DNS - Domain Name server - converts symbolic name to IP number.  Usually one 
  local DNS running on a network or known by IP to each network.  The DNS 
  service is structured as a hierarchical tree with the local DNS at the 
  lowest or leaf level.

  Service provided on Port 53. Client uses an ephemeral port.

  Server ID often provided as part of the DHCP look-up.

  Tiered structure with primarily a top down resolution for security.
  
  Uses UDP packets.
 

  See  : http://www.simpledns.com/help/v52/index.html
    http://blog.catchpoint.com/2014/07/15/world-dns-cache-king/

  When user uses a name, 
    The application hands the name to the node's resolver.

    The resolver looks through a local table it maintains to see if has
      already resolved the Name <-> IP relation.
 
    If look-up info is cached on the user's machine,
      IP returned.
 
  Else resolver contacts local domain's (ISP's) DNS server to see if 
    has recently looked up the name. 

    If found, 
      it uses that data.

      Some domains may have multiple DNS servers.
        especially if on a private network.
 
    # for each of the following steps, if information has been recently found,
    #   it will skip down to the next level. 
    Else the local DNS service asks one of the root DNS servers for 
      the IP of the TLD name-server of interest, .com, .edu, .net, etc..

      To simplify search, the Internet is broken up into non-overlapping 
        zones, com, edu, gov, etc., which are further broken into domains.

      It will cache this so it doesn't need to do it every time.

      The local DNS service then sends the DNS resolution query to the 
        chosen TLD name-server to find the target domain's DNS service.

      The TLD name-server should respond with the IP of the DNS server of the 
        domain of interest. 

        In some cases, a domain may have a tiered DNS structure. So, a 
        recursive query sequence may occur to further resolve the target IP.

      The DNS server of the targeted domain then returns the IP of the system 
        of interest. The DNS service of the local domain of the requester 
        then returns IP to requesting resolver. This ensures that the local 
        DNS service now has a copy for any new requests. 

  Top down resolution provides for a more secure result.

  It may be possible to hijack a DNS server within a local network. But only
    systems on that network will be affected.
    
  It may be possible to directly query a different site's DNS server for 
    information. This is not advisable.

   NIU runs a a pair of public facing DNS servers and a pair of private 
     sub-net DNS servers. 

   Network nodes on campus must use our private DNS servers to get a valid
     private network Id. Using an exernal DNS service directly will yeild
     invalid results.
 



TTL - time to live.  DNS entries at the TLD have a time to live after which
  they become stale.

  And the check will be performed even information is known. 
    24 hours common.  86400 seconds.

    https://en.wikipedia.org/wiki/Time_to_live

  Try :  dig +trace wikipedia.org