Question

what does port number mean here ?

Asked by: silverkid

my mobile operators ISP uses a proxy server. when i visit the site http://aruljohn.com . i can see the proxy server ip address and port number being displayed.

what does this port number mean ?

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Asked On
2009-09-21 at 03:46:23ID24747909
Tags

networking

,

mobile

,

ISP

,

ip address

,

port number

Topics

Cellular Phones & Carriers

,

TCP/IP

,

Network Operations

Participating Experts
1
Points
250
Comments
4

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Answers

 

by: Otto_NPosted on 2009-09-21 at 05:01:10ID: 25381963

It is the TCP/UDP port number associated with the particular data flow.  But let me explain:

IP packets carries information across IP networks, but it does not know how to handle continuous streams af data (such as the session data between you and the proxy-server).  This is where TCP comes in:  The Transmission Control Protocol (TCP) operates on Layer-4 of the OSI reference model, and handles the segmentation and reassembly of continuous streams of data between higher-level applications.

Because there can be more than one simultaneous connection to a specific server, even from the same host (i.e. having multiple browser windows open to the same site), TCP needs a way to distinguish between different data streams, even if the IP addresses are the same.  For this TCP implements a port number (also sometimes called socket number).  The port number is a 16-bit number that identifies the application/computer process that the specific packet is destined for.

Let me know if i need to point you to a couple of Internaet resources that can assist.

 

by: silverkidPosted on 2009-09-21 at 08:21:51ID: 25383822

i've understood to some extent but please be patient with me and clarify the doubts that i have below

i would like to further know if what you are saying means that each process / application requesting packets from a server on internet is assigned a specific port number . is that true ?
so if multiple tabs in a browser are browsing a yahoo site. each of these tabs have a different port number.
in that case each time i try to open a new url in my browser it gets assigned a new port number.

I am thinking that when we enter a new url in the browser , a new port number gets assigned to it . so all the ip packets generated by this process contain the same port number. the destination server , notes down this port number somewhere and when it sends a reply to the client browser it puts that port number in the ip packets.

is my understanding correct ?

 

by: Otto_NPosted on 2009-09-22 at 01:21:55ID: 25390560

More or less.  You need to keep in mind that you do have a source and destination port number, and that traffic is identified by source and destination IP address and port number.  The source port is usually chosen at almost random by the source application.

An example: HTTP servers listen for incoming requests on port 80.  So, when you type the website URL in the browser window, you application send a packet to the web-server with the destination IP the web-server IP, destination port 80, source IP your machine's IP address, and source port a randomly selected port number (usually upward of 10000).  Then, even if you open multiple sessions to the same website (all directed to the same IP and port), the web-server can distinguish between the sessions, based on the source IP and port.

Of course, when the data is delivered from the server to your PC, the source and destination data is reversed: Source IP and port becomes destination IP and port and vice versa.

If the URL in your browser windo display something like http://10.10.10.48:2034/, the browser is sending requests to dest. IP 10.10.10.48, dest. port 2034.  The source IP and port is not that easily visible, as it is embedded in the  software code.

You might wonder when I said HTTP servers listen on port 80, where the value 80 comes from?  Like all number ranges, the well-known port numbers are assigned by IANA (that also regulate public IP addresses).  You can see a list of these numbers at the following website: http://www.iana.org/assignments/port-numbers

For a very detailed overview of anything relating to TCP and IP, visit The TCP/IP Guide (http://www.tcpipguide.com).  This guide is very thourough, and goes into the detail mechanisms of IP and its associated protocols.  You can start of at http://www.tcpipguide.com/free/t_TCPIPPortsTransportLayerTCPUDPAddressing.htm, which covers much of the discussion to date.

 

by: silverkidPosted on 2009-09-22 at 03:13:13ID: 31631354

excellent support provide, thanks

20120131-EE-VQP-002

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