If you want to connect using SSH in a machine that is behind a NAT and have no wait to configure a port forward, here goes the way to do it.
#Execute this in machine behind NAT
ssh -o StrictHostKeyChecking=no -C -N -f -R 2222:127.0.0.1:22 user@remote_host_with_public_ip
#Execute this in server with public IP
ssh -p 2222 localhost -l root