Thank you for support,

If i try your commands on NIS+Client in NET_B, they don't work:
$ niscat -o "domainname"
"domainname": Error in accessing NIS+ cold start file... is NIS+
installed?.

$ nisupdkeys -a
Updating directory object ""domainname"" ...
ERROR: Unable to retrieve object.
"domainname": Error in accessing NIS+ cold start file... is NIS+
installed?.

I can run "niscat -o `domainname`" on NIS+Client on the same network
of NIS+Master, and I see the the real IP address:

....
Universal addresses (6)
[1] - udp, inet, "REAL_IP_NIS+Master".0.111
[2] - tcp, inet, "REAL_IP_NIS+Master".0.111
[3] - -, inet, "REAL_IP_NIS+Master".0.111
[4] - -, loopback, "Hostname_NIS+Master".rpc
[5] - -, loopback, "Hostname_NIS+Master".rpc
[6] - -, loopback, "Hostname_NIS+Master".rpc
.....

Is it possible to insert a row with also information of
NATTED_IP_NIS+Master?

I have just added entries in /etc/hosts of NIS+Client about natted_ip:
Natted_IP_NIS+Master Hostname_NIS+Master

Thank you