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