In the Network pane of System Preferences, there are two buttons below your list of network connections: Configure and Disconnect.
Click Configure. On the TCP/IP tab you have a button labeled "Renew DHCP lease." According to the help files (search for 'release ip address'), this is the same as ipconfig /release AND ipconfig /renew.
If you are set up to receive your IP address using DHCP, you can force your computer to release your current IP address and get a new one from your DHCP server.
Open System Preferences and click Network.
Choose your connection method from the Show pop-up menu (such as Built-in Ethernet or Airport).
Click TCP/IP and click Renew DHCP Lease.
This might be helpful in an environment where many computers use the same DHCP server, or the network administrator has set up a short DHCP lease time.
Check with your ISP or network administrator to find out if you need to renew your DHCP address.