Domain Name System servers, usually abbreviated as DNS servers, act as a type of network directory service that translates friendly, plain-language URLs into numeric IP addresses for computers and other devices to use.
Without a connection to a working DNS server, your computer would be unable to access anything on the internet. Because of the importance of having a working DNS server all the time, most computers support the use of an alternate DNS server in case the primary one is unreachable.
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DNS in a Nutshell
URLs (such as www.ehow.com) are used online since they are a lot more memorable than anonymous, phone number-like numerical IP addresses (for example, something like 188.8.131.52).
But to reach a website, your browser still needs to know the actual IP address of your destination. And that means that computers need some way of converting URLs into IPs. To do this, they send requests to their designated DNS server and ask for the IP address that corresponds to a web link, or vice versa. The following illustration is from an excellent infographic that describes the Domain Name System.
Why doesn't your computer just maintain the DNS list on its own? Well, by sending these requests to a remote server, local computers don't need to store large lists of IPs and URLs, which would be hard to keep up-to-date.
How Preferred and Alternate DNS Servers Work Together
Preferred DNS servers act as a device's "first choice" when making DNS requests. Alternate servers, on the other hand, are only used when the primary DNS server does not respond. They act as a backup for the DNS system as a whole.
It's important to note that if a primary server is functioning properly but it can't convert a particular URL into an address, the device will not contact the alternate DNS to see if it has better luck. This is because primary and alternate DNS servers share the same data.
The Benefits of Alternate DNS Servers
The main benefit of running two DNS servers is increased robustness. The presence of an alternate DNS means that a single DNS server failure does not prevent clients from accessing web pages. It allows devices to continue resolving addresses even if their primary server is down.
Alternate DNS servers can sometimes also be used to share the load of DNS requests on busy networks. If the volume of requests at a given time is greater than the primary DNS can handle, it can pass some of these requests off to the secondary server.
Keeping Servers in Sync
Ordinary users don't need to worry about this, but it is important that network managers keep alternate DNS servers up to date – otherwise, they might direct network clients to obsolete or inaccurate destinations.
Primary servers typically keep the master copy of the address data, which is transferred between the servers using a mechanism known as DNS zone transfer. This system automatically sends updated records from the primary server over to the alternate to keep them in sync. However, zone transfers increase load on the network, so they are occasionally performed in stages or overnight.