Which of the following TCP port numbers is used by Simple Mail Transfer Protocol (SMTP)?

A.
23

B.
21

C.
53

D.
80

E.
57

F.
25

Explanation:
TCP port 25 is assigned to SMTP. SMTP is a Transmission Control Protocol (TCP)/ Internet Protocol (IP)
protocol used to send and receive e-mail messages.
Important TCP port number assignments are as follows:
TCP port 23 is used by Telnet to allow remote logins.
TCP port 21 is assigned to File Transfer Protocol (FTP) for FTP control. FTP also uses port 20 to transmit
FTP data.
TCP and User Datagram Protocol (UDP) port 53 is assigned to Domain Name Service (DNS), which is used
to convert hostnames into Internet Protocol (IP) addresses.
TCP port 80 is used by Hypertext Transfer Protocol (HTTP), which is the base for transferring Web pages
over the Internet.
TCP port 57 is assigned to Mail Transfer Protocol (MTP).
TCP port 22 is used by Secure Shell (SSH).
UDP ports 67 and 68 are used by Dynamic Host Configuration Protocol (DHCP).
UDP port 69 is used by Trivial FTP (TFTP).
TCP port 110 is used by Post Office Protocol 3 (POP3).
UDP port 161 is used by Simple Network Management Protocol (SNMP).
TCP port 443 is used by Secure Sockets Layer (SSL).
TCP port numbers help to direct data to the appropriate application, service, or application window. TCP port
numbers ensure that data is displayed in the correct browser window when accessing Web data from multiple
sources, and ensures it is directed to the proper application or service when received.
Objective:
Infrastructure Security
Sub-Objective:
Configure, verify, and troubleshoot IPv4 and IPv6 access list for traffic filtering

Internetworking Technology Handbook > Internetworking Basics > OSI Model and Communication Between
Systems>Multiplexing Basics