DNS servers return destination IP addresses given a domain name. DNS is a distributed database. Separate, independent organizations administer their assigned domain name spaces and can break their domains into a number of subdomains. For example, given www.cisco.com, DNS returns the IP address DNS was first specified by RFCs 882 and 883. The current specifications are specified in RFCs 1034 and 1035.

DNS was implemented to overcome the limitations of managing a single text-host table. Imagine creating and maintaining text files with the names and IP addresses of all the hosts in the Internet! DNS scales hostname-to-IP-address translation by distributing responsibility for the domain name space. DNS follows a reversed tree structure for domain name space, as shown in Figure 7-6. IANA (http://www.iana.org) manages the tree's root.

244 Chapter 7: Internet Protocol Version 4 Figure 7-6 DNS Tree

.hr .manufacturing .sales .marketing

DNS uses TCP and UDP Port 53. UDP is the recommended transport protocol for DNS queries. TCP is the recommended protocol for zone transfers between DNS servers. A zone transfer occurs when you place a secondary server in the domain and transfer the DNS information from the primary DNS server to the secondary server. A DNS query searches for the IP address of an FQDN, such as www.cnn.com.

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