from the LAN host:
bash>ssh -N -f -R [new port on REAL-IP]:localhost:22 -l [REAL-IP user] [REAL-IP]
then on a computer anyware in the world:
connect on the REAL-IP computer
bash>ssh -l [LAN computer user] -p [new port on REAL-IP] localhost

peshka's reminder site – ver. 2.0
from the LAN host:
bash>ssh -N -f -R [new port on REAL-IP]:localhost:22 -l [REAL-IP user] [REAL-IP]
then on a computer anyware in the world:
connect on the REAL-IP computer
bash>ssh -l [LAN computer user] -p [new port on REAL-IP] localhost