Solaris:

route add host 192.168.1.12 192.168.1.1

Solaris, route persistent between reboots:

route -p add host 192.168.1.12 192.168.1.1

Linux RedHat:

route add -host 192.168.1.12 gw 192.168.1.1

Linux persistent routes:

/etc/sysconfig/network-scripts/route-XXX

where XXX - name of interface