Problem scenario
nmap shows that the host seems down. You cannot ping the IP address. But you can SSH to the IP address. How is this possible?
Solution
Possible Solution #1
A host server could have multiple IP addresses (e.g., with a virtual server hosted by a type 2 hypervisor utilizing KVM or QEMU). The networking could be done at the kernel level.
It's not the ssh client that decides through which interface TCP packets should go, it's the kernel. In short, SSH asks the kernel to open a connection to a certain IP address, and the kernel decides which interface is to be used by consulting the routing tables.
https://unix.stackexchange.com/questions/16057/use-ssh-with-a-specific-network-interface
Is the client related to the destination server via a type 2 hypervisor?
Possible Solution #2
Check the firewall rules. icmp packets could be dropped, but SSH could still be allowed. See this external posting for more information.
Possible Solution #3
Restart the firewall service or reboot. You may want to see this external posting for more information.
Possible Solution #4
If you searched online, you may want to read more about SSH with other postings on this site.