How does an open port scanner work

DNS tools

21The "FTP" port.
Used for file transfer via FTP.22The "SSH" port.
Used for access via Secure Shell.23The "Telnet" port.
Used for terminal emulation.25The "SMTP" port.
Is used for sending e-mails (see also port 465).53The "DNS" port.
Used to resolve domain names into IP addresses.79The "finger" port.
Used to display information about a user.80The "HTTP" port.
Used to communicate with the web server.110The "POP3" port.
Used for client access to email servers.115The "SFTP" port.
Is used for the "Simple File Transfer Protocol" for data exchange.135The "RPC" port.
Used to execute remote procedure calls.139The "NetBIOS" port.
Used by Netbios (Network Basic Input Output System).143The "IMAP" port.
Used to access and manage mailboxes.194The "IRC" port.
Used to access IMAP mailboxes.389The "LDAP" port.
Used for querying and modifying directory services.443The "HTTPS" port.
Used for encrypted web server transmissions.445The "SMB" port.
Used by Windows file sharing.465The "SMTPS" port.
Is used for secure sending of e-mails.1433The "MS SQL" port.
Is used for communication with an MS SQL server.1723The "PPTP" port.
Used for point-to-point tunneling (VPN).3306The "MySQL" port.
Used to access MySQL databases.3389The "Remote Desktop" port.
Used for Windows remote desktop access.5632The "pcAnywhere" port.
Used for remote access via pcANYWHERE.5900The "VNC" port.
Used by VNC for viewer access.6112The "Warcraft III" port.
Used by Warcraft III to host games.8080The "HTTP Alt" port.
Often used by proxy and caching servers.