Filter
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Port numbers
A filter can also match a packet’s port number attributes, but only if the filter’s protocol
type is set to TCP or UDP, since only those protocols use port numbers. The filter can be
configured to match the following:
• The source port number (the port on the sending host that originated the packet)
• The destination port number (the port on the receiving host that the packet is destined
for)
By matching on a port number, a filter can be applied to selected TCP or UDP services,
such as Telnet, FTP, and World Wide Web. The following tables show a few common ser-
vices and their associated port numbers:
Port number comparisons
A filter can also use a comparison option to evaluate a packet’s source or destination port
number. The comparison options are:
• No Compare: No comparison of the port number specified in the filter with the
packet’s port number.
• Not Equal To: For the filter to match, the packet’s port number cannot equal the port
number specified in the filter.
Internet service TCP port Internet service TCP port
FTP 20/21 Finger 79
Telnet 23 World Wide Web 80
SMTP (mail) 25 News 144
Gopher 70 rlogin 513
Internet service UDP port Internet service UDP port
Who Is 43 TFTP 69
World Wide Web 80 who 513
SNMP 161