2 Common TCP Ports and Services
Learn common TCP port and service pairings, what each service is used for, and why a port number is only a clue to the traffic it carries.
How to interpret ports
A identifies an application service on a host, but a number is only a clue: services can use nonstandard ports, and registered ports can carry other traffic. Learn each together with its transport protocol and purpose.
File transfer, remote access, and email
These pairings describe common uses; they do not prove which application is using a .
TCP
21— control: sends commands and responses. File data travels on a separate connection, and that connection’s depends on the transfer mode.TCP
22— : encrypted remote login and command-line administration; it also supports secure file-transfer tools such as SFTP.TCP
23— : older, unencrypted remote-terminal access. Avoid it for administration over untrusted networks; is the secure alternative.TCP
25— : transfers email between mail servers.
DNS and web traffic
DNS queries and responses can use TCP 53, including reliable transfers such as zone transfers. DNS also commonly uses UDP 53.
is commonly associated with TCP 80 and is web traffic without TLS by default. , commonly associated with TCP 443, is protected with TLS.
Remote graphical access
, commonly associated with 3389, provides remote desktop access to a Windows computer. IANA registers the service for both TCP and UDP.
, commonly used by VNC and associated with 5900, provides remote display and input so a user can view and control another computer’s desktop.
Compare access types and review pairings
, , and VNC are all used for remote administration, but they provide different kinds of access. is generally command-line access, while and VNC provide graphical desktop access. is also a remote-terminal protocol, but it does not provide ’s encryption.
For review, match each with its common service: 22—; 25—; 53—; 80—; 443—; 3389—. The broader set of common pairings is 21—, 22—, 23—, 25—, 53—, 80—, 443—, 3389—, and 5900—/VNC. A is a useful clue, not proof of the service or its security.