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iMac won't connect to Internet

Recently my father-in-law, who lives in Lexington, switched to Insight cable for his internet connectivity. He's been using an old Dell PC, with no problem obtaining an internet connection.

Today, we surprised him with an new iMac. The iMac would pull an IP from the cable modem. Spoke with Insight who said there was no special DHCP/network configuration required. Spoke with Apple, who had me try many things before finally reinstalling the operating system from scratch. Still, the iMac would not connect to internet. (Could only get a self-assigned IP.)

Brought iMac home to Louisville, thinking the ethernet card in the iMac would need to be replaced. Plugged it into my own Insight cable modem (same model as my fa-in-law's) and, whaddya know, the iMac got an IP address and connected to the internet just fine.

Cable modem is new Motorola Surfboard SB5120. Longest time we kept it unplugged was five minutes.

The Dell, again, has no problem obtaining an IP address.

My instinct is to try to things: (1) try the Lexington connection with my cable modem. (2) if that doesn't work, then to move the iMac to another room and try a different cable connection. (e.g., unplug a TV).

Appreciate any responses.
1 Solution
First off I would try unplugging the cable modem for a while. It is possible that it is caching the mac address of the Dell.

Secondly, get a terminal window open on the mac and issue the command:

sudo tcpdump -n -i en0 port 67 and port 68

This will monitor the ethernet network for DHCP packets. Then try and renew the DHCP address. This should let you see if the DHCP packets are going to and from the cable modem.


Also, verify with ISP that the UBR is not caching the Dell's MAC address.

The ISP can limit the modem to a given number of computers (the ISP I tiered-3 at only allowed 2). Any other computers would pull an IP, but not have connectivity to the Internet.

If Insight is limiting to 1 computer per connection (unless you pay for the 2nd, 3rd, etc) ... this might happen. You would have to call support, they'd log in to the UBR and clean your cable modem's clients.
jdmailAuthor Commented:
Thanks very much. I left the modem unplugged for an hour, and that did the trick.
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