 192.168.x.x, meaning there are 65,536 possible IP addresses – Andrew P. Mar 17 '16 at 16:49

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