If you own a wireless network and you live in Las Vegas, you may have a lot of unexpected guests this weekend.

During the first weekend of August, the US city is hosting the annual DefCon hacker conference which is running a competition to see who can find the most wireless networks.

During the conference, teams of eager hackers will be driving the streets touting laptops fitted with software that can spot the wireless networks.

If the networks in Las Vegas are anything like those in other cities around the world, then many of the networks the teams find will be doing little to protect themselves against unwanted visitors.

Packet play

Back in the days when hackers only had the phone network to play with, many curious technology enthusiasts conducted "wardialling" expeditions.

This involved making their computer modem dial through a long list of numbers to find out which ones responded with data dialtones.

Crisp cans are used as antennas to help find networks

With the advent of wireless technologies that let people connect up computers with radio, hackers have adapted and now embark on "wardriving" expeditions.

Wardriving involves scouring an area for the wireless networks. They can be easy to spot because many broadcast their presence to make it easy for computers to link in.

"Most wardriving is really done for sport," said Geoff Davies, managing director of security company i-sec. "It's very much become a pastime to see what kind of things you can pick up."

Earlier this year Mr Davies' firm showed how a crisp can could be used to pinpoint the location of wireless networks.

Software to spot wireless networks and capture key information about them is readily available online.

Collecting points

A wardriving competition is being held as part of the annual DefCon conference which takes place in Las Vegas between 2-4 August.

Under the rules of the competition teams of hackers will be given two hours to drive around and spot wireless networks.

Points will be awarded to teams for each network they find and extra points will be given for those found with their basic security systems turned off.

If a team finds a wireless net that no-one else does they get five points. The team with the most points at the end of the two hour drive around wins.

The location of each network must be logged using GPS co-ordinates so the DefCon organisers can check the find.

The competition is being judged by Pete Shipley, the man who invented wardriving.

Better software

Unfortunately surveys carried out by i-sec and others have found that many of the wireless networks are not doing enough to make them secure.

Mr Davies said many were unwittingly giving people high-speed access to the net and, in the worst cases, were potentially leaving sensitive company data open to scrutiny by outsiders.

He added that the publicity about insecure wireless networks was starting to make people work harder to protect them.

"Certainly a lot of people who are using wireless have woken up to it and have taken action," he said.

Now products are starting to appear, such as Bluesocket and Reefedge, which corral wireless networks behind their own firewall.