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CCENT/CCNA ICND1 100-101 Official Cert Guide by Wendell Odom

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Matching TCP and UDP Port Numbers

Extended ACLs can also examine parts of the TCP and UDP headers, particularly the source and destination port number fields. The port numbers identify the application that sends or receives the data.

The most useful ports to check are the well-known ports used by servers. For example, web servers use well-known port 80 by default. Figure 23-4 shows the location of the port numbers in the TCP header, following the IP header.

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Figure 23-4 IP Header, Followed by a TCP Header and Port Number Fields

When an extended ...

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