A security administrator has configured FTP in passive mode. Which of the following ports should the
security administrator allow on the firewall by default?

A.
20

B.
21

C.
22

D.
23

Explanation:
When establishing an FTP session, clients start a connection to an FTP server that listens on TCP port 21
by default.
Incorrect Answers:
A: FTP uses port 20, but it is not the default port.
C: SSH uses TCP port 22.
D: Telnet uses port 23.
http://compnetworking.about.com/od/tcpip/p/port-numbers-21-ftp.htm
http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers