Most wireless routers and access points are shipped with default SSIDs; see this list of common SSID values. For example, "linksys" is extremely common because many consumers just plug in their brand new Cisco Linksys routers without changing this factory-default SSID. This practice misleads users into accidentally connecting to the wrong wireless network -- for example, if you and your neighbor both own Cisco Linksys routers, you might end up connected to your neighbor's wireless network instead of your own.
Over the past few years, many wireless routers have started to ship with randomly-generated default SSIDs -- especially those that support Wi-Fi Protected Setup (WPS), a quick and easy way to set up WPA2-PSK security by entering the same PIN or pushing a button on both your router and a client you want to connect to your network. This practice not only improves overall WLAN security, but prevents those accidental connections that happened with default SSIDs.
Some wireless network administrators hide SSIDs to discourage outsider connections. In that case, you may see an "available wireless network" without a name. But those SSID values are not truly hidden; they can be easily obtained by running or many other Wi-Fi network discovery tools.
This was first published in March 2009