The most common TCP and UDP default ports
1.5 Identify common TCP and UDP default ports
As I mentioned, the Transport layer of the TCP/IP protocol suite contains two primary protocols: TCP and UDP. There are many protocols, most at the Application layer, that use one or the other, or even both, of these protocols as they work their way down the OSI model. The independent sessions that are created are possible because of a logical connection called aport.
If people performed only one task at a time with each computer, there might not be a need for ports, but we all know that computers can perform many tasks at one time. Because this is the case, you 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, the system assures 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 the following sections, I will discuss the various types of TCP and UDP ports and describe their general use. You should be able to identify the port number that each of the most common network protocols, services, and applications use, as shown in Table 1.6. You should know the port number when given a service as well as the service when given a port number.
Service, protocol, or application
|
Port assignment
|
Most often used with TCP, UDP, or both
|
|---|---|---|
File Transfer Protocol (FTP)
|
20, 21
|
TCP
|
Secure Shell (SSH)
|
22
|
TCP
|
Telnet
|
23
|
TCP
|
Simple Mail Transfer Protocol (SMTP)
|
25
|
TCP
|
Domain Name System (DNS)
|
53
|
TCP/UDP
|
DHCP
|
67, 68
|
TCP
|
Hypertext Transfer Protocol (HTTP)
|
80
|
TCP/UDP
|
Network Time Protocol (NTP)
|
123
|
TCP
|
Internet Message Access Protocol (IMAP)
|
143
|
TCP
|
Simple Network Management Protocol (SNMP)
|
161
|
UDP
|
RDP
|
3389
|
TCP
|
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