Single Client Access Name (SCAN) is s a new Oracle Real Application Clusters (RAC) 11g Release 2 feature that provides
a single name for clients to access Oracle Databases running in a cluster.

The benefit is that the client’s connect information does not need to change if you add or remove nodes in the cluster.
SCAN provides load balancing and failover for client connections to the database.

The SCAN is configured during the installation of Oracle Grid Infrastructure that is distributed with Oracle Database 11g Release2.
Oracle Grid Infrastructure is a single Oracle Home that contains Oracle Clusterware and Oracle Automatic Storage Management.
You must install Oracle Grid Infrastructure first in order to use Oracle RAC 11g Release 2.
During Oracle Grid Infrastructure installation, you will be prompted to provide a SCAN name.

There are 2 options for defining the SCAN:

1. Define the SCAN in your corporate DNS (Domain Name Service)

In the DNS a SCAN name would be defined ie a single name that resolves to 3 IP addresses using a round-robin algorithm.
You can check the SCAN configuration in DNS using “nslookup”.
If your DNS is set up to provide round-robin access to the IPs resolved by the SCAN entry, then run the “nslookup” command at least twice to see the round-robin algorithm work.

For each of the 3 IP addresses that the SCAN resolves to, a SCAN VIP resource is created and a SCAN Listener is created. The SCAN Listener is
dependent on the SCAN VIP and the 3 SCAN VIPs (along with their associated listeners) will be dispersed across the cluster. This means, each pair of resources (SCAN VIP + Listener) will be started on a different server in the cluster, assuming the cluster consists of three or more nodes.

2. Use the Grid Naming Service (GNS)

During the cluster configuration, three IP addresses will be acquired from a DHCP service (using GNS assumes you have a DHCP service available on your
public network) to create the SCAN and name resolution for the SCAN will be provided by the GNS.

Clients use SCAN name in tnsnames.ora to make the DB connection.
SCAN Listener can be defined on any node
SCAN listeners would be started from GRID Oracle Home, not the database/rdbms home.
SCAN Listener is provided in the REMOTE_LISTENER database parameter
Unlike in former database versions, it is not recommended to set your REMOTE_LISTENER parameter to a server
side TNSNAMES alias that resolves the host to the SCAN ( for example) in the address list entry,
but use the simplified “SCANort”
The LOCAL_LISTENER parameter should be set to the node-VIP, a node listener is created on each node in the cluster

When a SCAN Listener receives a connection request, the SCAN Listener will check for the least loaded instance
providing the requested service. It will then re-direct the connection request to the local listener on the node where the
least loaded instance is running. Subsequently, the client will be given the address of the local listener. The local listener
will finally create the connection to the database instance.