The Internet Storage Name Service (iSNS) is a central directory of iSCSI targets and iSCSI initiators. Similar to the idea of a DNS, clients and servers register to the iSNS server, and then perform a lookup for resources available on the network. Once the list of resources is returned, the clients can then log into the resources as needed.
In a small or highly-centralized environment, this type of configuration may not be needed. If your environment has only one or two iSCSI targets, then the built-in target portals will provide the required information. However, if your environment contains multiple iSCSI targets, or will eventually grow to include multiple iSCSI targets, the iSNS server provides a central lookup and registration tool.