127.0.0.0, is class A address,is all the series is used for loopbacking,or there are some other functions,?

Answer 1

127.x.x.x addresses are used for loopbacks only, no other useful functions. You can ping 127.x.y.z where x,y,z can be anything from 0 to 255 and you'll always get replies. Most common is 127.0.0.1, but you could ping 127.45.67.89 and you'd still get replies, doesn't matter. Note that these are "reserved" addresses, so one could argue if they're technically "Class A" or not. They are in that class A range but not useable addresses like regular unreserved addresses.