If you're assigning a private network to an existing machine (or deploying your own operating system), you'll need to configure the IP addresses on the private interface.
The control panel will assign you an IP address, you should replace '10.99.0.200' in the examples with that IP.
CentOS:
Replace /etc/sysconfig/network-scripts/ifcfg-eth1 with the following:
DEVICE=eth1
ONBOOT=yes
NM_CONTROLLED=no
BOOTPROTO=static
IPADDR=10.99.0.200
NETMASK=255.255.0.0
IPV6INIT=no
MTU=1450