There is a few things going on here. Everyone that has replied that I have seen so far, has missed a major thing. Source Port, and Destination Port swap for the return packet. Scenario #1, You want to browse a webpage. Your PC: 10.192.168.1 Web Server: 10.0.0.2 Your PC sends out a packet, this packet will look partially similiar to this layout: Source Address (Where it came from) : 10.192.168.1 Source TCP Port (Random number > 1023) : 31000 Destination Address (Where it is going) : 10.0.0.2 Destination Port (Well-known service port) : 80 When this packet comes to the webserver, it looks.. "My IP Address? Yes. Port number? tcp/80. Do I know what that port is? Yes, pass it to Apache." Apache then looks, "Source 10.192.168.1, new session. Source port 31000." It adds : 10.192.168.1,31000 to a table of active connections, and preps a packet with the information requested. It will look like this: Source Address (Where it came from) : 10.0.0.2 Source TCP Port (Random number > 1023) : 80 Destination Address (Where it is going) : 10.192.168.1 Destination Port (Well-known service port) : 31000 The packet is then sent through the network to your PC. Your PC looks at the packet, "For my IP? Yes. What port? tcp/31000. Do I know what that port is? Yes, I just sent a request in Netscape window #2 on that port to the source address of this packet." Pass it to Netscape, which opens the file. This process continues, allow with TCP ACK packets, since this is a tcp session, until that file is loaded. This is also done in several threads at once, to load your webpage faster. If this still confuses you, draw a picture on a big piece of paper.. two computers.. several arrows going left and right, and write it out visually. Just try to remember that for the scenario listed, your PC always uses tcp/31000 (only in this scenario, it is a random number between 1024-65536).. the webserver always uses tcp/80. Whether these numbers are "Source" or "Destination", depends on the direction of the arrow. 1 More attempt.. if the first didn't work. Think of it like an airplane flight, round trip. You have a source airport, and a destination airport. On the way home, they are swapped. Vancouver to Toronto: To Toronto, your ticket (packet) is like this: Source Airport (where I'm leaving) : YVR (just like an IP, it is unique) Source gate : Domestic, A30 Destination Airport (Where I'm going) : YYZ Destination gate : Domestic, I43 On the way home, for the sake of this example, your flight happens to use the same gates (since they do in a tcp session). This is a round trip ticket, so I'm not leaving from Vancouver, I'm going to Vancouver. Source Airport (where I'm leaving) : YYZ (just like an IP, it is unique) Source gate : Domestic, I43 Destination Airport (Where I'm going) : YVR Destination gate : Domestic, A30 Just an analogy to see if it helps.. some people get it, some don't. That's the way I thought of it at first, and now it's just second nature. Regards, Trevor Corness, CCNA MCSE MCP+I Network Systems Engineer, DataCom BMS Communications Ltd. http://www.bmscom.com -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of RAUNIYAR RAJEEV Sent: Friday, September 29, 2000 1:05 PM To: [EMAIL PROTECTED] Subject: Could someone help me ! Hi all, now i DO have a question. i'm reading up about ports used by TCP/UDP protocols but im having trouble visualizing where the source port and destination ports fit in. im thinking that the destination port (suppose on a www, http segment) of 80, would be on the server from which we will download the data right? and we would specify a port (called source port) to which we want the data to come into our machine right? but then how would the www server distinguish between many sessions if their port is always port 80?? another example... suppose a college closes a "napster" port... can't you just log onto the napster server using a different port from your college? hmm.. i really confused. could you somehow help me visualize where these ports are in the network. and who sets them and how destination servers and clients differ etc.. thanks,
BEGIN:VCARD VERSION:2.1 N:Corness;Trevor FN:Trevor Corness ORG:BMS Communications;DataCom TITLE:Network Systems Engineer TEL;PAGER;VOICE:604-631-7867 ADR;WORK:;;2880 Production Way;Burnaby;BC;V5A4T6;Canada LABEL;WORK;ENCODING=QUOTED-PRINTABLE:2880 Production Way=0D=0ABurnaby, BC V5A4T6=0D=0ACanada URL: URL:http://www.bmscom.com EMAIL;PREF;INTERNET:[EMAIL PROTECTED] REV:20000921T155409Z END:VCARD
