Loopback ping

w^rl0rd

On the Exam Cram practice test, it says that the loopback ping only verifies that TCP/IP is installed. I thought it tested the functionality of the NIC card as well. Is this true?

RussS

I guess it depends on how one looks at it. Mike Meyers states that the loopback address has a number of uses and one of the most common is to use it with the ping command to test a NICs ability to sedn and recieve packets.
Now consider - how can the NIC send a ping packet if it is not functional?

Paisleyskye

Ping does test the functionality of the card. Once you've proved the card works, you need to start looking for hardware conflicts or software problems. We used to use that old trick when I worked for Dell Canada - if Pinging the Loopback worked, they didn't get a new card - they got format/reintalled.

I know, I know....but Dell's plan of attack was always format/reinstall and diagnostics. We hated replacing parts - well I didn't hate it but we got in big trouble from the big wigs if we replaced too many parts in a week.

Drakonblayde

A loopback ping does not test the functionality of the card. It tests the functionality of IP-connectivity. A loopback ping doesn't actually put anything out on the network, so that does not test the cards ability to send or receive. Even if it did test the functionality of the card, a failed ping test doesn't necessarily indicate that the NIC is bad, it could simply be a TCP/IP stack being corrupt.

But the easiest way to see that a loopback ping doesn't touch the hardware is to go find a 95/98 machine with an external modem attached and no NIC in the machine. In the network panel, make sure only Client for Microsoft Networks, Dial-Up Adapter, and TCP/IP are installed. Turn off the external modem. Run a loopback ping. If the stack is installed and working correctly, you will get responses. Now the loopback ping may talk to the device driver. In this case, it's talking to a virtual NIC, the DUA, and no hardware, but I'm sure we've all run into instances where the driver for a device was loading but the device itself isn't working properly, or vice versa. In the case of a NIC, if some part of the transceiver were to go bad, the NIC wouldn't be able to send/receive, but the device driver would still likely load properly.

This one I know from the years working dial-up support, before most folks had NIC's in their machines. One of the first things we checked out was whether or not the stack was installed properly by pinging localhost (because most folks didn't have two phone lines, so they couldn't talk to us and be connected at the same time).

Now a ping to any other box on the network or outside the local subnet can tell you if the card is functional only if it succeeds, but that's only a by-product. If it fails, there is still the possibility of the local IP stack being corrupt, a bad cable, a bad hub/switch, misbehaving router, etc. That's why if a network ping fails and you want to check whether the NIC is bad or not (and can't tell from the lights on the NIC), the manufacturers diagnostic tools or replace suspected bad for good should be the next step.

curio

There are loopback tests and there are loopback tests.
Pinging 127.0.0.1 doesn't check the NIC only that the stack is functional. You can as stated by Drakonblayde ping 127 without even having a NIC. Try pinging 127.9.0.2

Webmaster

I agree, (thanks for this topic, I see that I forgot to add this essential item to the Network+ TCP/IP Suite TechNotes, will add it today or tomorrow.) only a hardware loopback will test the functionality of the NIC itself. The loopback address merely exists in software, successfully pinging the loopback address 127.0.0.1 will only proof that TCP/IP has initialized properly.

RussS

Correct. However if you can ping another machine on your segement or perhaps the gateway then that is an indicator that the NIC is working. Personally I have never bothered with a loopback plug as that usually means crawling on the floor to get around the back of the machine and I would rather be nice and comfortable sitting at the users workstation

curio

WTG RuSs - avoid crawling around on the floor if at all possible - the stuff you find down there!

jacev1

First off,
I know this is an old post, but I need your help. Another tech and I were discussing how the "ping localhost" works. I said it can test the TCP/IP stack and with some diags on a floppy you can test some of the nic's functions, but that you need a hardware looback adapter the test the nic itself. He says no, all you need to do is ping "127.0.0.1" and if you get a reply the nic works & you should look elsewhere for problems. According to this thread I'm right, he's wrong( and I should rule the world ..Oops wrong forum). Anyway, I could not find a link online that explains this so I can email it to him. Also does anyone know where I can find a hardware loopback adapter cheaper than $80.00? I tried Ebay, but couldn't find one. I thought I read somewhere that you can make your own, any ideas?

TIA,
Joe.

Webmaster

There are several ways to convince your friend, e.g.:
- let him try to pull a couple of pins out of the NIC's interface
- or let him disable the NICs interface (i.e. on Win XP)

and he'll see that he is still able to ping the ip loopback address successfully.