Why the first and the last address of each network is not used as host address?

First address of a network/subnet is a network/subnet id and last address is the broadcast address.

the first address is IP network and this is your ID network host. 

Dear Sir the last ip address of any subnet is the broadcast id and the first ip address of any subnet is network id which represent the subnet itself.

First address is the network ID and the last address is used for Broadcast purposes.

Because first IP address is the network a address commonly known as network  ID. The first IP address of a subnet may not be 0 also last IP address may not be 255 that depends on the network mask.