LOOPBACK

[obomheire]

Please can some one help me out. I'm studying for CCNA 200-120 & what I'll like to know is that, 127.0.0.1 IPV4 address range is reserved for LOOPBACK purposes right? Why then do we see a LOOPBACK interface configured with say 172.31.1.1or 10.23.3.2 for example i.e different from the 127.0.0.1 range in most cases? Thanks.

[andreas.huelsbeck]

Hi obomheire,

the Loopback interfaces are used for unnumbered Interfaces or Management reasons and some other reasons.

The thaught is a little different too the Hosts Loopback Interfaces. Take also a look here https://learningnetwork.cisco.com/thread/17213 

Regards Andreas

[obomheire]

If I understand you correctly sir, the 127.0.0.1 range is used only for Host LOOPBACK address while any other address can be choosing for any kind of LOOPBACK you want to create say for admin purpose. If this is right Thanks. I have also visited the link you sent Thanks very much. More Power to Your Elbow.

[LJ Gabrillo]

Don't be confused with the 'interface loopback' command and the loopback address range

Note that the 'interface loopback' command is simply creating an virtual interface inside the router, that is always active and it can be assigned any IP address

The loopback address 127.x.x.x on the other hand is used for hardware testing
if you want to test it out, with your computer (im assuming it's windows), disconnect all your network connections, disconnect the wire, or disconect from the wireless

now open your CMD prompt and ping 127.X.X.X e.g., 127.1.1.1
There will be a reply :)

[obomheire]

Thank you very much, you just hit the nail on the head. You just solve my problem. Once again Thanks so much. More Power to your Elbow.

[LJ Gabrillo]

If it's not much, don't forget to rate so we can close this query :D

Cheers,

[Oliver Kaiser]

The reason we create loopback interfaces on networking devices is so we could associate the address with certain functions.

A loopback interface may use ip address different from the 127.0.0.0/8 range which is reserved for host loopback addresses. (RFC 990)

To make it more clear imagine this...

You have 4 routers in a full-mesh topology. You got dynamic routing set up and also advertise your loopback address. If you want to reach any router you may use the ip address of its physical interfaces but in case the one you try to reach is down you would not be able to connect to the router. That is when a loopback address comes into play.

Because it is not bound to any physical link you will still be able to reach the router by its loopback address. The routing protocol detects the failure and uses a different path to reach the router.

[obomheire]

Your explanation & description is very detailed, sound & cleared Thanks so much. More Power to your Elbow.

[manukrishnanm93]

127.0.0.1 is conventionally a computer's loopback address. Messages sent to loopback IP addresses like 127.0.0.1 do not reach outside to the local area network but instead are automatically re-routed by the computer's own network adapter back to the receiving end of the TCP/IP stack.

[obomheire]

Excellent!!! Thanks so much Loud & Clear.