The 127.0.0.0 loopback

watermelonss

Do people use 127.0.0.1 to make sure their NIC is working?

Also, if you get a successful loopback ping, what layer of the OSI model does that satisfy?

Thanks!

theodoxa

Yes. It proves the NIC itself is working. It doesn't tell you anything about anything beyond the NIC. The connection to the switch could still be bad. The IP Addressing might be incorrect. The routing on the router may be wrong. This is why if you got a successful PING on 127.0.0.1, you would proceed on to additional testing. Personally, I don't PING 127.0.0.1 unless I can't even reach my gateway.

Interestingly, any address in the 127.0.0.0/8 network is loopback.

gncsmith

Researching your questions, this has been asked before on Cisco's Learning Network and there is a few answers.

A ping tests layers 3 through 1

https://learningnetwork.cisco.com/thread/4161

First off, even though we learn the OSI model as a basis for understanding the layered approach, most of the actual technologies developed by OSI aren't implemented. Hence, it is more appropriate to use TCP/IP model as this is what the industry actually uses. That said I understand your intent.

On a PC when you ping your Loopback the NIC is involved down to the NDIS(network driver interface specification), which is the driver software for the NIC and resides between Internet/Data Link layer. This driver is responsible for allowing multiple protocol stacks i.e. IPX/SPX, TCP/IP to coexist. So the ping doesn't actually get framed and sent out on the wire. It simply "polls" the appropriate stack driver to see if it is configured and able to respond to requests.


On a router if you ping your loopback address the ping does go out on the wire to the next hop and then returns to you. So in this case all Layers are involved.

Here is a link to HP that discusses two loopback tests...
1. Which is the normal Network level loopback "ping 127.0.0.1" will test your TCP/IP stack and thus layers 4 down..

2. Transport level loopback using internet services such as a browser, FTP or telnet etc. and this is how you will test the application layer!

http://docs.hp.com/en/B2355-90748/ch04s05.html

NOTE: They actually list a network loopback test as validating layers 1-3! This is true with the exception that you have verified the TCP stack as well which in my mind justifies layer 4 but technically I guess you have to establish a TCP connection to validate that 4 is up and thus need to perform the transport loopback test! The diagram is somewhat confusing so just read through the info! Good troubleshooting information here!

james43026

I would just like to say that the information that gncsmith has posted is correct. A loopback ping only tests the protocol stack being used for network communication, and does not test anything physical. This is actually a very common misknomer in the IT world, and you will hear a ton of people argue that a loopback test will verify that your NIC is working. The only way to verify the NIC is working is with a ping test / traceroute, or even better a loopback plug, this is if you are having connectivity issues. Generally if I do a loopback test and it fails, I immediately reset the TCP/IP stack and winsock, this usually resolves the issue.