Based on the many responses we got regarding the 'Packetslinger' diary, here a few notes on how to setup a penetration/cracking exercise.

As a remark: Laws change from area to area. Whatever you do, check your local laws and regulations. Corporate policies, university ethics guidelines and ISP contracts may have to be consulted.

1. Avoid the use of public networks if possible. Its just too easy to 'fat finger' an IP address. It is all too easy to unintenionally shut down a critical system using an attack as simple as a portscan.
2. If you have to use a public network, try to setup a VPN to isolate the sources and targets involved.
3. Ask participants to remove or turn off additional network interfaces (in particular wireless interfaces).

Any attack, even as simple as a portscan, should only be performed with written permission. Even in a lab environment, it may be a good exercise to go through the motions of obtaining written permission from the instructor. It is not always easy to identify the person who has to provide permission. But in general, this should be the 'network owner'. Remember that part of a corporate network may be owned by an ISP, and not the company (or university).

Can you go to jail for running a portscan? Unlikely. But the fact that you consider this question is a good hint that you should get written permission. Internal teams may be given permission  via policy documents. See http://www.sans.org/resources/policies/ for templates (e.g. the Audit Vulnerability Scanning Policy or the Risk Assessment Policy).

Couple additions submitted by readers:

- Setup the entire network (attacking systems and targets) in vmware. Use RFC1918 addreess to avoid 'leakage' and firewall the test network. Students can ssh into the network. (Thanks Mike and Nick!)