Hi,

Class A network addresses 1.0.0.0-126.0.0.0
Class B network addresses 128.0.0-191.0.0.0
in between 127 is left, where it is being used?

well, the IP 127.0.0.0-127.255.255.255 refer to the local host and network – loopback addresses.

Similarly, you will find:
224.0.0.0-239.0.0.0 is reserved to be used by multicast protocols
240.0.0.0-255.0.0.0 is reserved, you cannot used them
10.0.0.0-10.255.255.255 is reserved for private networks
172.16.0.0-172.31.255.255 reserved for private networks
192.168.0.0-192.168.255.255 is also reserved for private networks