File transfer and remote access: FTP, SSH, Telnet, RDP 15
A firewall log shows a server opening connections from its own port 20. Which service is sending data?RFC 959, File Transfer Protocol, section 3.2
A help desk cannot open a remote desktop session to a Windows PC that answers pings. Which port should be checked on its firewall first?Microsoft Learn, Change the Remote Desktop listening port on your computer
A port scan of a Windows PC lists the service name "ms-wbt-server" on port 3389. Which protocol on the A+ list is that?IANA Service Name and Transport Protocol Port Number Registry, port 3389
A port scan reports the service name "ftp" on an open port. Which port number is it?IANA Service Name and Transport Protocol Port Number Registry, port 21
A router accepts encrypted command-line logins on TCP port 22. Which protocol is in use?IANA Service Name and Transport Protocol Port Number Registry, port 22
An old switch is managed through a remote terminal session on port 23. Which protocol is that?IANA Service Name and Transport Protocol Port Number Registry, port 23
FTP's control connection follows the conventions of which other protocol on the A+ list?RFC 959, File Transfer Protocol, section 2.3
Show all 67 questions (60 more)
File transfer and remote access: FTP, SSH, Telnet, RDP, continued
Which of these is a stated objective of FTP?RFC 959, File Transfer Protocol, section 1
Which port does FTP use for its control connection?IANA Service Name and Transport Protocol Port Number Registry, port 21
Which port does SSH use?RFC 4253, The Secure Shell (SSH) Transport Layer Protocol, section 4.1
Which port does Telnet use?RFC 854, Telnet Protocol Specification, Port Assignment
Which port does the Remote Desktop Protocol (RDP) listen on by default?Microsoft Learn, Change the Remote Desktop listening port on your computer
Which port is registered for FTP's default data connection?IANA Service Name and Transport Protocol Port Number Registry, port 20
Which protocol is described as providing secure remote login over an insecure network?RFC 4251, The Secure Shell (SSH) Protocol Architecture, abstract
Why is SSH on port 22 preferred to Telnet on port 23 for managing a device?Wikipedia, Telnet
Email: SMTP, POP3, IMAP 11
A mail client downloads messages from TCP port 110. Which protocol is it using?IANA Service Name and Transport Protocol Port Number Registry, port 110
A mail server relays messages to other mail servers over TCP port 25. Which protocol is it speaking?IANA Service Name and Transport Protocol Port Number Registry, port 25
A phone and a laptop show the same mail folders, kept on the server and reached on TCP port 143. Which protocol is in use?IANA Service Name and Transport Protocol Port Number Registry, port 143
A user can receive mail but outgoing messages never leave the mail server. The server's firewall most likely blocks which outbound port?RFC 5321, Simple Mail Transfer Protocol, section 4.5.4.2
What is the stated objective of SMTP?RFC 5321, Simple Mail Transfer Protocol, section 1.1
Which mail protocol lets a client work with mailboxes on the server as if they were local folders?RFC 9051, Internet Message Access Protocol (IMAP), Version 4rev2, abstract
Which port does IMAP use for a cleartext connection?RFC 9051, Internet Message Access Protocol (IMAP), Version 4rev2, section 2.1
Which port does POP3 use?RFC 1939, Post Office Protocol, Version 3, section 3
Which port does SMTP use?RFC 5321, Simple Mail Transfer Protocol, section 4.5.4.2
Which transport protocol do SMTP, POP3 and IMAP all run over?RFC 1939, Post Office Protocol, Version 3, section 3
With POP3, what normally happens to mail once the client has fetched it?RFC 1939, Post Office Protocol, Version 3, section 1
Web, DNS and DHCP 15
A browser shows a padlock and connects to TCP port 443. Which protocol is in use?IANA Service Name and Transport Protocol Port Number Registry, port 443
A capture shows a new laptop sending UDP from port 68 to port 67. What is the laptop asking for?RFC 2131, Dynamic Host Configuration Protocol, section 4.1
A firewall allows outbound TCP 80 and blocks TCP 443. What will users notice?RFC 9110, HTTP Semantics, sections 4.2.1 and 4.2.2
A PC opens a website by its IP address but not by its name. Traffic to which port is most likely failing?RFC 1035, Domain Names: Implementation and Specification, sections 3.2.2 and 4.2
A services file lists port 53 under the name "domain". Which protocol on the A+ list uses it?IANA Service Name and Transport Protocol Port Number Registry, port 53
A services file names ports 67 and 68 "bootps" and "bootpc". Which protocol on the A+ list uses them?IANA Service Name and Transport Protocol Port Number Registry, ports 67 and 68
A web server answers unencrypted requests on TCP port 80. Which protocol is that?IANA Service Name and Transport Protocol Port Number Registry, port 80
To which port does a DHCP client send its messages?RFC 2131, Dynamic Host Configuration Protocol, section 4.1
To which port does a DHCP server send its replies?RFC 2131, Dynamic Host Configuration Protocol, section 4.1
What does DHCP add to the older Bootstrap Protocol (BOOTP)?RFC 2131, Dynamic Host Configuration Protocol, abstract
What is the size limit of a DNS message carried by UDP under the original specification?RFC 1035, Domain Names: Implementation and Specification, section 4.2.1
Which DNS activity must use TCP on port 53 rather than UDP?RFC 1035, Domain Names: Implementation and Specification, section 4.2.1
Which port does DNS use?RFC 1035, Domain Names: Implementation and Specification, section 4.2
Which port does HTTP use by default?RFC 9110, HTTP Semantics, section 4.2.1
Which port does HTTPS use by default?RFC 9110, HTTP Semantics, section 4.2.2
Directory and file sharing: NetBIOS, LDAP, SMB 13
A scan lists the service names netbios-ns, netbios-dgm and netbios-ssn. Which ports are open?IANA Service Name and Transport Protocol Port Number Registry, ports 137 to 139
A scan of a file server lists the service name "microsoft-ds" on port 445. Which protocol on the A+ list is that?IANA Service Name and Transport Protocol Port Number Registry, port 445
A Windows PC must share files with a very old system that knows only SMB over NetBIOS. Which TCP port carries those sessions?Microsoft Learn, Direct host SMB over TCP/IP, More information
An administrator disables NetBIOS over TCP/IP on a Windows PC. Which port carries all of its SMB traffic from then on?Microsoft Learn, Direct host SMB over TCP/IP, More information
An application looks up user accounts in a directory server on TCP port 389. Which protocol is it using?RFC 4511, Lightweight Directory Access Protocol (LDAP): The Protocol, abstract
In Microsoft's list of the ports NetBIOS over TCP traditionally uses, which service is listed with UDP only?Microsoft Learn, Direct host SMB over TCP/IP, More information
What does LDAP give a client access to?RFC 4511, Lightweight Directory Access Protocol (LDAP): The Protocol, abstract
What kind of traffic does Windows carry with the SMB protocol?Microsoft Learn, Direct host SMB over TCP/IP, Summary
Which port does LDAP use?IANA Service Name and Transport Protocol Port Number Registry, port 389
Which port does SMB use when it is hosted directly on TCP, without NetBIOS?Microsoft Learn, Direct host SMB over TCP/IP, More information
Which port does the NetBIOS datagram service use?RFC 1002, NetBIOS Service on a TCP/UDP Transport: Detailed Specifications, section 6, Defined Constants and Variables
Which port does the NetBIOS name service use?RFC 1002, NetBIOS Service on a TCP/UDP Transport: Detailed Specifications, section 6, Defined Constants and Variables
Which port does the NetBIOS session service use?RFC 1002, NetBIOS Service on a TCP/UDP Transport: Detailed Specifications, section 6, Defined Constants and Variables
TCP vs. UDP 13
An application needs ordered, reliable delivery of a stream of data. Which transport does the UDP specification tell it to use?RFC 768, User Datagram Protocol, Introduction
How wide is the port number field in a TCP or UDP header?RFC 9293, Transmission Control Protocol (TCP), section 3.1
What does the Window field in a TCP header tell the other side?RFC 9293, Transmission Control Protocol (TCP), section 3.1
What is the procedure that opens a TCP connection called?RFC 9293, Transmission Control Protocol (TCP), section 3.5
What is the range of the System Ports, also known as the Well Known Ports?RFC 6335, Service Name and Port Number Procedures, section 6
Which field does a TCP header carry that a UDP header does not?RFC 9293, Transmission Control Protocol (TCP), section 3.1
Which port on the A+ list lies outside the Well Known Ports and in the Registered Ports?RFC 6335, Service Name and Port Number Procedures, section 6
Which port range is never assigned to a service and is left for dynamic, private or ephemeral use?RFC 6335, Service Name and Port Number Procedures, section 6
Which protocol on the A+ list uses UDP as its transport?RFC 2131, Dynamic Host Configuration Protocol, section 4.1
Which statement describes TCP?RFC 9293, Transmission Control Protocol (TCP), section 2.2
Which statement describes UDP?RFC 768, User Datagram Protocol, Introduction
Which TCP control bit is set in the segment that asks to open a connection?RFC 9293, Transmission Control Protocol (TCP), section 3.1
Which TCP control bit tells the other side that the sender has no more data?RFC 9293, Transmission Control Protocol (TCP), section 3.1
How this list was compiled: Own questions, written to objective 2.1 of CompTIA's A+ Core 1 (220-1201, V15) exam objectives, document version 4.0: compare and contrast TCP and UDP ports, protocols and their purposes. The objective lists FTP 20-21, SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67/68, HTTP 80, POP3 110, NetBIOS/NetBT 137-139, IMAP 143, LDAP 389, HTTPS 443, SMB/CIFS 445 and RDP 3389, and TCP vs. UDP. The objectives are linked from https://www.comptia.org/en-us/certifications/a/core-1-v15/ (read 2026-10-12). Every port number was read on 2026-10-12 in the IANA Service Name and Transport Protocol Port Number Registry (https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml, last updated 2026-10-09). What each protocol does, and how TCP and UDP differ, was read the same day in the protocol's own specification at rfc-editor.org (RFC 768, 854, 959, 1002, 1035, 1939, 2131, 4251, 4253, 4511, 5321, 6335, 9051, 9110, 9293), in two Microsoft Learn articles (Remote Desktop listening port; direct hosted SMB) and, for one question, in Wikipedia's Telnet article. Each question names the text and section it was written from and links to it. Licence: Own work. Every question, wrong choice and explanation was written for this site from the public texts named in each question. Nothing is copied or reworded from a CompTIA exam, a released or sample question, or another practice product.
How it works
Sixty-seven port questions for CompTIA A+ Core 1, unofficial and our own, each marked at once with the registry entry or specification it rests on. CompTIA does not sponsor or endorse this page, and no real exam question is here.
Read the full guide
One objective, not the whole exam
Objective 2.1 of Core 1 (220-1201, V15; CompTIA's objectives read October 12, 2026) is to compare and contrast TCP and UDP ports, protocols and their purposes. It names fourteen protocols, then TCP vs. UDP, and nothing off that list is asked here.
Asked both ways
Some questions give the protocol and ask for the port. Others start from what a technician sees, a port in a firewall log or a service name in a scan, and ask for the protocol. Numbers come from the IANA port registry, purposes from each protocol's own specification.
Learn the pairs together: 20 and 21 for FTP, 67 and 68 for DHCP, 137 to 139 for NetBIOS.
Scanners print registry names: domain is DNS on 53, microsoft-ds is SMB on 445, ms-wbt-server is RDP on 3389.
Port 3389 is the only one on the list above 1023, so it is a registered port, not a well-known one.
DHCP runs over UDP on 67 and 68. DNS uses UDP 53 for standard queries and TCP 53 for zone transfers. NetBIOS uses UDP 137 and 138 and TCP 139. SSH, Telnet, SMTP, POP3 and IMAP listen on TCP.
What is the difference between port 139 and port 445?
Both carry SMB file sharing. Port 139 is the NetBIOS session service, used with older systems. Port 445 is SMB hosted directly on TCP, with no NetBIOS layer. Windows tries both when both are enabled.
What port does RDP use?
3389. Remote Desktop listens on port 3389 by default, and the IANA registry names that port ms-wbt-server. An administrator can change the listening port in the Windows registry.