引用那個Burleson的文章部分內容，簡單概括下：

Public IP: The public IP address is for the server. This is the same as any server IP address, a unique address with exists in /etc/hosts.
public IP：client透過這個ip訪問資料庫伺服器，也就是資料庫伺服器所在的伺服器.
Private IP: Oracle RCA requires "private IP" addresses to manage the CRS, the clusterware heartbeat process and the cache fusion layer.
private IP: 節點間通訊專用，包括心跳和Cache fusion等.
Virtual IP: Oracle uses a Virtual IP (VIP) for database access. The VIP must be on the same subnet as the public IP address. The VIP is used for RAC failover (TAF).
Virtual IP: 和public IP 屬於同一個子網，每個節點都有一個VIP.當某個節點出現故障的時候，其所對應的那個VIP將會轉移到其他剩下的可用節點上。

詳細的內容請閱讀下面的E文。

- Public IP ：Configured before installation for each node, and resolvable to that node before installation.

Role: Allow communication/configuration of Virtual/SCAN between the nodes of the cluster. Without Public Interface/IP the Clusterware don't start.
The Virtual/SCAN will work as alias IP on Public Interface over Public Network.

- Private or Interconnect

Configured before installation, but on a separate, private network, with its own subnet, that is not resolvable except by other cluster member nodes
Role: Clusterware uses the interconnect for cluster synchronization (network heartbeat) and daemon communication between the the clustered nodes.
RAC uses the interconnect for cache fusion (UDP) and inter-process communication (TCP).
Cache Fusion is the remote memory mapping of Oracle buffers, shared between the caches of participating nodes in the cluster.

- Virtual IP：Configured before installation for each node, but not currently in use. On the same subnet as all other public IP addresses, VIP addresses, and SCAN addresses.

Role: The goal is application availability. If add or remove nodes it's necessary make config your client adding/removing VIP (with SCAN it's not necessary)
When a node fails, the VIP associated with it is automatically failed over to some other node.
Without using VIPs or FAN, clients connected to a node that died will often wait for a TCP timeout period (which can be up to 10 min) before getting an error.
As a result, you don't really have a good HA solution without using VIPs and FAN. The easiest way to use FAN is to use an integrated client with Fast Connection Failover (FCF) such as JDBC, OCI, or ODP.NET.

-SCAN IP

Three Static IP addresses configured on the domain name server (DNS) before installation so that the three IP addresses are associated with the name provided as the SCAN, and all three addresses are returned in random order by the DNS to the requestor
Configured before installation in the DNS to resolve to addresses that are not currently in use. On the same subnet as all other public IP addresses, VIP addresses, and SCAN addresses
Role: The goal is application availability before clients establish communication with RAC and make the Cluster-Wide totally transparent.
SCAN IP is a new "layer" network (oracle) with high availability that allows you to change the characteristics of your cluster (i.e add/remove nodes) without the need to make configuration changes in their clients "Grid concept".

2. What is the relationship between SCAN IP and Virtual IPs ?

SCAN IP is used to receive new connection requests and redirects to the VIP IP.
The Virtual IP establishes and allow failover of the connections after connection being established.

When the client requests a connection, Oracle Client 11gR2 find for IPs available for this host-scan and create a list of all SCAN IP the first connection attempt on RAC is using one of SCAN IPs avaliable.
The SCAN Listener will receive this connection and re-direct to one of avaliable nodes using LOCAL_LISTENER from this point the the connection is established using Virtual IP (VIP).

All SCAN/VIP must be resolved by DNS.

The client knows that there is only the SCAN Hostname, which is configured in the connection string.
Once the connection is requested Oracle Clusterware redirects the connection to one of the VIP hostname which must be resolved by DNS.

Role of Public,Private,Virtual, and SCAN IPs

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