[barbarian]

Is there a simple way to check and see how many people are logged onto a particular airport or ethernet network?

[Mithras]

Not exactly. Some machines will let themselves be seen, and others may not.

But here's what you can do as a first effort:
1. Open Terminal
2. Type ifconfig en0 (for Ethernet) or ifconfig en1 (for Airport)

The third line of the output reads
inet 192.168.x.x netmask 0xffffff00 broadcast 192.168.1.255

or something similar. The part in bold (broadcast 192.168.1.255) is called the broadcast address. That's the address to send things to if you want everyone on the network to receive the packet.

3. Ping the broadcast address.
Pinging sends a packet that requests, "please reply to let me know if you're there." Machines don't have to reply if they don't want to.

Type ping 192.168.1.255 (or whatever your broadcast address is)
Press control-C to stop the pinging after a bit.

You'll get something like the following:

Code:
64 bytes from xxx.xxx.245.95: icmp_seq=0 ttl=64 time=0.376 ms
64 bytes from xxx.xxx.245.1: icmp_seq=0 ttl=255 time=51.563 ms (DUP!)
64 bytes from xxx.xxx.245.95: icmp_seq=1 ttl=64 time=0.36 ms
64 bytes from xxx.xxx.245.70: icmp_seq=1 ttl=64 time=1.969 ms (DUP!)
64 bytes from xxx.xxx.245.112: icmp_seq=1 ttl=64 time=6.158 ms (DUP!)
64 bytes from xxx.xxx.245.95: icmp_seq=2 ttl=64 time=0.365 ms
64 bytes from xxx.xxx.245.70: icmp_seq=2 ttl=64 time=1.974 ms (DUP!)
64 bytes from xxx.xxx.245.112: icmp_seq=2 ttl=64 time=6.164 ms (DUP!)

Looking at the replies, you can get a sense of how many unique machines are out there. But remember this only finds machines that are willing to be found.


Here's a command that will do the whole thing at once:
ping -c 10 $(ifconfig en0 | awk '/broadcast/ {print $6}') | awk '/bytes from/ {print $4}' | sed -e 's/://' | sort -u -n -t. +3 +2 +1 +0

Or, if you just want the count:
ping -c 10 $(ifconfig en0 | awk '/broadcast/ {print $6}') | awk '/bytes from/ {print $4}' | sed -e 's/://' | sort -u -n -t. +3 +2 +1 +0 | wc -l


One other thing you can do is, after you've pinged the broadcast address and asked machines to tell you that they're there, consult your ARP table, which is the table your machine uses to keep track of how to reach machines on the network.
Type arp -a to see the list of machines your Mac presently knows about on the network.

[barbarian]

The reason I ask is that I have a home in NY with a very fast connection. I'm not there very often (live in CA), and while I'm not there I leave an airport network open to whoever finds it. When I returned recently I noticed several users itunes libraries showing up via rendezvous and started wondering just how many people were logging on. It would be cool to have a little app that tracked users as the came on and off and gave you a summary at the end of the day via email...

Many thanks for the terminal goodness.

[bborofka]

Are you using an Airport router? If so, you can probably just look at the DHCP client list. I've never seen the Airport config tool so I don't know where to look for it. That should tell you all the names of the computers, their LAN ips, and MAC addresses too.