CCNA 5.1: Describe DNS lookup operation

Overview:

A DNS lookup is the process by which a host queries a domain name and a DNS server returns an IP address

Study Notes:

• There are two types of DNS lookups:
  • Forward lookup - the host asks about a domain name and an IP address is returned
  • Reverse lookup - the host asks about an IP address and a domain name is returned
• The DNS lookup operation
  • A host requests a domain name: www.example.com
  • The host looks at its own hosts file first to see if it has that domain listed with an IP address
  • If it does not have it in its own hosts file, the host contacts its local DNS server as configured in the hosts network settings.
  • The local DNS server is typically configured as a recursive server, which means that it will handle the remaining queries for this request until the A Resource Record is received.  The A Resource Record contains the hostname-to-IP addrses mapping.
  • If the local DNS does not have an A Resource Record for the requested www.example.com domain then it first sends a request to a Root DNS server.  There are 13 Root DNS servers worldwide.
  • The Root DNS server will respond to the Local DNS server with an NS Resource Record containing the nameserver and IP address of the .com top-level domain TLD DNS server.
  • The Local DNS server will then send a request to the .com TLD DNS server to resolve www.example.com
  • The .com TLD DNS server responds with an NS Resource Record containing the nameserver and IP address for example.com.  Let’s say that nameserver is called authority.example.com.
  • The Local DNS server will then send a request to authority.example.com to resolve www.example.com
  • The authority.example.com nameserver will respond with an A Resource Record telling the Local DNS Server the hostname and IP address of the webserver where www.example.com is located.  Let’s say the website is hosted on a server called server.example.com
  • The Local DNS server then responds to the host with the IP address for www.example.com.  The host can now go to the IP address for www.example.com and view the webpage.

• PacketTracer has a sample DNS topology.  We step through that in detail in the lab.

PacketTracer Lab: CCNA-5.1-Describe-DNS-lookup-operation.pkt

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