What causes ssh to return Network is down? I'm trying, without success, connect to a local network ssh server from macOS by doing:

$ ssh user@1111:ab1:123:ab1:011b:12bc:ab12:1a2b


and then I get:

ssh: connect to host 1111:ab1:123:ab1:011b:12bc:ab12:1a2b port 22: Network is down


I actually can connect to that server from iPhone app Termius (and there are also other reasons to think the server side is ok) so I believe there is a problem on the client side. I tried that same app on macOS without success.

Also, ssh to a virtual machine works well, as well as to an amazon aws server.

Also tried verbose:

$ ssh user@1111:ab1:123:ab1:011b:12bc:ab12:1a2b -vvv
OpenSSH_7.6p1, LibreSSL 2.6.2
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 48: Applying options for *
debug2: ssh_connect_direct: needpriv 0
debug1: Connecting to 1111:ab1:123:ab1:011b:12bc:ab12:1a2b port 22.
ssh: connect to host 1111:ab1:123:ab1:011b:12bc:ab12:1a2b port 22: Network is down


And IPV6:

$ ssh -6 -vvv user@1111:ab1:123:ab1:011b:12bc:ab12:1a2b
OpenSSH_7.6p1, LibreSSL 2.6.2
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 48: Applying options for *
debug2: ssh_connect_direct: needpriv 0
debug1: Connecting to 1111:ab1:123:ab1:011b:12bc:ab12:1a2b port 22.
ssh: connect to host 1111:ab1:123:ab1:011b:12bc:ab12:1a2b port 22: Network is down


And ping:

$ ping 1111:ab1:123:ab1:011b:12bc:ab12:1a2b
ping: cannot resolve 1111:ab1:123:ab1:011b:12bc:ab12:1a2b: Unknown host


What might be causing the Network is down? How should I debug this?
SSH is just the messenger, and is likely calling strerror(3) or equivalent on a bad exit code from some socket call:

$ find /usr/include -name "*.h" -exec grep 'Network is down' {} +
/usr/include/apr-1/apr_errno.h:#define SOCENETDOWN             (SOCBASEERR+50)            /* Network is down */
/usr/include/sys/errno.h:#define        ENETDOWN        50              /* Network is down */
$ 


If we dig around in the system man pages for ENETDOWN this will turn up documentation...

$ find /usr/share/man -type f -exec grep -l ENETDOWN {} +
/usr/share/man/man2/connect.2
/usr/share/man/man2/connectx.2
/usr/share/man/man2/intro.2
/usr/share/man/man2/send.2
...


Though not much as connect(2) or intro(2) merely indicate

 [ENETDOWN]         The local network interface is not functioning.
...
 50 ENETDOWN Network is down.  A socket operation encountered a dead net-
         work.


Is IPv6 working on this host? You'll probably need to debug with ping6 (not ping) and maybe check the IPv6 settings via some clicky interface or instead ifconfig and netstat -rn.

To avoid the GUI, the network settings for IPv6 can be adjusted with something like

sudo ipconfig set en0 AUTOMATIC-V6


see the ipconfig(8) manual for details.