The organization which network this server is a part of is changing its ISP. When I switch this server to new values of IP, gateway, etc., I can't connect to it via ssh anymore. I went to one of the port-checking tools and when I checked port 22 it returned "I could not see your service on _new_IP_value_ on port 22".
New IP is static. There's no router on the server in question, no port-forwarding -- the network cable goes straight into the server.
Naturally, I thought the organization's network or the new ISP was blocking port 22 so I phoned network's administrator and asked to open it or tell ISP to open it. He said he would. But nothing changed. I called him a day later with the same question, but you could tell he was busy -- said that he did what I asked and basically hung up on me.
So, now I'm thinking perhaps there's something wrong with the server's settings after all. But all the Linux tools such as lsof, netstat, nmap, etc. tell that sshd is up and running and listening on port 22, port 22 is open, etc. and I still can ssh into the server when I switch it back to our old ISP's network -- all I switch is IP, gateway, dns, nothing more.
Could there be something on this server that is blocking ssh connections when the server is on the new ISP's network, but not the old one's? The OS is Linux Mint.