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Port 23 is the default port for Telnet, an old protocol for unencrypted remote access. Here you’ll find key details about the port and firewall commands to block it or allow access selectively.
Built by Justin Schuessler
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Remote access · Well-known port · OSI layer 7
Legacy remote command-line access, completely unencrypted. Replaced by SSH.
IANA: officially registered as telnet
Firewall commands
Open port
sudo ufw allow 23/tcpFor Ubuntu and Debian. ufw creates the rule for both IPv4 and IPv6. If ufw is disabled, allow SSH first (sudo ufw allow 22/tcp), then enable it with sudo ufw enable, or you'll lock yourself out.
On a ZAP-Hosting VPS with an active network firewall, you also need to allow the port in the Customer Panel.
Port 23 explained
Telnet opens a text-based connection to a remote device where you can enter commands. It used to be the standard way to manage servers and network devices remotely. Today, you may still find Telnet on older switches, routers, industrial equipment and embedded devices. Admins also use a Telnet client to quickly check whether a TCP port responds at all.
Telnet is one of the oldest internet protocols, dating back to the ARPANET era. Its specification, RFC 854 from 1983, is still in effect today. The IANA lists port 23 under the name “telnet” as a well-known port. SSH was later developed as a replacement that performs the same task securely.
Telnet transmits everything unencrypted, including usernames and passwords. Anyone who can intercept the traffic can see your credentials in plain text. Never open port 23 to the internet; use SSH on port 22 instead. Open Telnet ports are scanned extensively for default passwords—the Mirai botnet compromised numerous IoT devices this way in 2016. If a device supports only Telnet, access it exclusively through a separate management network or a VPN.
More ports
These ports are often part of the same setup. Click one to see its explanation and firewall commands.
View all well-known ports and the complete IANA list
Port list source: IANA Service Name and Transport Protocol Port Number Registry, as of October 22, 2026. iana.org
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FAQ
Quick answers about port 23.
No. Telnet transmits all data in plain text, including usernames and passwords. Use SSH on port 22 for encrypted remote access.
To block the port with ufw, use sudo ufw deny 23/tcp. Better yet, do not install the Telnet service, or disable it so nothing listens on port 23.
Telnet uses TCP. Although the IANA has also reserved port 23 for UDP, Telnet runs over TCP in practice.
As a server service, Telnet is now mainly used for older network and industrial devices. You can still use the Telnet client to check whether a port responds, for example with telnet example.com 25.
Any more questions?
Contact SupportWith a ZAP-Hosting VPS, you have root access, can set up services and firewall rules yourself, and can immediately see which ports are open.
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