prerequisite concepts: prelude
If you're not already using a config file (~/.ssh/config) you should peruse the documentation to see what it offers; an ongoing benefit I enjoy is that it allows me to accomplish more while typing less. Suppose, for example, you need to access two mail servers which are both behind a firewall and sharing a single public IP address. One server uses NAT (port forwarding) to provide user access via IMAP-SSL, POP3-SSL, and perhaps even webmail, all on default ports; similarly SSH can be accessed on port 22. The other server happens to be a mail relay, which handles all of the spam and virus scanning for inbound and outbound mail; while the SMTP, SMTPS, and submission services all enjoy a NAT configuration using default ports, SSH access is on port 23 because port 22 already forwards to the IMAP server and the sysadmin hasn't read this series of articles.