I did a project with a bank where we had to connect our stuff to theirs via a VPN, but they required that the addresses inside the VPN were notionally public -

I did a project with a bank where we had to connect our stuff to theirs via a VPN, but they required that the addresses inside the VPN were notionally public - so no 10.x. We borrowed 51.x, which at the time was used by the UK's Department of Work and Pensions but not actually routable. Occasionally new people on the team would ask why the VPN was called VPN-DWP-1.