I am installing Ubuntu 16.04.3 on a desktop. When I get to the step where I am prompted for a hostname, I randomly type in "fortknox" but the Ubuntu installation says "That name already exists on the network."
Here is some background that may be relevant. My fiber connection to the Internet was installed by AT&T a few days ago. The modem is an Arris BGW210-700. All of my computers are behind the router's firewall and are assigned IPs by the router's dhcp such as 192.168.1.84 and so on. I confirmed the IP address 192.168.1.84 was assigned to the new Ubuntu box by looking at the modem's HTTP device webpage.
The router/modem is located in another room. I am using a USB wifi dongle to connect to the router on the Ubuntu box. When I got the message saying the hostname "fortknox" already exists on the network, I went to another Linux box and did 'ping fortknox' and got a response back from IP 18.104.22.168.
Did an ARIN lookup (https://whois.arin.net/rest/net/NET-104-239-128-0-1/pft?s=22.214.171.124) and found that the IP address is owned by a big datacenter here in town named Rackspace. This raises a number of questions for me.
Is my wifi dongle on my new Ubuntu box getting spoofed? Since I am able to confirm the IP was assigned by the ATT router/modem, I don't think so.
Is my new Ubuntu box visible from the Internet?
I don't have in-depth understanding of networking protocols but it seems very suspicious that my new Ubuntu installation inside of my home network can't use a hostname on an entirely different network.