Rare Connectivity Issue TCP/IP Stack?

Hi all,

I had a customer today with a seemingly rare connectivity issue with a Mac OS X 10.4.x. I "fixed" the issue, more like a band-aid, but my own curiosity has me researching this, and will report back to him. He switched to our internet/phone service today and his Mac would not pull an IP directly from our Ambit phone/cable eMTA (combo modem). After verifying it worked on my Windows laptop and obvious things like, ethernet cable, swapping modems, etc. I triple checked all the settings in the built-in ethernet port. It would only auto-assign itself a 169.x.x.x. I manually changed duplexing, TX speeds, flow control, disabled air-port, etc. basically every imaginable configuration to no avail. I then decided to try a standard linksys N router between our modem and the Mac. Boom, got on instantly both hard-wired and via air-port. The only conclusion I can draw is that a lesser protocol in the TCP/IP stack would not work properly directly to this modem, therefore not completing the DHCP handshake, but did with the router. We see quite a few Macs with very few problems, and never an issue where a modem would not work hard-lined directly to a Mac vs. through a router. Perhaps an issue in the link layer, ARP, OSPF? Maybe UDP in the transport layer? TFTP? Just doesn't make sense to work with one and not the other since these protcols are universal across platforms, but apparently it liked the linksys DHCP service. Anyone seen an issue like this before? Any insight would be appreciated.

These issues make me want to get back into IT world, just can't afford any more lay-offs!

Thanks,