By default, Linodes use DHCP to acquire their IP address, routing and DNS information. DHCP, however, will only assign one IP to your Linode so if you have multiple IPs you'll need to use a static configuration. Even if you only have one IP, you can still do a static assignment, but it's not necessary. Note, you must reboot after adding an IP address to your Linode before the changes will take effect.
CentOS:
/etc/sysconfig/network-scripts/ifcfg-eth0
# Configuration for eth0
DEVICE=eth0
BOOTPROTO=none
# This line ensures that the interface will be brought up during boot.
ONBOOT=yes
# eth0 - This is the main IP address that will be used for most outbound connections.
# The address, netmask and gateway are all necessary.
IPADDR=12.34.56.78
NETMASK=255.255.255.0
GATEWAY=12.34.56.1