Suppose there is an SSH server inside a remote network that does not have its SSH port exposed to the Internet (named "internal.hostname.tld"). If there is an SSH gateway host that you can SSH to (that has the ability to reach "internal"'s SSH port), you can use the netcat (nc(1)) command to Proxy your SSH session to "internal" through "gateway".
Put something like the following in your ~/.ssh/config (or /etc/ssh/ssh_config):
Host internal.hostname.tld internal User merdely HostName internal.hostname.tld ProxyCommand ssh firstname.lastname@example.org nc %h %p 2> /dev/null
Then, make connect to "internal" as if you could directly: