Ethernet not working on Ubuntu with Realtek RTL8111/8168B

Thanks to rvdavid, the network on my new PC is now working, despite Ubuntu misrecognising it:

How to Get Gigabit Speeds from RTL8111/8168B PCI Express Gigabit Ethernet controller on Ubuntu Linux.

This is the kind of thing I had to do 10 years ago. I thought we'd moved on...