2.12. Identifying Well-Known Ports Associated with Services and Protocols
Now that we have discussed the general nature of ports, let's look at much more specific information about well-known ports. Although there are 1,024 well-known ports, only a handful of these are commonly used on networks. The ones that are used most frequently are not arranged in any logical order in regard to their use, so unfortunately the only way to remember most of them is to memorize them. In this section, we will pair up each of the most commonly used well-known ports with its protocol, service, or application.
2.12.1. Critical Information
You should be able to identify the port number that each of the most common network protocols, services, and applications use. You should know the port number when given a service as well as the service when given a port number.
2.12.1.1. Well-Known Port Numbers
As we said before, well known is the name given for the port designation, but not all of the numbers between 0 and 1,023 have a well-known service assigned to them. The good news is that you don't have to memorize 1,024 port assignments! The bad news is that you do have to mem-orize the port assignments in Table 2.8. As you can see, some of the ports are TCP (connection-oriented) ports and others are UDP (connectionless) ports. HTTP is an application that uses both types of ports.
TABLE 2.8 The Most Common Well-Known Port Numbers and Associated Services
| Service, Protocol, or Application | Port Assignment | TCP, UDP, or Both |
|---|---|---|
| FTP (File Transfer Protocol) | 20, 21 | TCP |
| SSH (Secure Shell) | 22 | TCP |
| Telnet | 23 | TCP |
| SMTP (Simple Mail Transfer Protocol) | 25 | TCP |
| DNS (Domain Name System) | 53 | UDP |
| TFTP (Trivial File Transfer Protocol) | 69 | UDP |
| HTTP (Hypertext Transfer Protocol | 80 | Both |
| POP3 (Post Office Protocol version 3) | 110 | TCP |
| NNTP (Network News Transfer Protocol) | 119 | TCP |
| IMAP4 (Internet Message Access Protocol version 4) | 143 | TCP |
| SNMP (Simple Network Management Protocol) | 161 | UDP |
| HTTPS | 443 | TCP |
You should memorize the port numbers in Table 2.8 so that you can recognize them when configuring servers, routers, switches, and other network equipment. You might use them to configure a service or protocol. In addition, you might use them to filter a protocol or service on a firewall. In either case, a familiarity with the port numbers will assist you in configuration as well as in communication about the services and applications themselves.
2.12.2. Exam Essentials
Understand the use of ports. Ports are used so that a computer can work with many applications at the same time and through the same physical interfaces. A port is a logical location in a computer where a packet is sent for processing based on the information contained in packet. In this way, the packets are sent to the logical place in the computer that has the correct resources to process the packet.
Memorize the most common port numbers. You should memorize the most common ports, such as HTTP, FTP, SMTP, and others. Be able to relate the port number to the application, and vice versa. This is essential information for troubleshooting as well as for general network knowledge.