Dear Jack, you will see loopback advertised as a /32 in ospf, since ospf treats loopback as a stub network meaning no other routers can connect to it :-), in other routing protocols like rip and eigrp you will usually see original mask that is configured, its just the case in ospf, if you want to revert it to its original mask then run this command on loopback
Dear jack, may be i was confused with your question, i think you meant to ask that when we are assigning ip addresses then in that case sometimes we use /32 and sometimes /24 ( or any other valid mask ), see, loopback is normally used to simulate LANS, now for example, in your practice you would like to implement an access-list that denies ip packet from 10.0.0.1 but not from 10.0.0.2 ! now you just have routers connected and no PC, how will you test it now ? if i use loopback 0 to assign 10.0.0.1/24 then i cannot assign this same network on any other interface ( including other loopbacks ) so what i will do, is that i will assign 10.0.0.1/32 and 10.0.0.2/32 to 2 different loopback interfaces to simulate my testing, also remember loopback interface are definetly used for some useful configuration in OSPF, BGP and others but mainly its purpose is for testing different situations as well
The main difference between the 2 implementations is that with the first one you are wasting a whole Class C address on an interface while on the 2nd one, you are assigning a host IP, hence you can use 192.168.1.2 - 254 for loopbacks in other routers .
That's the main reason why loopbacks are assigned /32, for IP address preservation.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...