Cannot access internal network server from internal network device using public IP

I cannot access server2 from my laptop under the office network using public external IP address. And here is my experiment Case From To Command Connection Case 1 L server1 ssh server1@1.2.3.4 -p 4001 OK Case 2 L server1 ssh server1@192.168.1.10 -p 4001 OK Case 3 L server2 ssh server2@1.2.3.4 -p 2001 Time out Case 4 L server2 ssh server2@192.168.1.11 -p 2001 OK Case 5 D server1 ssh server1@1.2.3.4 -p 4001 OK Case 6 D server2 ssh server2@1.2.3.4 -p 2001 OK I saw the same situations on another laptop. I got some hint that NAT hairpin should be supported in my router (Linksys mx5500). But I found

Question by Olivia on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.