Mysterious chalk symbols have appeared almost overnight in London, believed to be created by a gang of nerds set on revealing the city's wireless hot spots.

"Warchalking", as it is known, derives from the practice of tramps in 1930s depression-hit America leaving chalk messages to each other to indicate where they could get food and shelter.

Today, the set of symbols tells other geeks, or "Wibos" as they are known, where they can get a free wireless internet connection.

Symbols written on the pavement indicate whether the wireless network is open, closed or encrypted. Above the symbol is the network's Service Set ID (SSID), which is used to identify the particular wireless Lan to be accessed. Below the symbol is the amount of bandwidth on offer.

Anyone with knowledge of the symbols would be able to set up a laptop or PDA with the relevant settings and connect to a company's network to surf the internet or pick up email for free.

The new wave idea of warchalking was invented by web designer Matt Jones, who got the idea after seeing some architectural students chalk up a life-size office plan in a London square.

"The chalk plan had door and window symbols and the URL of the student's website depicted. I thought it was fun, but not that useful," said Jones. "It just illustrated the possibility to passers-by that outdoor wireless net access could happen, but didn't tell them how to join in."

After talking with some friends about how this idea could be turned into something more useful, a friend mentioned the symbols used by hobos to get meals. Jones then set up a weblog to expose the idea to the world. Since then he has been overwhelmed by emails from people eager to get involved with the project.

There are concerns that systems administrators could be at risk from having their networks exposed to the general public. But Jones believes this could be a positive thing in promoting the need for wireless security.

"If you see the chalk symbols appearing, then you know that you have an exposure to the public - you can deal with this how you will," he said. "Some enlightened companies might provide an open node for the public, others may choose to limit access," he said.

Matt Jones's warchalking web site can be found here.