Since most network applications run on the same machine, the computer must be able to ensure that the software program on the destination machine can get the data packet from the source machine, and the source computer can receive the correct reply. This is done by using the "port number" of UDP. For example, if a workstation wants to use the domain name service system on workstation 128. 1.123.1,it will give the packet a destination address128.1and insert the destination port number 53 in the UDP header. The source port number identifies the application that the local machine requests domain name service, and all response packets generated by the destination station need to be assigned to this port of the source host.