TCP and UDP use port numbers. Port numbers are used to indicate a network process or service from the perspective of both the source of the message and the destination.
Video – Transport Layer Port Numbers (15.2.1)
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TCP and UDP Port Numbers (15.2.1)
There are many services that we access through the internet in the course of a day. DNS, web, email, FTP, IM and VoIP are just some of these services that are provided by client/server systems around the world. These services may be provided by a single server or by several servers in large data centers.
When a message is delivered using either TCP or UDP, the protocols and services requested are identified by a port number, as shown Figure 15-3. A port is a numeric identifier within each segment that is used to keep track of specific conversations between a client and server. Every message that a host sends contains both a source and destination port.
Figure 15-3 Port Numbers Identify the Application in Use
When a message is received by a server, it is necessary for the server to be able to determine which service is being requested by the client. Clients are preconfigured to use a destination port that is registered on the internet for each service. An example of this is web browser clients which are preconfigured to send requests to web servers using port 80, the well-known port for HTTP web services.
Ports are assigned and managed by an organization known as the Internet Corporation for Assigned Names and Numbers (ICANN). Ports are broken into three categories and range in number from 1 to 65,535:
• Well-Known Ports – Destination ports that are associated with common network applications are identified as well-known ports. These ports are in the range of 1 to 1023.
• Registered Ports – Ports 1024 through 49151 can be used as either source or destination ports. These can be used by organizations to register specific applications such as IM applications.
• Private Ports – Ports 49152 through 65535 are often used as source ports. These ports can be used by any application.
Table 15-1 displays some common well-known port numbers and their associated applications.
Table 15-1 Well-Known Port Numbers and Applications
Some applications may use both TCP and UDP. For example, DNS uses UDP when clients send requests to a DNS server. However, communication between two DNS servers always uses TCP.
Search the IANA website for port registry to view the full list of port numbers and associated applications.