In Mac OS X, to see if a computer is on the network, use the Network Utility to ping the remote host. A ping is a small packet of data sent to the remote host and then sent back by the recipient computer. If the computer is not on the network, it will not be able to respond to the packet.
To use the Network Utility:
1. From the Applications folder, open the Utilities folder, and then open the Network Utility application. Alternatively Select GO in the Finder menu to access the Utilities folder.
2. In the Network Utility window, click the Ping tab.
3. In the first field, enter the IP address or domain name of the computer you want to ping.
4. Select Send only 4 Pings.
5. Alternatively to send a continuous ping then select Send Unlimited Pings
6. Click Ping.
The Network Utility will begin pinging the host. If the host is on the network, the packets will be listed in the Network Utility as they are returned. If no packets are returned, the computer may be off the network or there may be network problems between your computer and the target computer address.
It’s also possible that the computer has been configured not to respond to pings.