From what you are explaining, it seems that when you connect to the internet, your DNS is routing you to the internet for helpdesk.com. Do you have a Domain Controller on your LAN? If so, you can make a DNS entry in there for helpdesk.(your domain) that your users can go to. Another option is you can just have them go to the ip address and then save it into their favorites with the name “help desk”. If you provide more information on your LAN setup and what kind of servers and services you provide on your LAN, other options might be available as well.