X marks the spot for hackers
The signs tell others where these networks are and what, if any, security they have.
It's called "war-chalking", a term derived from "war-dialling", a computer hacker practice dating back to the beginning of the Internet.
War-dialling hackers set up their computers to dial phone numbers until they find one that gives them access to a network.
Wireless networks enable hackers to use their computers on the road. Soon they were "war-driving" and "war-walking" city streets with notebook computers, often hooked to makeshift antennae, looking for unprotected wireless networks through which to log in.
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