DNS - Domain Name System
  An Internet wide hierarchical database that provides name assignment to IPs.

  see : https://en.wikipedia.org/wiki/Domain_Name_System

  When implemented correctly, provides a fairly secure way of correctly
    resolving names to IP and IP to name.

 
Naming protocol.
  Names often start with type of protocol www.cs.niu.edu (web), ftp.cdrom.com
   (ftp site), etc.  Although a good idea, this is not a requirement.  
   Some sites reserve several alternative names. Also, some browsers will
   supply www if unqualified name a miss.

  Right most domain qualifier must be one of the recognized top-level domains.

  Each label or qualifier (between dots) can be up to 63 characters.

  Total length <= 253 characters - www.completelyfreesoftware.com.
  
  Names must consist of at least 2 qualifiers long but can be up to ~125. 
    3 or 4 total most common.  www.cs.niu.edu.

  ASCII - letters, numbers, hypen - ICANN now allows international characters
    (Unicode) to be remapped to ASCII. (2009) The ASCII remap may read like
    giberish but provide a unique identifier.

  There is no relation between name and ip other than a name must resolve
    to a single ip.

  At the top level under root servers, the DNS systems are somewhat isolated. 
    .com handled by one system, .edu by another, etc. 

  At this level, ownership of each top-level domain by individual organizations
    or shared between a few to encourage some competition.

    See InterNIC.net for a list
    of many of the DNS registry providers.

    If new top level domains become available, they will be for sale/lease for
    $185000 initial / $25000/yr. Owner will then be able to charge for domain
    names under that top domain.   


Originally, there were 8 TLD Ids.

.com - commercial
.edu - education, 4 yr. accredited universities.
.gov - U.S. government sites.
.mil - U.S. military.
.org - non-profit organizations.
.net - sites that dealt with networking, usually non-profit.
.int - international domains created based on treaty between nations.

.arpa - (advanced research project agency), 
   now used to support Internet structure.

Later, 2 letter names added for each country in the world.

Now, new TLDs can be requested.  

List of IANA recognized Top Level Domain Names"

dig +trace evaluations.cs.niu.edu
  eval.cs.niu.edu
  www.microsoft.com
  
netstat -a
ss
nmap 131.156.145.51
nmap -n -sP -v 131.156.145.0/24
arp

Resolving an name/IP.


DNS record types
See : https://en.wikipedia.org/wiki/List_of_DNS_record_types