The NIC is a wired NIC, so I need to plug it into that obsolete ethernet thingy. But I have no spare addresses on my wired network, nor can I temporarily use one from any of the machines because they are all busy doing things. And to cap it all, I don't have any spare switch ports so I can't even be naughty and run another address range on the same subnet. But, I realised, my Powerbook, which lives on my wireless network, does have an ethernet port.
So I ran a cross-over cable between the Libretto and the Powerbook. The Powerbook does NAT and routes the packets out over its wireless interface. Which connects to an Airport base station which does NAT and routes the packets out onto the wired network, where another router picks them up and does the DSL dance to get them to the rest of the world.
Looks like I'm losing about 20% of the network capacity with all the gratuitous NATting.