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Full Discussion: Help me understand ports and port forwarding please
Special Forums IP Networking Help me understand ports and port forwarding please Post 302840477 by DGPickett on Monday 5th of August 2013 04:44:39 PM
Old 08-05-2013
Ports are fields in UDP and TCP packet headers that allow the flow to be divided on a host to 65K different apps. For instance tcp cpnnections could be made from 63K different apps on one host to port 80 web server on the next. Sometimes port numbers imply a protocol, like 80 for http, 25 for smtp, etc. Servers listen on ports and clients get random ports to identify their socket from al others on the host, In IPV4, you have 2^32 hosts and 2^16 ports, so there are 2^96 possible connections. UDP is connectionless, so a "connection" is just a filter on remote host+port and default remote host+port destination on a socket.

IP packets are identified by Host and protocol (such as TCP), and for tcp and udp, by port. Firewalls like iptables key off the host and port. With tcp, you can tell which end is the client (connecting) and which is the server (listening) in the first two packets (syn and syn+ack bits on, respectively). So, you can allow clients inside to connect everywhere outside but not vice-versa. ICMP is an IP sub-protocol that supports IP, TCP, UDP with control and diagnostic messages Some ICMP messages can be toxic if counterfeit.

IPTables also has NAT, the ability to rewrite packets for a new host, port or both going "out", and back to the original host/port for packets coming "in". This is handy if inside hosts are unroutable, like 10.*, or just to hide inside hosts. Some protocols like FTP (which runs on top of, or inside, TCP) put hosts and port numbers in the data stream as well, and some of these NAT knows how to rewrite. All packet rewriting include adjustment of checksums.
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LEARN ABOUT ULTRIX

mdc

mdc(4)							     Kernel Interfaces Manual							    mdc(4)

Name
       mdc - DS5100 serial line driver

Syntax
       device	 mdc0	  at ibus?     vector mdcintr
       device	 mdc1	  at ibus?     vector mdcintr
       device	 mdc2	  at ibus?     vector mdcintr

Description
       A  DS5100 base system (mdc0) provides four asynchronous ports.  An optional asynchronous card (mdc1 and mdc2 ) can be added to supply eight
       additional ports.

       The mdc ports are used as follows:

       Port    Usage			  Connector

       0       Console port		  6-pin MMJ on CPU board
       1       Local terminal		  6-pin MMJ on CPU board
       2       Full modem control port	  25-pin D-Sub on CPU board
       3       Local terminal		  6-pin MMJ on CPU board

       An optional asynchronous board provides an additional eight lines:

       4       Local terminal		  6-pin MMJ
       5       Local terminal		  6-pin MMJ
       6       Full modem control port	  25-pin D-Sub
       7       Local terminal		  6-pin MMJ
       8       Local terminal		  6-pin MMJ
       9       Local terminal		  6-pin MMJ
       10      Local terminal		  6-pin MMJ
       11      Local terminal		  6-pin MMJ

       The console baud rate is set to 9600 baud by default.  You can change this by setting the baud environment variable  from  console  prompt.
       Each  communication  port  from	the  serial  line controller behaves as described in the reference page and can be set to run at any of 16
       speeds.	For the encoding, see

       You can establish the state of port 2 (port 6) by specifying either modem or nomodem as part of the file entry.

Restrictions
       The speed default is to 9600 baud on the console port.  The console device must be set for 8-bit character length with one stop bit and	no
       parity. The driver enforces these restrictions; that is, changing speeds with the command may not always work on console ports.

Files
       Console terminal

       Full modem control or local terminal

See Also
       console(4), devio(4), tty(4), ttys(5), MAKEDEV(8)

								       RISC								    mdc(4)
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