On Centos 6.8 +++++++++++++
To add static route add following line to file /etc/sysconfig/network:
GATEWAY=192.168.1.1
this will make the route permanent across reboots, however you can also add default route using standard route command:
route add default gw 192.168.1.1
On SmartOS/Solaris
++++++++++++++++++
Prior to Solaris 11 the /etc/defaultrouter was used, but nowadays route command with -p is used:
route -p add default 192.168.1.1
-p makes this change permanent across reboots.