How can i set static ip in linux softether client

[n3n5n1]

Just set a static IP for the virtual interface on the client. It's not something SoftEther controls.

For example, in Debian, you would edit /etc/network/interfaces file and add something like:

allow-hotplug vpn_sstp1
iface vpn_sstp1 inet static
address 10.0.0.51
netmask 255.255.255.0

...where "vpn_sstp1" is the name of the virtual interface you've specified in SoftEther Client settings.

You have to restart /etc/init.d/networking service after you edit the file to apply changes. Keep in mind you can lose network connectivity if you make a mistake there, so be careful.