1. Connection redirected from external ssh server X to external ssh server Y.

Destination nat: internal host A connects to X but gets redirected to Y.

192.168.0.2->158.110.28.25 becomes 192.168.0.2->158.110.144.168

Internal host 192.168.0.2 wants to connect to 158.110.28.25, port 20024 (Udine's department of management, electric and electronic engineering ssh server). The gateway 192.168.0.1, redirects connection towards 158.110.144.168, port 22 (ssh server of Udine's computer science deparment, host primergy.diegm.uniud.it). Gateway's work is transparent to 192.168.0.2, who communicates to 158.110.144.168 thinking it is 158.110.28.25.

In the picture above, you can see what happens in IPFIRE nat filter gateway.