Common Network Port Numbers Flashcards: Protocols, TCP/UDP & Uses

Cards in this deck

1. Card 1

   Question

   What does a transport-layer port number identify?

   Answer

   An application endpoint within a host for one transport protocol. The IP addresses, transport protocol, and source and destination ports together identify a communication flow.

2. Card 2

   Question

   Are TCP port 53 and UDP port 53 the same port?

   Answer

   No. TCP and UDP have independent 16-bit port namespaces, so 53/TCP and 53/UDP are separate transport endpoints.

3. Card 3

   Question

   What is the IANA System Port range?

   Answer

   0–1023. These are also called Well-Known Ports and are assigned by IANA under the stricter System Port procedures.

4. Card 4

   Question

   What is the IANA User Port range?

   Answer

   1024–49151. These are also called Registered Ports and may be assigned by IANA to services.

5. Card 5

   Question

   What is the IANA Dynamic or Private Port range?

   Answer

   49152–65535. IANA does not assign ports in this range; systems commonly use it for temporary or private endpoints.

6. Card 6

   Question

   Does an IANA port assignment prove what observed traffic is—or that it is safe?

   Answer

   No. A registration is a naming convention, not traffic identification, authorization, or a security verdict. Verify the actual protocol and context.

7. Card 7

   Question

   Must a service use its registered or conventional port?

   Answer

   No. Administrators and applications can choose another port, negotiate one dynamically, or discover it through mechanisms such as DNS SRV records.

8. Card 8

   Question

   What port roles are typical in a client-server connection?

   Answer

   The server usually listens on a known destination port, while the client uses a temporary source port. Either side can be configured differently.

9. Card 9

   Question

   SSH and SFTP — common server port and transport?

   Answer

   TCP 22. SSH provides encrypted remote access, and SFTP normally runs as a subsystem inside the same SSH connection.

10. Card 10

    Question

    53/UDP or 53/TCP — which service?

    Answer

    DNS. Routine queries often use UDP, while full DNS implementations also support TCP; zone transfers use TCP.

11. Card 11

    Question

    SMTP relay between mail servers — common port and transport?

    Answer

    TCP 25. SMTP uses it for server-to-server mail transfer; user mail submission normally uses port 587 or implicit TLS on 465.

12. Card 12

    Question

    HTTP without TLS — conventional port and transport?

    Answer

    TCP 80 for conventional HTTP/1.1 and HTTP/2 connections. The service can be configured on another port.

13. Card 13

    Question

    67/UDP — which DHCPv4 endpoint?

    Answer

    The DHCPv4 server endpoint. Clients send server-directed DHCP messages to UDP port 67.

14. Card 14

    Question

    21/TCP — which service and channel?

    Answer

    FTP control. It carries commands and replies; file data uses a separate connection.

15. Card 15

    Question

    SNMP requests and responses — common agent port and transport?

    Answer

    UDP 161. Managers commonly send queries to an SNMP agent there; other SNMP transports exist.

16. Card 16

    Question

    1433/TCP — which database service?

    Answer

    Microsoft SQL Server's default instance. TCP 1433 is the default Database Engine port, but named or custom instances may use another port.

17. Card 17

    Question

    23/TCP — which remote-access protocol?

    Answer

    Telnet. It provides a plaintext remote terminal session and does not protect credentials or traffic with encryption.

18. Card 18

    Question

    NTP — usual port and transport?

    Answer

    UDP 123. NTP normally uses UDP for time synchronization; IANA also has a TCP 123 registration, but that is not the usual NTP transport.

19. Card 19

    Question

    Why must a general-purpose DNS implementation support both UDP and TCP?

    Answer

    UDP handles most routine queries efficiently, while TCP handles cases such as zone transfers and responses that do not fit the chosen UDP path. Both use port 53.

20. Card 20

    Question

    POP3 — conventional port and transport?

    Answer

    TCP 110. POP3 retrieves mail; it can upgrade with STLS, while implicit TLS uses TCP 995.

21. Card 21

    Question

    443/TCP or 443/UDP — which web service?

    Answer

    HTTPS. HTTP/1.1 and HTTP/2 normally use TLS over TCP 443; HTTP/3 uses QUIC over UDP 443.

22. Card 22

    Question

    DHCPv4 client endpoint — port and transport?

    Answer

    UDP 68. Servers send client-directed DHCPv4 messages to port 68.

23. Card 23

    Question

    69/UDP — which file-transfer protocol?

    Answer

    TFTP. A client sends the initial read or write request to UDP 69; the transfer then continues with a server-selected UDP transfer identifier.

24. Card 24

    Question

    162/UDP — which network-management traffic?

    Answer

    SNMP notifications, traditionally called traps. Notification receivers commonly listen on UDP 162.

25. Card 25

    Question

    MySQL classic protocol — default port and transport?

    Answer

    TCP 3306. It is the default for classic MySQL client-server connections and can be reconfigured.

26. Card 26

    Question

    How do FTP control, active-mode data, and passive-mode data ports differ?

    Answer

    Control uses TCP 21. The server side of the default active-mode data connection uses TCP 20, while passive mode negotiates another server port.

27. Card 27

    Question

    3389/TCP or 3389/UDP — which remote-desktop service?

    Answer

    Microsoft Remote Desktop Protocol (RDP). TCP and UDP 3389 are the standard defaults, and the listening port can be changed.

28. Card 28

    Question

    Syslog's conventional UDP transport — port?

    Answer

    UDP 514. Classic syslog over UDP has no delivery guarantee; TCP 514 is registered to the shell service, not syslog.

29. Card 29

    Question

    How do DHCPv4 server and client ports pair up?

    Answer

    UDP 67 is the server port, and UDP 68 is the client port. Client-to-server messages target 67; server-to-client messages target 68.

30. Card 30

    Question

    IMAP — conventional port and transport?

    Answer

    TCP 143. IMAP accesses and manages mail on a server; implicit TLS uses TCP 993.

31. Card 31

    Question

    445/TCP — which file-sharing service?

    Answer

    Direct-hosted SMB. Modern SMB servers commonly listen on TCP 445 without the older NetBIOS session layer.

32. Card 32

    Question

    88/UDP or 88/TCP — which authentication protocol?

    Answer

    Kerberos. Clients and Key Distribution Centers support both transports; TCP also handles requests or replies too large for UDP.

33. Card 33

    Question

    FTP's default active-mode data connection — server-side port and transport?

    Answer

    TCP 20. This is the server's default source port for active-mode data; passive FTP negotiates another server port.

34. Card 34

    Question

    BGP — standard operational port and transport?

    Answer

    TCP 179. RFC 4271 defines BGP peer sessions over TCP; IANA separately retains a UDP 179 registry entry.

35. Card 35

    Question

    5432/TCP — which database service?

    Answer

    PostgreSQL. TCP 5432 is the customary default server port, but an installation can choose another port.

36. Card 36

    Question

    IKE for IPsec — initial port and transport?

    Answer

    UDP 500. IKE normally begins there; NAT traversal commonly moves subsequent IKE and encapsulated ESP traffic to UDP 4500.

37. Card 37

    Question

    NetBIOS Name Service — common port and transport?

    Answer

    UDP 137. It supports NetBIOS name registration and lookup; direct-hosted modern SMB does not require it.

38. Card 38

    Question

    587/TCP — which email role?

    Answer

    SMTP message submission. Mail clients normally submit outgoing mail to a submission server there, often with STARTTLS; server relay remains on TCP 25.

39. Card 39

    Question

    389/TCP — which directory protocol?

    Answer

    LDAP. Ordinary connection-oriented LDAP commonly uses TCP 389; IANA also registers UDP 389 for connectionless LDAP.

40. Card 40

    Question

    L2TP — common port and transport?

    Answer

    UDP 1701. L2TP carries tunnel control and data but does not provide encryption by itself; deployments often pair it with IPsec.

41. Card 41

    Question

    138/UDP — which NetBIOS service?

    Answer

    NetBIOS Datagram Service. It carries connectionless NetBIOS datagrams, historically used in NetBIOS over TCP/IP environments.

42. Card 42

    Question

    IMAPS — default port and transport?

    Answer

    TCP 993. It starts IMAP with an immediate TLS handshake for encrypted mail access.

43. Card 43

    Question

    636/TCP — which directory-service convention?

    Answer

    LDAPS, the commonly used implicit-TLS port for LDAP. LDAP can also negotiate TLS on TCP 389.

44. Card 44

    Question

    1723/TCP — which VPN control protocol?

    Answer

    PPTP control. PPTP uses TCP 1723 for control, while tunneled PPP data travels in GRE—not on TCP or UDP port 47.

45. Card 45

    Question

    NetBIOS Session Service — common port and transport?

    Answer

    TCP 139. It provides connection-oriented NetBIOS sessions and historically carried SMB before direct-hosted SMB on TCP 445.

46. Card 46

    Question

    995/TCP — which mail-access service?

    Answer

    POP3S. It starts POP3 with an immediate TLS handshake for encrypted mail retrieval.

47. Card 47

    Question

    RADIUS authentication and authorization — standard port and transport?

    Answer

    UDP 1812. RADIUS accounting normally uses the separate UDP port 1813.

48. Card 48

    Question

    IPsec NAT Traversal — port and transport?

    Answer

    UDP 4500. It carries IKE and UDP-encapsulated ESP when NAT traversal is in use.

49. Card 49

    Question

    5060/UDP or 5060/TCP — which signaling protocol?

    Answer

    SIP. It commonly uses UDP or TCP 5060 for session signaling; SIP over TLS conventionally uses TCP 5061.

50. Card 50

    Question

    Which common dedicated ports distinguish plaintext or STARTTLS-capable services from implicit-TLS variants?

    Answer

    HTTP 80 ↔ HTTPS 443; IMAP 143 ↔ IMAPS 993; POP3 110 ↔ POP3S 995; LDAP 389 ↔ LDAPS 636. A port alone does not prove that TLS is configured correctly.

51. Card 51

    Question

    22/TCP — which secure remote-access service?

    Answer

    SSH. It supports encrypted remote login, command execution, tunneling, and file-transfer tools such as SFTP.

52. Card 52

    Question

    DNS — registered port and required general-purpose transports?

    Answer

    Port 53 over UDP and TCP. Most ordinary queries start with UDP, while TCP is required for full implementations and used for zone transfers and other larger exchanges.

53. Card 53

    Question

    25/TCP — which email function?

    Answer

    SMTP mail relay and delivery between servers. End-user clients normally submit new mail on TCP 587 or TCP 465 instead.

54. Card 54

    Question

    80/TCP — which conventional web service?

    Answer

    HTTP without TLS. It is the conventional port for HTTP/1.1 and HTTP/2, although servers can listen elsewhere.

55. Card 55

    Question

    DHCPv4 server endpoint — port and transport?

    Answer

    UDP 67. DHCPv4 clients direct server-bound messages to that port.

56. Card 56

    Question

    FTP control — server port and transport?

    Answer

    TCP 21. Commands and replies use the persistent control connection; data travels on a separate connection.

57. Card 57

    Question

    161/UDP — which network-management traffic?

    Answer

    SNMP requests and responses. Managed agents commonly listen on UDP 161 for manager queries.

58. Card 58

    Question

    Microsoft SQL Server default instance — default port and transport?

    Answer

    TCP 1433. Named instances may use dynamic or custom ports, with SQL Server Browser commonly using UDP 1434 for discovery.

59. Card 59

    Question

    Telnet — conventional port and transport?

    Answer

    TCP 23. Telnet carries terminal traffic in plaintext, so SSH is the usual secure replacement.

60. Card 60

    Question

    123/UDP — which time service?

    Answer

    NTP. Network Time Protocol commonly synchronizes clocks over UDP 123.

61. Card 61

    Question

    110/TCP — which mail-access protocol?

    Answer

    POP3. It downloads or retrieves mail from a server; implicit TLS uses TCP 995.

62. Card 62

    Question

    HTTPS — common port and transports?

    Answer

    Port 443: TCP for HTTP/1.1 and HTTP/2 over TLS, and UDP for HTTP/3 over QUIC.

63. Card 63

    Question

    68/UDP — which DHCPv4 endpoint?

    Answer

    The DHCPv4 client endpoint. Servers direct client-bound DHCP messages to UDP 68.

64. Card 64

    Question

    TFTP — initial request port and transport?

    Answer

    UDP 69. Only the initial request targets 69; the server chooses another UDP transfer identifier for the data exchange.

65. Card 65

    Question

    SNMP traps and other notifications — common receiver port and transport?

    Answer

    UDP 162. SNMP notification receivers commonly listen there.

66. Card 66

    Question

    3306/TCP — which database protocol?

    Answer

    The classic MySQL protocol. TCP 3306 is its default client-server port and is configurable.

67. Card 67

    Question

    RDP — standard port and transports?

    Answer

    TCP and UDP 3389. Microsoft Remote Desktop uses them by default, though administrators can change the listening port.

68. Card 68

    Question

    514/UDP — which logging protocol?

    Answer

    Syslog over UDP. It is a conventional lightweight transport without delivery guarantees; secure syslog commonly uses another transport and port.

69. Card 69

    Question

    143/TCP — which mail-access protocol?

    Answer

    IMAP. It manages mail on the server; implicit TLS uses TCP 993.

70. Card 70

    Question

    Direct-hosted SMB — common server port and transport?

    Answer

    TCP 445. It carries SMB directly over TCP without the legacy NetBIOS Session Service on TCP 139.

71. Card 71

    Question

    Kerberos KDC traffic — port and transports?

    Answer

    UDP and TCP 88. Conforming clients and Key Distribution Centers support both, with TCP available for larger exchanges.

72. Card 72

    Question

    20/TCP — which FTP role?

    Answer

    The server side of FTP's default active-mode data connection. Passive mode negotiates a different server port.

73. Card 73

    Question

    179/TCP — which routing protocol?

    Answer

    BGP. Border Gateway Protocol peers use a TCP session to exchange routing information.

74. Card 74

    Question

    PostgreSQL — customary default port and transport?

    Answer

    TCP 5432. PostgreSQL servers normally use it unless configured or compiled with another default.

75. Card 75

    Question

    500/UDP — which IPsec setup protocol?

    Answer

    IKE, the Internet Key Exchange protocol. It negotiates IPsec security associations and commonly moves to UDP 4500 when NAT traversal is used.

76. Card 76

    Question

    137/UDP — which NetBIOS service?

    Answer

    NetBIOS Name Service. It handles name registration and lookup in legacy NetBIOS over TCP/IP environments.

77. Card 77

    Question

    SMTP message submission — normal port and transport?

    Answer

    TCP 587. It separates authenticated client submission from server-to-server SMTP relay on TCP 25; STARTTLS is commonly used.

78. Card 78

    Question

    LDAP — conventional connection-oriented port and transport?

    Answer

    TCP 389. Connectionless LDAP can use UDP 389, and implicit-TLS LDAP commonly uses TCP 636.

79. Card 79

    Question

    1701/UDP — which tunneling protocol?

    Answer

    L2TP. It tunnels Layer 2 traffic but needs a separate security layer such as IPsec when confidentiality is required.

80. Card 80

    Question

    NetBIOS Datagram Service — common port and transport?

    Answer

    UDP 138. It provides connectionless NetBIOS datagram delivery.

81. Card 81

    Question

    993/TCP — which mail-access service?

    Answer

    IMAPS. It begins IMAP with implicit TLS for encrypted access to mail stored on a server.

82. Card 82

    Question

    LDAPS — commonly used implicit-TLS port and transport?

    Answer

    TCP 636. LDAP can instead use TCP 389 and negotiate TLS with StartTLS.

83. Card 83

    Question

    PPTP control — port and transport?

    Answer

    TCP 1723. The control channel uses that port; PPP payloads travel through GRE, which is IP protocol 47 rather than port 47.

84. Card 84

    Question

    139/TCP — which NetBIOS service?

    Answer

    NetBIOS Session Service. It historically carried SMB sessions; direct-hosted SMB commonly uses TCP 445 instead.

85. Card 85

    Question

    POP3S — default port and transport?

    Answer

    TCP 995. It uses implicit TLS for encrypted POP3 mail retrieval.

86. Card 86

    Question

    1812/UDP — which AAA service?

    Answer

    RADIUS authentication and authorization. RADIUS accounting normally uses UDP 1813.

87. Card 87

    Question

    4500/UDP — which IPsec function?

    Answer

    IPsec NAT Traversal. It carries IKE and UDP-encapsulated ESP across network address translation.

88. Card 88

    Question

    SIP signaling without implicit TLS — common port and transports?

    Answer

    UDP or TCP 5060. SIP over TLS conventionally uses TCP 5061, and media streams use separately negotiated ports.

88 cards

Common Network Port Numbers Flashcards: Protocols, TCP/UDP & Uses

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