Is scanning a LAN for devices connected to it illigal?

I got this app on my iPhone called Fing - Network Scanner by Overlook Soft, and when I connect to Wi-Fi, it lets me see what devices are connected to the same Wi-Fi. It also gives me that device's IP address and MAC address. Is this considered Port Scanning? Please enlighten me, I don't want to get banned by the Wi-Fi or get that Wi-Fi banned by its ISP. If intent is important, my intent is just to know out of curiosity.

The app has 4439 Ratings and who knows how many downloads. If it was illegal it would be down by now.

If it is your own LAN (your Wi-Fi), perfectly legal since everyone connection should have your permission to use the LAN.

If it is not your LAN, you would want to make sure to check with competent legal counsel but in all likelihood would need written permission to use this in this way first at a minimum. Certainly number of downloads is not sufficient to determine legality, and I cite napster as an example of a popular app that was ruled illegal. Most network scanners do some sort of port scan, and from the info on the app page cited in my source, it looks like this version of Fing can do both a port scan and use documented network discovery protocols to determine information.

Technically the program is packet sniffing not port scanning to get IPs and MAC addresses. Media Access Control (MAC) not Macintosh Mac! The phone is intercepting packets on the Wi-Fi and collecting a list of each packets to and from addresses. Port scanning is examining a device to find available service ports on a device or "holes in the firewall". A hacker can then exploit those open ports to penetrate the target machine and install rootkits to take control of the device. We use packet sniffers to locate devices and then port scanners to locate their vulnerable ports.

Fing can be used for "war driving" or just walking around to locate open networks and devices. It is just snooping. Hackers then maintain lists of open networks and devices on the internet. Snoop, identify, and report. The next step would be http://fastandeasyhacking.com/manual it gives you framework from which to mount standardized attacks across networks.

In the 80's exploring networks was a benign activity and how we learned network technologies. The internet was designed on the friendlier world. Hacking was good and respectable way of advancing state of the art computing. Unfortunately this open technology has been adopted world wide handling commercial and private activities beyond the design scope of the 80's internet. Our privacy, the privacy of our friends and our identities are now exposed to the world.

Hacking is now a shady activity at best and those caught at it have a lot of explaining to do. Increasingly companies are seeking the maximum possible sentencing on these fringe activities to discourage the activities. That's because we now depend on networks to live our lives. Lost data can now ruin lives and cost billions of dollars to fix.

The issue is complicated because a lot of us have always been hackers (in the good sense), and now 20, 30 years later "big government and big business" want to take that away from us. Some have dedicated their lives to challenging the government kind of like green peace, the tea party or occupy movements. Remember these guys have powerful legal defense funds behind them.

So how you ask can I download this software if it is not legal. Well, it is like buying a gun. You can, but don't take it to your elementary school, wave it in your front yard, or pack it in your travel bag before you board a plane.

If you are really curious take a network security class at the local community college. Create your own home network. Find "shooting ranges" were it is legal and allowed to practice with your weapons. Remember each time you log into a public or work based network there's a terms of service agreement which prohibits "snooping" that you must agree to before you can use the network. Remember that businesses and government will prosecute to the fullest extent of the law to protect our privacy and the trillion dollar economies we depend on to live.