Computing desk
< October 1	<< Sep \| October \| Nov >>	October 3 >

Welcome to the Wikipedia Computing Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

October 2[edit]

Telnet port numbers[edit]

I'm pointing out here and now that this is a homework question. Don't you wish all homework questions were that blatant about it? The question asks if clients A and B initiate telnet connections to server S at about the same time, what source and destination port numbers would A, B, and S use. Wouldn't A & B both use port 23 since that's the default port for telnet? It then goes on to ask whether the ports would be the same or different based on whether the connections (A and B) were coming from the same or different hosts. Wouldn't it work in a similar fashion to HTTP and its use of port 80? I can't seem to find the text relating to this and don't recall reading it. Thanks for any assistance, Dismas|^(talk) 02:54, 2 October 2011 (UTC)[reply]

Hi Dismas. Clients A and B would send packets to port 23 on Server S. However, they would each send these packets from a different randomly-generated source port. You can see this if you open two command prompts by going to Start → Run... → cmd twice and typing telnet towel.blinkenlights.nl into one of the windows. In the other window, you type netstat -an and you'll see each TCP/IP connection to your computer. Look for the connection with the destination port like this: 94.142.241.111:23. That number is a socket — an IP address followed by a colon and the port number. But, in short, port numbers are pre-determined for the server (for the listening application). They are not pre-determined for the client application. As for the second question, they would also be different, even if they were coming from the same computer. Operating systems determine the return path by using different port numbers. One telnet session may have the return address of 192.168.1.100:5000 and another 192.168.1.100:5001. If the telnet server sends a packet to 192.168.1.100:5000, your computer will know that the packet is destined for the first telnet session.—Best Dog Ever (talk) 04:59, 2 October 2011 (UTC)[reply]

For more about the client port selection, see Ephemeral port. Unilynx (talk) 13:39, 2 October 2011 (UTC)[reply]

Thanks to you both! A much better explanation than what my teacher normally gives me! Dismas|^(talk) 02:55, 3 October 2011 (UTC)[reply]

What may be missing from a complete picture is that while the server is listen()ing on port 23, as soon as it accept()s the connection, a new ephemeral port is created to handle that particular TCP connection. So while connections are initiated at port 23, none of them occupies it permanently. --Stephan Schulz (talk) 20:15, 3 October 2011 (UTC)[reply]

What? The connection is permanently identified by the (sourceaddr,sourceport,destaddr,destport) tuple. One of those numbers is 23. Run netstat if you don't believe me. 67.162.90.113 (talk) 23:29, 3 October 2011 (UTC)[reply]

I always assumed that the new socket returned by accept() also implied a new port. Seems that I was wrong. Thanks for enlightening me. BTW, I ran netstat on an ssh connection, given that the number of open telnet ports is really quite limited nowadays ;-) --Stephan Schulz (talk) 13:26, 4 October 2011 (UTC)[reply]

YPbPr Problem[edit]

Hello all.

I've got a slight problem related to my old TV. See, for a few years now, I've been playing my Xbox on this fairly old TV, on standard resolution. As the TV's about 3 years old, I'd assumed it couldn't display HD. But this morning, I noticed a little sticker on the front that says 'HD Ready'. Now the problem is, I've got an Xbox HD cable, which I worked out is a YPbPr cable. The table on HD Ready says that if the TV says it is HD Ready, it must be able to accept YPbPr input, and indeed, when I cycle through the channels, there is one labelled 'YPbPr'. The problem is, I can't see where I should plug my YpbPr cables in. Here's a picture of the back: http://www.flickr.com/photos/68190540@N04/6203422208/lightbox/ (Apologies for the terrible quality). But basically, there's an input marked DVI (I understand that is an HD computer cable), One marked D-SUB, one called PC Audio, A yellow (I think S-Video?) and two sound input, and A scart input. There's no YPbPr input that I can see. Do I need to buy a signal transformer of some sort? Thanks. 92.7.30.242 (talk) 10:56, 2 October 2011 (UTC)[reply]

Also, here's the product details: http://www.ciao.co.uk/Swisstec_J19_1__6615521 . Any help is appreciated! 92.7.30.242 (talk) 10:58, 2 October 2011 (UTC)[reply]

Are you sure the cable is YPbPr? The colour of the three plugs should be a guide so if you have red, white and yellow that is just sound + composite analogue video. Do the TV's menus or the user manual give you any information about switching one of the labelled inputs to YPbPr? Sometimes, the SCART connector can be set to receive composite or some kind of component video (though perhaps not YPbPr). It also depends on which type of Xbox you have: this one: Xbox, or this one: Xbox 360 as to what output it is capable of producing. Astronaut (talk) 16:05, 2 October 2011 (UTC)[reply]

It seems that your TV doesn't actually have a YPbPr input. I've helped someone set up an XBox before. I think it has an HDMI output that carries digital video & audio to a TV that has an HDMI input. In your case, you have only a DVI input but no HDMI. Maybe you can look into getting a HDMI-to-DVI adapter. The audio may not work, as DVI is video-only, at least the older implementations are. If the audio doesn't work, you can use analog or optical audio, but there's some complication. Once you plug in an HDMI cable, you cannot plug in the original analog A/V connector. There are slim audio adapters that can plug into the analog A/V output connector even with an HDMI cable plugged in. People have invented hacks to work around the plug interference problem without an adapter, but that involves removing the plastic housing of the standard A/V cable. See [1]. If this seems a bit complicated, I'll admit it is, but I don't have another suggestion. Good luck. --72.94.148.76 (talk) 16:19, 2 October 2011 (UTC)[reply]

Some TVs have YPbPr via the scart. I would check your manual to see if there is any mention of it. BTW, does your TV say HD ready or does it have the HD ready logo in the above article? If it's the former, this may not mean the same thing as implied by the logo. Nil Einne (talk) 16:40, 2 October 2011 (UTC)[reply]

On closer inspection, although on flicking through the channels it just says 'YPbPr', when you look in the menu, it says 'D-Sub (YPbPr)'. So i guess I'm going to have to get one of these : http://forums.bit-tech.net/showthread.php?t=154360 . Incidentally, it's a 360, and definitely YPbPr cables; they're red, green, and blue. 92.7.30.242 (talk) 17:39, 2 October 2011 (UTC)[reply]

I once owned a television in which the component inputs were striped downward across the same plugs as the composite inputs (which ran across). Looking at your photos, though, this doesn't seem to be the case. And as an aside (mainly to other answerers, not the OP), early XBox 360s did not have HDMI, it was added with the Zephyr series motherboards. See Xbox 360 hardware for details. gnfnrf (talk) 04:07, 3 October 2011 (UTC)[reply]

Right, the cables have arrived, I have hooked it all up, and my Xbox is now displaying in glorious HD! Thank you everyone who answered this question, I know that having to do tech support must be pretty annoying. Could someone please mark this as resolved? 92.7.31.251 (talk) 18:51, 4 October 2011 (UTC)[reply]