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What is a loopback cable ?

Posted on 2004-03-21
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Last Modified: 2007-12-19
after i assign IP address(Ex: 192.168.0.1) to my pc, I can't ping using the ip that i have assigned unless i connect my computer to a hub.

I heard that we can make a loopback cable using UTP and we just connect one end of that cable to the pc and no need hub and then we can ping the ip that we assigned to that pc.

So i followed their instruction to make the cable, but it doesn't work after i connect this cable to my pc.

Please help,


Thanks,
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Question by:nsophat
7 Comments
 
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Expert Comment

by:rafael_acc
ID: 10648024
A loopback cable is also called "cross over cable". It's used for directly connecting 2 computers using a STP (UTP) network cable. STP cable is the acronym for "Shielded Twisted Pair Cable" and UTP is for "Unshielded Twiste Pair Cable". These both cable types are usualy used for connecting computers to a hub/switch/firewall through an RJ45 connector. However, when you don't have a switch/hub/firewall you can invert the wires in a way that you could connect 2 computers directly.

You can also find adaptors wich make posible switching from one type to the other.

Example 1 (over a STP/UTP cross-over cable)
······················································
                 Computer 1 ----------------------- Computer 2

Example 2: (over a standard STP/UTP cable)
······················································
                 Switch ------------Computer 1
                           |
                            ----------- Computer 2

See these links:

· http://gccprinters.com/support/doc/crossover.html
· http://www.johnscloset.net/wiring/crossover.html
· http://www.cabletron.com/support/techtips/tk0231-9.html
· http://www.makeitsimple.com/how-to/dyi_crossover.htm

So, my answer is that you surely didn't make your cable corectly! That's for sure. Try buying the cable on the market or an adaptor.

Cheers.
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Expert Comment

by:pseudocyber
ID: 10648312
Sorry, but I have to disagree - a Loopback cable is NOT a crossover cable.  They are related, but not the same thing.

A cross over cable would cross the Transmit Pair over to the Receive Pair and vice versa (TX-RX, RX-TX) which would allow a cross over cable to link two similar devices Data Communication Equipmnent (DCE-DCE (Like two switches)) or Data Terminal Equipment (DTE-DTE (Like two PCs)).  

A Loopback cable - typically referred to as a "Loopback plug" is simply a small stub of cable which takes the Transmit Pair and brings it right back into the Receive Pair - so that the device is transmitting and receiving to itself.  It's typically used for testing situations.

In a 568B pinout - one end of a cross over would have this color scheme (Pin 1 at the top of the list and 8 at the bottom)
White Orange
Orange
White Green
Blue
White Blue
Green
White Brown
Brown

If you want to cross (or loopback) then reverse pins 1 to 3 and 2 to 6 so that the other end would have this scheme:
White Green
Green
White Orange
Blue
White Blue
Orange
White Brown
Brown

A Loopback would have White Orange (pin 1) to White Green (pin 3) and Orange (pin 2) to Greeen (pin 6)

HTH
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by:kabaam
ID: 10648454
I agree with pseudocyber
Have you tried to ping 127.0.0.1......... that is a loopback address for t/s the NIC and tcp/ip
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Expert Comment

by:rafael_acc
ID: 10648890
Ok guys. I realy didn't know that. Thank you for your comments. I'm now reacher in knowledge :)
Here is one more link on this, if needed.
http://www2.adtran.com/support/technotes/t1ddsadptxvr/


Thanks again,
Cheers.
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Author Comment

by:nsophat
ID: 10655470
According to pseudocyber comment, i did make my cable right.

If i ping using 127.0.0.1, yes it works right away.

But if i ping using an ip address that i assign to the network card 192.168.0.1, it doesn't work at all.

When i ping, i have the loop back cable connected to my pc.

So it want to make sure whether i can ping using the 192.168.0.1 address with just the loopback cable connected ?

Thanks,
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Expert Comment

by:iwontleaveyou
ID: 10656503
You cant ping a your computer without connecting it to the hub
becoz the hub recieves the request for ping then broadcast the ip to all the ports including the one it recieved the request from, and this is how your computer respond to the broadcast.

the only way to ping your own PC without any hub is to ping using

ping 127.0.0.1
   OR
ping loopback

hope you understand.
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Accepted Solution

by:
pseudocyber earned 50 total points
ID: 10656904
Ping (ICMP Request) is not a broadcast.  It's a Unicast.

You CAN ping your own computer without connecting it to anything.  If you can ping the loopback address (127.0.0.1) this indicates your TCP/IP stack is functioning.  If you cannot ping the NIC, with a static address set, this indicates the NIC is malfunctioning in some way.  You do not need a loopback plug, or to have it connected, in order to ping it.

You DO need the loopback plug in order to perform certain types of diagnostic tests - usually included with the NIC driver.

Nsophat, you may want to try swapping out the NIC.
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