Network Defense Essentials (NDE) Practice Exam 2026 - Free Network Defense Practice Questions and Study Guide

The reason for selecting the filter that specifies tcp.port eq 25 is that this filter directly corresponds to the standard port used by the Simple Mail Transfer Protocol (SMTP). SMTP is the protocol primarily used for sending emails on the Internet, and it operates over TCP. The default port for SMTP is indeed port 25, making it the proper filter to isolate SMTP traffic in Wireshark.

In contrast, the other options reference ports associated with different protocols: port 110 is used for Post Office Protocol version 3 (POP3), port 443 is used for Hypertext Transfer Protocol Secure (HTTPS), and port 80 is used for Hypertext Transfer Protocol (HTTP). Each of these protocols serves a different purpose in network communication, which is why they are not suitable for filtering SMTP traffic.