You can lookup the IP address by hostname. Assuming the hostname of your Pi is "raspberrypi"

Here is how you obtain the IP address using ping command and hostname. 

From Linux: **ping raspberrypi**<br>
From Windows: **ping raspberrypi**<br>
From OS X: **ping raspberrypi.local**<br>

In order for the command to work from Windows, you need to have Samba running on your Pi. You can install Samba using the command

**sudo apt-get install samba**