I have a 2008 24" iMac running Snow Leopard that was my primary work machine, and I keep it around and running for those times that I need to fiddle with some Old Stuff. My routine access method has been to use the standard Share Screen capability from my current main machine (a 27" iMac running Mavericks) over my wired LAN, which has worked just fine until a few days ago. I hadn't accessed this machine in a while, and when I did I was unable to keep a connection open for more than a few seconds. After some fiddling around I discovered that I couldn't "see" the SL machine, if I tried to ping it (by name OR IP) it wasn't there. However, if I ran "arp -a" the machine and IP address were listed but the MAC address was gone. Since I run my DHCP and DNS from OS X Server (on a network Mac Mini) I first suspected that, but could find nothing wrong there. After more fiddling I discovered that if I switched from the wired NIC connection to wireless everything started working just fine again after I mucked out all the various DNS caches. So, it sounds like the wired ethernet connection on my iMac is shot. Is there any way to test this hypothesis?