Applications are types of traffic for which protocol standards exist. Each application has a transport protocol and destination port number(s) associated with it, such as TCP/port 21 for FTP and TCP/port 23 for Telnet. When you create a policy, you must specify an application for it.
You can select one of the predefined applications from the application book, or a custom application or application set that you created. You can see which application you can use in a policy by using the show application CLI command.
Note: Each predefined application has a source port range of 1-65535, which includes the entire set of valid port numbers. This prevents potential attackers from gaining access by using a source port outside of the range. If you need to use a different source port range for any predefined application, create a custom application. For information, see Understanding Custom Policy Applications.
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