In a computer so many processes are runs each processes has a port number,
port number is used for identify processes on the host
port numbers are divided into two types
1.fixed port(ex:25,80)
2.os assigned dynamic port nos(0-65535)
so the port no’s range can be 0-65535

the host attaches source port num,dest port num with the data at transport layer so after the destination computer recieves this data packet it knows which port to send data by placing the destination port number