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

Question 1

What does a transport-layer port number identify?

Accepted 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.

Question 2

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

Accepted Answer

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

Question 3

What is the IANA System Port range?

Accepted Answer

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

Question 4

What is the IANA User Port range?

Accepted Answer

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

Question 5

What is the IANA Dynamic or Private Port range?

Accepted Answer

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

Question 6

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

Accepted Answer

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

Question 7

Must a service use its registered or conventional port?

Accepted Answer

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

Question 8

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

Accepted Answer

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

Question 9

SSH and SFTP — common server port and transport?

Accepted Answer

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

Question 10

53/UDP or 53/TCP — which service?

Accepted Answer

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

Question 11

SMTP relay between mail servers — common port and transport?

Accepted Answer

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

Question 12

HTTP without TLS — conventional port and transport?

Accepted Answer

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

Question 13

67/UDP — which DHCPv4 endpoint?

Accepted Answer

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

Question 14

21/TCP — which service and channel?

Accepted Answer

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

Question 15

SNMP requests and responses — common agent port and transport?

Accepted Answer

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

Question 16

1433/TCP — which database service?

Accepted Answer

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

Question 17

23/TCP — which remote-access protocol?

Accepted Answer

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

Question 18

NTP — usual port and transport?

Accepted 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.

Question 19

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

Accepted 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.

Question 20

POP3 — conventional port and transport?

Accepted Answer

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

Question 21

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

Accepted Answer

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

Question 22

DHCPv4 client endpoint — port and transport?

Accepted Answer

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

Question 23

69/UDP — which file-transfer protocol?

Accepted 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.

Question 24

162/UDP — which network-management traffic?

Accepted Answer

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

Question 25

MySQL classic protocol — default port and transport?

Accepted Answer

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

Question 26

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

Accepted 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.

Question 27

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

Accepted Answer

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

Question 28

Syslog's conventional UDP transport — port?

Accepted Answer

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

Question 29

How do DHCPv4 server and client ports pair up?

Accepted 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.

Question 30

IMAP — conventional port and transport?

Accepted Answer

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

Question 31

445/TCP — which file-sharing service?

Accepted Answer

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

Question 32

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

Accepted Answer

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

Question 33

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

Accepted Answer

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

Question 34

BGP — standard operational port and transport?

Accepted Answer

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

Question 35

5432/TCP — which database service?

Accepted Answer

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

Question 36

IKE for IPsec — initial port and transport?

Accepted Answer

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

Question 37

NetBIOS Name Service — common port and transport?

Accepted Answer

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

Question 38

587/TCP — which email role?

Accepted Answer

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

Question 39

389/TCP — which directory protocol?

Accepted Answer

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

Question 40

L2TP — common port and transport?

Accepted Answer

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

Question 41

138/UDP — which NetBIOS service?

Accepted Answer

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

Question 42

IMAPS — default port and transport?

Accepted Answer

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

Question 43

636/TCP — which directory-service convention?

Accepted Answer

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

Question 44

1723/TCP — which VPN control protocol?

Accepted Answer

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

Question 45

NetBIOS Session Service — common port and transport?

Accepted Answer

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

Question 46

995/TCP — which mail-access service?

Accepted Answer

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

Question 47

RADIUS authentication and authorization — standard port and transport?

Accepted Answer

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

Question 48

IPsec NAT Traversal — port and transport?

Accepted Answer

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

Question 49

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

Accepted Answer

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

Question 50

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

Accepted 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.

Question 51

22/TCP — which secure remote-access service?

Accepted Answer

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

Question 52

DNS — registered port and required general-purpose transports?

Accepted 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.

Question 53

25/TCP — which email function?

Accepted Answer

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

Question 54

80/TCP — which conventional web service?

Accepted Answer

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

Question 55

DHCPv4 server endpoint — port and transport?

Accepted Answer

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

Question 56

FTP control — server port and transport?

Accepted Answer

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

Question 57

161/UDP — which network-management traffic?

Accepted Answer

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

Question 58

Microsoft SQL Server default instance — default port and transport?

Accepted Answer

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

Question 59

Telnet — conventional port and transport?

Accepted Answer

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

Question 60

123/UDP — which time service?

Accepted Answer

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

Question 61

110/TCP — which mail-access protocol?

Accepted Answer

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

Question 62

HTTPS — common port and transports?

Accepted Answer

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

Question 63

68/UDP — which DHCPv4 endpoint?

Accepted Answer

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

Question 64

TFTP — initial request port and transport?

Accepted Answer

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

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