Uh, Don't Interrupt. RUDE

Today, I went to a customer's house who was having internet problems. When I tried to explain to him why he was having an issue, he rudely stated that I was wrong and that he knew what he was doing because he was a network engineer. I have never met such a dumb network engineer who couldn't figure out the basic principle of IP addresses. He insisted that the computer picks up the same address whether it's connected to the router or directly to the modem. I simply said, "Okay sir, let me do my job and fix it for you." I reset his modem and problem solved. Dumb ASS.