A NAS is "Network Attached Storage" and is essentially a thing on your local network making some storage space available over the network to clients. This may be a dedicated server, or a dedicated appliance or it may be nothing more than your home computer with "file sharing" turned on.
A SAN is a "Storage Area Network" and consists of at least two redundant controllers, and whatever networking kit is required to make that network access fully redundant.
Authentication/authorisation don't distinguish between SAN and NAS. You might or might not need credentials.
Protocol sort-of indicates SAN or NAS, but there is a heap of crossover. Some NAS devices offer iscsi, but few SANs offer NFS or SMB/CIFs access without some kind of host to do the sharing.
Controller redundancy is the one thing that makes a device a NAS and not a SAN. If your gear has two or more independent controllers, so that one can be restarted and all services are moved to the other, then you may have a storage device to use in a SAN. If you only have one controller such that service must stop to upgrade, then you have a NAS.
Note multiple ethernet ports, using a LAGG or etherchannel or bond are not the same as redundant controllers.
Example NAS devices I have used
- Iomega/Lenovo (it has a facebook uploader!)
- FreeNAS distro installed on a generic PC
- Synology NAS
- Thecus NAS
- Promise NAS
- Linux box running nfsd
- Linux box running samba (cifs)
- Linux box running iscsid
- Windows host with a shared drive or directory.
- Drobo - this was an 11 bay device with 8x2TB drives and 3x200GB SSDs, and it had 3x 1Gbit ethernet ports but only one controller. Upgrades stopped service for minutes, so its not truely redundant even though it had 3 links in multipath.
Example SANs I have used:
- HP lefthand SAN (2004ish) This was 4 separate 3RU boxes with many cables and switches and disks. It would have cost more than a new car.
- Dell MD3600 family
- Compellent sc4020
- Netapp monster thing of doom that needed Java to do anything.
This is a network plan for someone's virtualisation project, and the blue lines are the iSCSI network.
Everything is redundant, with multiple paths, and redundant PSUs on each component of the SAN.
So cost is a good second-order indicator of NAS vs SAN component. If you can buy a device without requiring capital asset approval from benacounters, its probably just a NAS.