We're the group of volunteers who run the network at DEFCON. It's our job to design, plan, architect, implement, and secure the show network. We arrange bandwidth, we handle wireless, we provide secure connections for show staff, speakers, and press. Finally, we provide an environment where DefCon Attendees can share and be creative.

From the DEFCON FAQ:

It would be fair to describe the network as "hostile". It has been described as 'the worlds most hostile network,' but such descriptions are just attempts at flattery. It is recommended that if you want to connect to the DEFCON network pretend that you are sharing out your entire hard drive to 5000 hackers. You may want to bring a 'clean' computer that you don't mind being infected/hacked/etc. It is considered very poor form to attempt to DoS the network; while the DEFCON staff may not do anything about such attempts it is reasonable to assume that 'peer justice' may be metted out. If you're unhappy about the possible risks associated with connecting to DEFCON networks there are a couple of options: refrain from computer use for a few days or connect using another network elsewhere in Vegas (another hotel or something).