So DNS is A client-server application that maps domain names into their corresponding IP addresses with the help of name servers. Mapping domain names into their corresponding IP addresses is called name resolution or Address Resolution or name translation or name mapping D.N.S(Domain Name System )

Root Name Servers “. “ Servers As of February 2013, there are 13 root name servers in the world Top Level Name Server (Intermediate Name Server): Authoritative name server: Stores host’s IP address, name. So can perform name resolution Local name servers: Each ISP, company has name server known as Local NS Types Of Name Servers

Root NS Top Level NS Authoritative NS Local NS HOST Query Finally Host will get IP Address through Local NS 1 2 Two things to keep in mind

Mapping domain names into their corresponding IP addresses. This technique of Name Resolution is of two types : 1) Recursive resolution:-Recursive servers know how to ask others for information 2) Iterative resolution:- Iterative servers don’t know how to ask others for information they can only answer if they have information in its cache or in its database Name Resolution

Which one is better Iterative resolution or recursive resolution? Recursive is better because Due to its caching property If distance is more than iterative takes very much time to resolve

Q.1 Can we have more than one IP address for one domain name? i.e myhost.cit.cornell.edu Q.2 Can we have more than one domain name for one IP address? Q.3 Is this possible ? abc.cit.cornell.edu xyz.cit.cornell.edu Questions???

Ans1. Yes, we can have more than one IP address for one domain name. This thing is managed by A(Address) record. It is known as round-robin DNS. DNS will cycle from one IP address to the next each time the domain name is called. A record An A (Address) record translates a domain name to an IP address myhost.cit.cornell.eduIN A myhost.cit.cornell.eduIN A abc.cit.cornell.eduCNAMEmyhost.cit.cornell.edu xyz.cit.cornell.eduCNAMEmyhost.cit.cornell.edu

Ans 2. No, we cannot have more than one domain name for one IP address. The CNAME maps the aliases (different name of single host) to the "official" name of a host. The official name is stored in the A record. myhost.cit.cornell.eduIN A myhost.cit.cornell.eduIN A abc.cit.cornell.eduCNAMEmyhost.cit.cornell.edu xyz.cit.cornell.eduCNAMEmyhost.cit.cornell.edu Ans 3. Yes, we can have like this We have one domain name and multiple aliases for that one domain name. This thing is managed by CNAME CNAME( canonical name ) abc.cit.cornell.edu xyz.cit.cornell.edu