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.
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
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 :)
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.
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.