However, Altos was more exclusive than the average underground BBS. Wanna-be hackers had trouble getting into it because of the way X.25 networks were billed. Some systems on the network took reverse-charge connections--like a 1-800 number--and some, including Altos, didn't. To get to Altos you needed a company's NUI (Network User Identifier), which was like a calling card number for the X.25 network, used to bill your time on-line. Or you had to have access to a system like Minerva which automatically accepted billing for all the connections made. X.25 networks are different in various ways from the Internet, which developed later. X.25 networks use different communication protocols and, unlike the Internet at the user-level, they only use addresses containing numbers not letters. Each packet of information travelling over a data network needs to be encased in a particular type of envelope. A `letter' sent across the X.25 network needs an X.25 `stamped' envelope, not an Internet `stamped' envelope. The X.25 networks were controlled by a few very large players, companies such as Telenet and Tymnet, while the modern Internet is, by contrast, a fragmented collection of many small and medium-sized sites. Altos unified the international hacking world as nothing else had done. In sharing information about their own countries' computers and networks, hackers helped each other venture further and further abroad. The Australians had gained quite a reputation on Altos. They knew their stuff. More importantly, they possessed DEFCON, a program which mapped out uncharted networks and scanned for accounts on systems within them. Force wrote DEFCON based on a simple automatic scanning program provided by his friend and mentor, Craig Bowen (Thunderbird1). Like the telephone system, the X.25 networks had a large number of `phone numbers', called network user addresses (NUAs). Most were not valid. They simply hadn't been assigned to anyone yet. To break into computers on the network, you had to find them first, which meant either hearing about a particular system from a fellow hacker or scanning. Scanning--typing in one possible address after another--was worse than looking for a needle in a haystack. 02624-589004-0004. Then increasing the last digit by one on each attempt. 0005. 0006. 0007. Until you hit a machine at the other end. Back in 1987 or early 1988, Force had logged into Pacific Island for a talk with Craig Bowen. Force bemoaned the tediousness of hand scanning. `Well, why the hell are you doing it manually?' Bowen responded. `You should just use my program.' He then gave Force the source code for his simple automated scanning program, along with instructions. Force went through the program and decided it would serve as a good launchpad for bigger things, but it had a major limitation. The program could only handle one connection at a time, which meant it could only scan one branch of a network at a time.