User of the web site: our IP is 192.168…
Me: that's your local network address, we'll need to know the external address our web site sees.
Them: our IT department assure me that's the right address, can you help?
Me: I can't help them since they don't know the basics of Internet networking, but maybe I can help you…

@RogerBW

A quick explanation of TRACERTE then follows. 3:O)>

(This moose is still more used to the IBM Mainframe spelling, even though I retired a couple of years ago.)

@Cadbury_Moose Nah, we send them a custom URL, they load it, and I look at the logs and see that it's been loaded six times from different addresses. But apparently this is legitimate business practice, not a half-arsed DDoS.