TCP port 110 is used by which protocol?

• A. Simple Mail Transfer Protocol (SMTP)
• B. Internet Message Access Protocol (IMAP)
• C. File Transfer Protocol (FTP)
• D. Post Office Protocol version 3 (POP3)

Answer: (D)

Port numbers identify the specific service a host offers, with well-known ports assigned to common protocols. For email retrieval, the standard choice using a reliable, connection-oriented transport is POP3, which operates over TCP and has 110 as its traditional port. So when you see TCP port 110, think of the Post Office Protocol version 3 used to download mail from a server. (Note that a secure variant commonly uses a different port, 995, while other protocols use their own standard ports: SMTP on 25/587/465, IMAP on 143 and 993, and FTP on 20/21.)