Sub-nets
    A particular network block can be further subdivided by using a
      sub-net mask by the owner of that address range.   

    A class B address has default sub-net mask of 255.255.00.00
      which indicates that all systems in that network are on a single 
      network.

    By using mask 255.255.255.00 : 
      Network block can have up to 256 separate local networks of 253 machines.
      (Reserved :  0 (identifier), 1(gateway), and 255(broadcast) 
      * 1 does not have to be gateway but usually is.
      ** and a sub-net can have multiple gateways.

    Sub-net division is the choice of addresses' owner.  
      Division does not have to be on octet boundaries.

    NIU uses a sub-net mask of 255.255.248.0 
      The 1st octet is set at 10, 
        Private  Sub-net ID      ( node ID    ) 
         ID 
       (00001010).bbbbbbbb.bbbbb(bbb.bbbbbbbb)
      11 bit node Id ~ 2048 nodes.
      13 bit sub-net Id ~ 8K sub-nets.

   check out : http://www.subnet-calculator.com/cidr.php

     Nodes may be assigned to a particular sub-net because of physical location.
  
     Or because of logical function.  
     10.158.56.ddd is reserved for systems with static pubic IPs in PsychMath.

       turing.cs.niu.edu : 10.158.56.120
       hopper.cs.niu.edu : 10.158.56.24
       babbage.cs.niu.edu : 10.158.56.53
       blitz.cs.niu.edu : 10.158.56.43
       

IPs connect two end points on the Internet.
  Implemented at the IP packet or datagram level.
  But they don't identify a particular established session.
  For that, nodes also have ports. 

Ports
  Each IP node has up to 65 thousand ports. 2^16
    Allows multiple connections and sessions for a single node.
    Implemented at the TCP/UDP packet level.

    Ports <= 1024, called "well-known ports"  are reserved for predefined 
      or system level connections and assigned to standard services : 
      httpd, sftpd, sshd, etc. * Used to contact listening servers or daemons.
      Services using these ports usually require admin perimission to start.
     
    Ports between 1025 and 49151 are 'registered ports'. 
      These ports have been registered with IANA.
      Some service applications may use ports in this range without 
        official permission which can cause conflicts.

    Ports 49162 and above are "ephemeral" ports 
      used on the client side of a IP connection between a user and service 
      being accessed.

    There is nothing stopping a system from using any port on the client 
      side but it increases the chance of port conflicts.
    
Some better known well-known ports

  ftp-data 20 TCP - file transfer protocol
  ftp      21 TCP
  * Becoming obsolete because of lack of security.
  * Replaced with sftp (secure file tranfer protocol) 
  * Provide many of the ftp features using ssh functions.  

  ssh      22 TCP - secure terminal

  telnet   23 TCP - terminal
    * becoming obsolete because of lack of security.

  * IMAP, Internet Message Access Protocol, allows multiple connections to 
    a user's email Inbox and email is left in user's inbox on server unless 
    moved.

  * POP - Post Office Protocol, one connection per user at a time. When mail
    is read, it is moved from system's inbox area to user's directory. Older
    protocol.
  
  smtp     25 TCP - simple mail transfer protocol (SMTP) - sending, unsecure.

  pop     110 TCP - post office protocol - receiving, unsecure.
                    Mail retrieved and deleted from server.
                    * Neither smtp or pop provide encryption or other security.

  IMAP    143 TCP - Internet Message Access Protocol - unsecure method
                    for fetching email, leaves copy on server.

  domain   53 TCP/UDP - associates an IP@ with a Domain Name.

  bootps   67 UDP -  BOOTP/DHCP server
  bootpc   68 UDP -  BOOTP/DHCP client

  web service  80 - unencrypted web services.

  secure web   443 - HTTPS, web service over TLS/SSL.

  secure SMTP  587 - securly (TLS/SSL) sending email from client.
          also 465  # older version. Most emails servers accept both.

  imaps        993 - for fetching email from server over SSL.

  pop3s        995 - for fetching email from server over SSL.
   
  It is common for a service to have a different set of reserved ports for
    secure connections.

 Port listing (local file)
 IANA listing