The delivery of an IP packet is similar in concept to the delivery of a letter in the postal system. The destination address on a letter consists of two parts used to deliver the letter to the final destination. The state, city, and street name are used to route the letter to the proper street. This is analogous to the network portion of an IP address used to route an IP packet to the proper network. The street number is used to determine the proper house while the host portion of an IP address is used to determine the destination host. The name on the letter determines the recipient of the letter, and on the Internet, the UDP or TCP port number serves a similar function. The port number is used to deliver the data to the proper application.
Although the concepts of mail and IP packet delivery are similar, the addressing details of IP are much more complicated and difficult to master. If you are planning on becoming a network professional, it is imperative that you master the details of IP addressing, subnetting, and summarization.