2.11. Defining the Function of TCP/UDP Ports

If we performed only one task at a time with each computer, we might not need ports, but the fact is that computers can perform many tasks at one time. Therefore, we need a way to identify packets so that they will be processed by the computer in the correct manner. By identifying each packet with a port number, we ensure that the computer will direct the packet to the right area within it where the appropriate processes can be performed.

TCP and UDP port numbers are used to identify packets in regard to the services that they require. You can also filter traffic using these port numbers to restrict only specific types of traffic from a network. You should understand how TCP and UDP ports can be used to facilitate and control traffic. In this section, we will discuss the various types of TCP and UDP ports and describe their general use.

2.11.1. Critical Information

You should be able to define the function of TCP and UDP ports. You should be able to list the three main designations of ports. In addition, you should understand the general use of each designation of ports.

2.11.1.1. Port Designations

TCP/IP has 65,536 ports available. As you can imagine, some ports are used much more than others. Ports are divided into three main groups or designations as follows:

Well-known ports

These port numbers range from 0 to 1,023. These are the most commonly used ports that have been used for the longest period of time. When CompTIA states that you should know the definition of well-known ports, they are referring to these ports.

Registered ports

These port numbers range from 1,024 to 49,151. Registered ports are used by applications or services that need to have consistent port assignments. These ports, like the well-known ports, are agreed upon by most organizations for standardization of use.

Dynamic or private ports

These port addresses range from 49,152 to 65,535. These ports are not assigned to any particular protocol or service and can therefore be used for any service or application.

It is common for applications to establish a connection on a well-known port and then move to a dynamic port for the rest of the conversation. It's important that you understand port num-bers, because you may be configuring them for communication purposes as well as to provide filtering and therefore prevent communication of specified applications or services. In the next section, we will examine the most common specific port assignments more closely.

2.11.2. Exam Essentials

Know the port numbers and understand the use of well-known ports. The range of addresses for well-known ports is from 0 to 1,023. The well-known ports are the most common port designation.

Be able to list the port numbers and describe the use of registered ports. The range of addresses for registered ports is from 1,024 to 49,151. Registered ports are not used as often as well-known ports but are agreed upon for use by a specific service or application.

List the port numbers and undertand the use of dynamic or private ports. The range of addresses for dynamic, or private, ports is from 49,152 to 65,535. These port numbers are not assigned to any specific protocol, service, or application and can therefore be used by any service or application.