How to Set a Manual IP Address in Mac OS X

1. Launch ‘System Preferences’ from the Apple menu  (or Spotlight)
2. Click on the “Network” icon
3. In the lower right, click on the ‘Advanced’ button
4. In the pulldown menu next to “Configure IPv4” select “Manually” (or DHCP with manual address, if that’s what you need)
5. Fill in the IP address, subnet mask, and router, as appropriate to the network you are accessing
   • Note: If you do not have these details necessary to be compatible with a specific network, ask the network administrator for the proper assigned IP, subnet, and router to use
   • Tip: Picking your own manual IP address? The key is to pick a manual IP address which does not conflict with existing IP addresses on the joined network. For example, if the network router IP is “192.168.1.1” and the network has 5 computers in total, manually pick an IP far outside the range of those potential computers (which are likely to be 192.168.1.1 through 192.168.1.6, and so forth, as IPs are typically assigned in a sequential order), so a potential IP address could be “192.168.1.75” or some other number which will not conflict or match an existing network resource
6. Click ‘OK’
7. Click ‘Apply’ to set the change

Cr : http://osxdaily.com/2010/01/26/how-to-set-a-manual-ip-address-in-mac-os-x/