Final answer:
The default TCP port number for SMTP is 25. SMTP is the protocol for sending emails, and while port 25 is the traditional default, ports 587 and 465 are also used for secure email submission.
Step-by-step explanation:
The default TCP port number for SMTP is a. 25. SMTP stands for Simple Mail Transfer Protocol, which is the protocol used for sending emails across the internet. Port 25 is the standard port that has been used for this purpose traditionally.
However, due to issues with spam and security, alternative ports like 587 for submission (often with encryption and authentication in place) and port 465 (SMTPS for SMTP over SSL) are also commonly used today. Port 80 is commonly associated with HTTP web traffic, while port 110 is used for POP3, which is the protocol used for receiving emails.
The default TCP port number for SMTP (Simple Mail Transfer Protocol) is 25. SMTP is a standard protocol for electronic mail transmission. Port 25 is dedicated to email transmission, allowing mail servers to communicate and relay messages across networks. When an email client sends a message, it connects to the SMTP server on port 25 to initiate the transfer.
therefore correct option is a. 25