Never expose to the internet
Port 23 — Telnet
Unencrypted remote shell from the era before SSH. Everything, including the password, crosses the network in plain text.
Port
23
Service
Telnet
Protocol
TCP
Security guidance
If this is open, close it today. There is no safe use for Telnet on the public internet.
Is port 23 open on your server?
Test it from outside your network — the only view that matters.
Other common ports
20 · FTP data
21 · FTP control
22 · SSH / SFTP
25 · SMTP
53 · DNS
80 · HTTP
110 · POP3
123 · NTP
143 · IMAP
161 · SNMP
389 · LDAP
443 · HTTPS
445 · SMB
465 · SMTPS
587 · SMTP submission
993 · IMAPS
995 · POP3S
1433 · Microsoft SQL Server
1521 · Oracle Database
2082 · cPanel
2083 · cPanel (TLS)
3000 · Node.js / dev server
3306 · MySQL / MariaDB
3389 · RDP
5432 · PostgreSQL
5900 · VNC
6379 · Redis
8080 · HTTP alternate
8443 · HTTPS alternate
9200 · Elasticsearch
27017 · MongoDB