I keep forgetting how to switch to static from dynamic on Linux. I found the correct steps here Switch to Static IP on Ubuntu Server - david winter

In short edit /etc/network/interfaces
Change dhcp to static in the line   -->
iface eth1 inet dhcp
address w.x.y.z
netmask a.b.c.d
gateway m.n.o.p

and then /etc/init.d/networking restart

I would also try a reboot immediately after setting this, to test that it is permanent