I got up this morning to find my entire network down. I did a few checks, then went down to the Data Shed to see if I could spot the problem. I saw, almost immediately, one of the big gigabit switches (32 ports) was flashing all ports. I powered that switch down, and my network came back to life.
I think the switch had simply gone gaga, and was throwing packets onto the network as fast as it could. I doubt if it's something I can fix. So I replaced it with one of my old 100mbit switches, and now everything works.
I use gigabit on most servers, because it makes throwing huge amounts of data from one server to another, very fast. I need that when I'm backing up terabytes of data. But for external access, the limit is 100 mbit, because that's the speed of my line.
I would like to use a gigabit switch instead of that temporary 100 mbit switch. So I went on to Ebay, and found a 48-port gigabit switch for £30. Which is not bad, since a 48 port gigabit switch is, when new, over £200.
... later ... So then I bought another one for £35.