On Thu, 13 Dec 2012 14:58:38 -0000, RG wrote:

And what if the office router insists on a single Ethernet wire having
a single Internet address?

I have not found a modern one that does.

Nor me the switches in domestic kit are unmanaged so there is no way to
lock down a given ethernet port to a given MAC address let alone a IP
one.

Managed switches as found in commercial enviroments are very different
kettle of fish.

--
Cheers
Dave.