Following command will ssh through host in the middle. Unreachable_host is unavailable from local network, but it’s available from reachable_host’s network. This command creates a connection to unreachable_host through “hidden” connection to reachable_host.
ssh -t reachable_host ssh unreachable_host
That command is awesome and you can not only ssh to “unreachable_host”, but any other command, for example telnet etc.