DNS (Domain Name System) is an internet service that translates domain names into IP addresses. Domain names are alphabetic. It is much easier for us to remember a word than a series of numbers. DNS serves as “phone directory” for the internet by translating the name into the corresponding IP addresses. Without DNS resolution the internet would be a very inconvenient place.
Now what’s an IP address? 
Every device connected to the public internet is assigned a unique number known as an Internet Protocol address. It participates in computer network that uses internet protocol for communication. An IP address is analogous to a street address or telephone number in that it is used to identify a network device to deliver mail message. Redundancy actually means repetition of parts or subsystems to provide a backup in case of primary system failure. If you have websites or internet services that are critical or server fails for any reason, there are a number of different redundancy schemes available. DNS records are held on two or more name servers registered to the domain which point to different servers. The name servers are served in turn and thus the servers providing the service will share the internet traffic. To provide redundancy, the DNS servers for the domain should be on the same physical machine. Moreover the individual DNS records should have IP addresses for the machine.