Baffling networking question

Question

Bit of a random question (and I know a lot of the technology invovled is not described) but are there any networking gurus out there that can answer what is probably a simple M$ networking question that has me baffled?

Connected my laptop to the corporate network today in the same manner I do eveyday. I was allocated an IP address as per normal but unlike normal, I got no connectivity to the internet.

After a reboot and a ipconfig /release /renew the situation was the same. IP address but no connectivity.

So I called in the Techs who replaced my ethernet cable between my machine and the network port and everything sprung into life -- full connectivity restored.

My question: how did it get an IP address using that broken cable?

Answer 1

There's too many variables to keep a good educated guess. The best guess I can come up with the facts you've given is this:

The cable wasn't broken when it was initially plugged in. Maybe you were able to get an IP address before the cable broke.

Answer 2

Which IP did you get? Windows computers get assigned dummy IPs when they do not get one from a DHCP server, starting with 169 if I recall correctly.

Answer 3

As MrValdez already said, there are really to many unknowns to make a good guess. But one thing to keep in mind is, that just because one event follows another it is not necessarily caused by that event. This is also known as "Correlation does not imply causation".

It might well be, that the time to replace the cable was long enough that the real problem (router, ISP or some other thing along the path) could be solved by someone else (without knowing about your problem) or just vanished on its own.

Answer 4

Most probably, your laptop assigned itself the last IP it was given by DHCP when it failed to get one from the DHCP server.

Answer 5

If the cable was broken, you would get an error network "Not Connected" in the icon in the system tray. If you network connection was up, means the cable was OK. If the cable was not of the right type, then you could have had connection up, but Ethernet negotiated to 10Mpbs/Half Duplex, which can cause a lot of pain to the use who's not aware of it, including symptoms like no connectivity, dropped packets and hence slow connection, etc. After that, if you got an IP address beginning 169.184.X.X, that would mean that DHCP failed for whatever reason, and your laptop assigned itself a default configured IP address. If the IP address you got was what you normally get in your LAN environment (the once with which you usually connect to the internet), then there might be some firewall issues, or something else wrong with your corporate network's router, which the Tech guys might not be too willing to share with you :-)

Answer 6

It's impossible to be sure but here are a few:

• Network card driver: I've experienced very random things with HP laptops and ethernet cards. Usually a reboot fixes that. Green connectivity light doesn't always mean 'go', sometimes the drivers are responsible for turning the light on.
• Loss of connectivity in the cable or the network plug in the compute.
• Duplex negotiation problems between the switch and the computer
• Maybe trying gigabit ethernet on a 4 wire cable?

If it was me I'd try an ethernet sniffer (like tcpdump or wireshark) if this happens again, see if you're getting anything in. Ping results would also be nice (to IP addresses, not hosts).

Answer 7

The cable and/or connectors were flaky. DHCP is done using very small UDP packets, and if enough of them are sent, eventually they'll get through. The DHCP retry mechanism is very robust, so I wouldn't be surprised if you lost 95% of the packets, and it was still enough to get an IP.

However, once you start sending larger packets for TCP/IP transactions you'll find that the 95% loss drops enough packets that even with durable TCP/IP, it's too tough to get anything useful through.