32.3. Utilizing a Static IP Address

While each node in the cluster is able to discover other nodes in the cluster using the GOOGLE_PING protocol, all network traffic is over the internal private network. It is recommended to configure an external static IP address for a single node, so that a consistent address is available for configuring the cluster, such as through the Administration Console, across restarts. If no static address is configured each instance will contain a randomized IP address on its public network whenever it is started.

Full instructions for configuring an external static IP address may be found in Google's Configuring an Instance's IP Address documentation.