I have a 10-year-old Dell XPS laptop that is connected to my Internet router via a hub. Output of lshw tells me that the Ethernet card is a Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller. On my Arch-based distro, the driver is listed as r8169.

The other computers connected to the hub connect to the Internet without a problem. I've tried 3 different Ethernet cords, which all work fine with the other computers. Neither OS on the Dell detects the Internet connection. So it seems the problem is the Dell's network card.

The Dell has been losing the connection intermittently for some time, but would detect the network again after I restarted the laptop. Now the laptop detects no connection at all, except I will occasionally get an IPV6 address (but no IPV4 address) if I bring the Ethernet interface down and then back up.

Last time this happened, I fixed the problem by updating the system time with ntpd. But the Dell's system time doesn't seem to be off now (it matches the system time of one of the other computers).

What else can I try to make sure the network card isn't malfunctioning?

You can try any live distribution to see what happens but it appears to be a hardware failure. You can manually set an IP address and route using the ip command but if something like the transceiver hardware failed the OS would may not detect anything is wrong.

Since ntp requires networking to get time it would seem just to be a coincidence.

I agree with michaelk that it sounds like a hardware problem. Stuff does wear out.

You might try testing with a different, known-good ethernet cable. I once had a similar issue that, thanks to the coaching of an extremely competent support rep at my ISP's help desk, turned out to be a failing cable. (I have some gripes about my ISP, but their support staff and service techs set the standard.)