What Is the Difference Between a Preferred DNS Server & an Alternate DNS Server?by Walter Franz, Ph.D.
Internet service providers and office IT teams shield computer users from the technical details of accessing the Internet, so many users remain unaware that Domain Name Services mediate how their machines access resources located on other computers. Preferred and alternate DNS servers are your computer's most important resource in navigating to the information you seek on the Internet. Without specific DNS server settings, your computer would fail to load the website you expect to see when you enter the URL in your browser's address window.
What Is DNS?
DNS was invented to solve a problem. Computers connect with each other in a network through Internet Protocol addresses that consist of four binary octets -- for example, 10110011.11010110.1001001101.11110001. Because binary math is nonintuitive to human minds, each networked computer also has a host/domain name through which human users more commonly know it. DNS software keeps track of which host/domain names are associated with which IP addresses. The computer on which DNS software runs is called a DNS server. Anytime your computer or its software does not know what IP address goes with a specific domain name, it requests that information from your preferred DNS server.
Preferred and Alternate DNS Servers
Like any other network computer, a DNS server has both a host/domain name and an IP address. However, when we tell a specific computer where to seek DNS information, we do so by specifying the DNS server's IP address. If your preferred DNS server is unavailable, your computer requests the same information from the alternate DNS server. If both servers are unavailable, your computer fails to accomplish the things you are trying to do over the Internet -- access a website or get a directory of files on an FTP server -- and the software through which you are attempting to obtain the current information will usually let you down easily with a polite error message.
DNS at Home
If a computer connected to your home network needs DNS answers, then the modem or router provided by your Internet service provider starts the finding. Your cable modem or router has the preferred and alternate DNS server settings that specify the IP addresses of the DNS servers where any browser on your computer should send requests to resolve a host/domain name into the relevant IP address.
DNS at Work
The IT team where you work sets up company-issued computers with the IP addresses of the preferred and alternate DNS servers that answer for the local network. If the computer asking for DNS receives an error from the preferred DNS server, then the computer next sends the request to the alternate DNS server. That is the only difference between the preferred DNS server and the alternate DNS server. The preferred server always gets all the requests unless it is down; and the alternate server takes over if the preferred server is down.