201-450 Exam - Question 57

The port number in the excerpt is 21, which is commonly associated with FTP (File Transfer Protocol). However, the specific tcpdump output snippet provided shows typical communication patterns for HTTP over port 80, which is usually used by web traffic. The traffic flags such as 'ack' and 'seq' followed by data (indicated by flags like '[P.]') are characteristic of HTTP traffic. The HTTP service utilizes TCP, and the flags in the communication indicate a session with data or web content transfer, typical in HTTP. Therefore, the correct network service or protocol illustrated in this tcpdump output is HTTP.