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08.8 Domain Name System
9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/8/pages/8 1/3 Domain Name System Section 8: Understanding the TCP/IP Internet Layer The DNS provides an efficient way to convert humanreadable names of IP end systems into machinereadable IP addresses that are necessary for routing. On TCP/IP networks, hosts are assigned their unique 32bit IP addresses in the familiar dotted quad notation (x.x.x.x) so that they can send and receive messages over the local network and the Internet. Although not every IPv4 address has been assigned, there are billions of possible destinations. If there were no DNS, you would have to remember the IP address of every host that you would like to reach. Imagine having to remember the IP addresses for even the top 10 websites you visit. Domain Name System Examples DNS Hostname IP Address www.cisco.com 220.127.116.11 www.emc.com 18.104.22.168 www.microsoft.com 22.214.171.124 www.netapp.com 126.96.36.199 www.redhat.com 188.8.131.52 www.vmware.com 184.108.40.206 www.gmail.com 220.127.116.11 www.wikipedia.com 18.104.22.168 www.wunderground.com 22.214.171.124 www.thinkgeek.com 126.96.36.199 DNS uses a distributed database hosted on several servers, located around the world, to resolve the names that are associated with dotteddecimal IP addresses. The DNS 9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/8/pages/8 2/3 protocol defines an automated service that matches resource names with the required numeric network address. An easy way to observe DNS in action can be performed in a command window in Microsoft Windows, Apple MacOS X, or your favorite Linux distribution. Once the command window is open, enter nslookup www.cisco.com. This causes your IP host to make a DNS query. The result will appear below your query. The DNS transaction represented in the illustration is a simplification for the purposes of demonstration. In practice, there are DNS transactions, external to the local DNS server, that are necessary to receive an answer to the host query. Your host sends a DNS query for the IP address of www.cisco.com. If your DNS server has the answer cached, it returns the answer directly. 9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/8/pages/8 3/3 Up Next: Verifying the IPv4 Address of a Host