GATE | Gate IT 2005 | Question 77

Assume that “host1.mydomain.dom” has an IP address of 145.128.16.8. Which of the following options would be most appropriate as a subsequence of steps in performing the reverse lookup of 145.128.16.8? In the following options “NS” is an abbreviation of “nameserver”.
(A) Query a NS for the root domain and then NS for the “dom” domains
(B) Directly query a NS for “dom” and then a NS for “mydomain.dom” domains
(C) Query a NS for in-addr.arpa and then a NS for 128.145.in-addr.arpa domains
(D) Directly query a NS for 145.in-addr.arpa and then a NS for 128.145.in-addr.arpa domains


Answer: (C)

Explanation: Background:

When you type a web address into a web browser like www.google.com your computer needs to convert that into an IP address so that it can contact that web server & deliver the web page to you.
In a forward DNS lookup client first asks ISP(Internet Service Provider) for the dns of the domain name.ISP maintains a cache for the IP address of the domains whose lookup has been already made.If IP is not found in the cache then the ISP asks the root level dns server,which is a network of hundreds of servers in many countries around the world.Nearest root level DNS for India is in Tokyo.

The root level server then redirects it to another server ,say we are searching for geeksforgeeks.org then root level server will redirect isp to org server which will in turn redirect to geeksforgeeks server and from here ISP get the particular IP address which is then sent to the  client. Forward dns lookup-
domain name – IP
Reverse dns lookup-
IP-domain name

special address domain – in.addr.arpa (this domain basically converts to inverse domain)
IPv6 addresses use the ip6.arpa domain.
Now,ISP maintains a list of IP addresses corresponding to domain name and also domain names corresponding to the IP addresses.
in-addr.arpa is the domain name server
For IP address say 192.19.20.1(which is say IP address of geeksforgeeks) the corresponding entry will be
“1.20.19.192.in-addr.arpa. IN PTR www.geeksforgeeks.org.”
which will point back to www.geeksforgeeks.org .This helps in checking the ip address of the sender.
The IP in the entry is in reverse order .

This solution is contributed by Shashank .

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