to make changes permanent, edit the file /etc/network/interfaces

iface bge0 inet static address 10.91.3.5 netmask 255.255.252.0 gateway 10.91.3.1 

To make changes permanent:

Ensure default gateway is configured in /etc/defaultrouter

The netmask is configured in /etc/netmasks

And /etc/hostname.bge0 is configured with the static IP address