How to know the IP address of a Mac when it is in network

Fastest way: Using Spotlight (CMD + Space) type

Network Utility

which will open the OSX Network Utility Window

It will show the IP address(es) for all your adapters on the first pane.

In addition to @DaveParillo's answer, you can use the Terminal command ifconfig, which'll give output like this:

en0: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
    inet6 fe80::21d:4fff:fe47:f4f8%en0 prefixlen 64 scopeid 0x4 
    inet 10.6.29.76 netmask 0xffffff00 broadcast 10.6.29.255
    ether 00:1d:4f:47:f4:f8 
    media: autoselect (100baseTX <full-duplex>) status: active
    supported media: autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 10baseT/UTP <full-duplex,flow-control> 100baseTX <half-duplex> 100baseTX <full-duplex> 100baseTX <full-duplex,hw-loopback> 100baseTX <full-duplex,flow-control> 1000baseT <full-duplex> 1000baseT <full-duplex,hw-loopback> 1000baseT <full-duplex,flow-control>

'en0' is my Ethernet connection, and my IP address appears after 'inet'.

Use the "Network Utility" app found in the Applications -> Utilities subdirectory (or launch it using Spotlight).

Choose the network interface (wired/wireless) on the "Info" view and it will show you the IP address and other details of your network connection.

Or

Open the System Preferences app and click on Network. Click on the appropriate connection on the left hand side and it will show you the IP address right under the connection's status.

If I understand your question, can do this:

To find the IP address of a Mac OS or Mac OS X computer:

OS X 10.5

1. From the Apple menu, select System Preferences... . In System Preferences, from the View menu, select Network.

2. In the Network preference window, click a network port (e.g., Ethernet, AirPort, modem). If you are connected, you'll see its IP address under "Status:".

OS X 10.4 or 10.3

1. From the Apple menu, select Location, and then Network Preferences... .

2. In the Network Preference window, next to "Show:", select Network Status. You'll see your network status and your IP address displayed.

From http://kb.iu.edu/data/aapa.html#mac