Forward port thought nat using public server
+--------------------------------------------+
| Public server (1.2.3.4) |
+------- | -public -in 0.0.0.0 4455 -out 0.0.0.0 4444 | -------+
| +--------------------------------------------+ |
+----------+---------+ +----------+---------+
| NAT | | NAT |
+----------+---------+ +----------+---------+
| |
+--------------+-------------+ +------------------+-----------------+
| C1 (ssh client) | | c2 (ssh server) |
| $ ssh root@1.2.3.4 -p 4455 | | -in 1.2.3.4:4444 -out localhost:22 |
+----------------------------+ +------------------------------------+