TCP/IP addressing IP4.
  Originally designed to view the world as a collection of network domains
    which in turn can be divided into sub-nets at the owner's discretion.

    Individual systems (nodes) on sub-nets are assigned specific IP numbers.
    
  A node is given a numeric address (IP #)
    4 Octet value ( octet = 8 bit or 0-255) x.y.z.q

    The IP number has no relation to the MAC address of the node's hardware
      or even the type of hardware (token, Ethernet, etc).


    IPv4 specified 5 classes :

       bbbbbbb - binary,  ddd - decimal, xx - hexadecimal.

    Class A - 1st octet - network(domain), other 3 - 16M units at site.
      0bbbbbbb.ddd.ddd.ddd
         1-126 - all are in use.
      Network can be subdivided and sparse. IBM
      Currently all assigned.  (ATT, IBM, etc.)

      00.ddd.ddd.ddd and 127.ddd.ddd.ddd used for self-identification.
        0111111b - 127 - loop-back, used internally on each node.

      10.ddd.ddd.ddd - specifies private network. 
      (0000 1010b).ddd.ddd.ddd
        Address not visible outside of those directly connected to that 
        network. Requires network address translation to connect to the real 
        world. Our wireless uses this. 
        * Allows for > 16 million internal connections.
          But is usually broken up into many sub-nets.
          Systems that need to be assigned an 'alias' public IP usually draw
            from a small set of publicly recognized IPs set aside for this i
            reason.
          NAT - network address translation.

    Class B - 1st 2 octets (14 bit) - networks 16,384, 
      other 2 - node addresses (65 thousand)

      10bbbbbb.ddd.ddd.ddd  

       128-191.xx - Most in use.
      Occasionally come back into circulation.

      Ex.  NIU 131.156.d.d  - public domain ID.
        Currently, we use a 10.xx.xx.xx private network with a NAT when needed
          to connect with the outside. 
        * In part for security. Anonymous users on NIU's network
        * in part because of IP exhaustion. With 20K+ users, 65K IPs means only
           3 IPs per user. 
           
       172.16.0.0 - 172.31.255.255, private ~ 1 million private IPs.
         1010 1100 0001 bbbb bbbb bbbb bbbb bbbb
         Same as 10.x.x.x only a smaller internal block.
         Usually broken into 64K sized sub-nets.

    Class C - 1st 3 octets - network, last - units (254)
      110bbbbb.ddd.ddd.ddd, 192-233
      Small companies (may have more than one domain address).
      ddd.ddd.ddd.000 and ddd.ddd.ddd.255 are reserved.

      192.168.ddd.ddd - reserved for private sub-net behind a routing firewall.
        Allows for ~ 65,536 private IPs, usually as 256 unit sub-nets.
        However, most devices performing NAT only provide for 256 IPs.
          and some of those are reserved or pre-assigned. 
        The 3rd octet sometimes can be used to id the manufacturer of the firewall
          hub. 
       
    Class D - special broadcast - multi-cast.  1110b
      
    Class E - Future use and testing. 11110b

    Domain exhaustion and Classless IPs.

    Classless - introduced in early 90's as specific large network IPs become 
      occasionally available.
      Use of classless designation where IP followed by a mask 

         ddd.ddd.ddd.ddd/yy

      Where yy indicates which bits specify the network (domain) as a whole.

         ddd.ddd.ddd.ddd/18

      If this had been a Class B domain IP, with 64K IPs in it.
      The /18 allows four additional bits from the sub-net portion to be used
      to ID a domain. 

      A single Class B domain to be split into 4 domains with 16K IPs in each.
      Each domain could be assigned to a different ISP provider.
 
    IPs ending (last octet or subset) in :
     0 - used by a newly connected device to ask for help on the local sub-net.
     All 1s (usually 255) - broadcast to all devices on local sub-net.
     1 - usually the @ of the gateway system out of the local network.

NAT - network address translation.
   Uses an alias table to assign a private IP to a public IP/port combination.
   IP exhaustion  
     Across Internet (sparse) - IPv6.
     Within Domain - NAT

   Where a special router uses a single IP on the "public" side and 
     192.168.x.y on the private side. Class C

   x usually ids the NAT hardware manufacturer.
   y is one about 254 available ids. IPs 0, 1, and 255 special use.

   Ports - secondary number used to id specific session or transaction.

   Each system on the private side is assigned one or more ports on the public
     side to uniquely id it. 

   Generally, not useful when systems behind NAT router are servers. 

  Special use addresses.
  10.0.0.0/8, 172.16.0.0/12, 192.168.x.y/24 - private networks, lower octets 
    for local sub-nets.  Not visible to the world, has to go through a NAT 
    router or proxy server.  

  169.254.0.0/16 - Link-local, IPs only valid on the local network, usually
    systems only of interest to other systems on the local network, such as
    the DHCP server. Gateway ignores.

  192.88.99.0/24 IPv6 to IPv4 - gateway(?) to help with moving IPv6 packets 
    over IPv4 networks.

  224.0.0.0/4 - IP multi-cast - type broadcast, useful for situations like 
    live streaming video where many systems want to receive the same data.