What is SSID- By: Musarrat Sheikh

Description : The SSID also known as the Service Set Identifier is a token which identifies an 802.11 Wi-Fi network. The SSID is a secret key which is set by the wireless network administrator and you must know the SSID to connect to an 802.11 wireless network. However, the SSID can be discovered by network sniffing or scanning. By default, the SSID is part of the packet header for every packet sent over the WLAN.
SSID access points continuously broadcast radio signals which are received by client machines if enabled. Based on the automatic or manual configuration, the client can connect to the access point. A SSID is generally 32 bit long, but when displayed to the user, it is projected into a human readable ASCII format. Multiple access points can possibly share same SSID if they are for the same wireless network. Many wireless access points support broadcasting multiple SSIDs, permitting the formation of Virtual Access Points. Such Virtual Access Points partition a single physical access point into many logical access points, each of which can have a special set of security and network settings.
The fact that the SSID is a secret key instead of a public key creates a key management problem for the network administrator. Every user of the network must configure the SSID into their system. If the network administrator seeks to lock a user out of the network, the administrator must change the SSID of the network, which will require reconfiguration of the SSID on every network mode. Some 802.11 NICs allow you to configure several SSIDs at one time.
Most 802.11 access point vendors allow the use of an SSID of "any" to enable an 802.11 NIC to connect to any 802.11 network. This is known to work with wireless equipment from Buffalo Technologies, Cisco, D-Link, Enterasys, Intermec, Lucent, and Proxim. Other default SSID does include tsunami, 101, Roam About Default Network Name, Default SSID, and Compaq so that every time a client connects to the wireless network, the SSID is communicated in plain text format, which can easily be sniffed by eavesdropper using sniffing applications like Kismet. Hence, additional security techniques are required to be implemented in order to enhance the wireless security or a lot of hacking into the system will happen often.
Many Wireless Access Point also known as WAP vendors have added a configuration option which lets you disable broadcasting of the SSID. This adds little security because it is only able to prevent the SSID from being broadcast with Probe Request and Beacon frames. The SSID must be broadcast with Probe Response frames. In addition, the wireless access cards will broadcast the SSID in their Association and Reassociation frames. Because of this, the SSID cannot be considered a valid security tool.
The SSID is also referred to as the ESSID Extended Service Set Identifier. While some vendor refer to the Service Set Identifier as the network name. The Service set Identifier can also be set on the Many Wireless Access Point.

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Author Resource : This article touched the basics of the topic. I have 2 more resources related to the above. They are ssid and ssid. They are worth a read.