How to release/renew IP address over Wifi Connection

Hi guys,
When First time connecting to my router+firewall.
It get IP address with something like 192.168.0.200
So I assign my Samsung Galaxy S MAC address to my firewall to become 192.168.0.100
I tried to forget the network on my Samsung Galaxy S but when join again it show still same IP address .200 not .100
In windows we can do /release and /renew.
But how do I do /release or /renew on my Samsung Galaxy S?
Anyway if I put static address on my Galaxy S it work.
But that not I want.

Thanks I tried this, it didn’t work.
The problem is my router+firewall has DHCP lease time which been set default to maximum.
In no time I can change and modify my router+firewall by changing DHCP lease time more shorten time. I put it as 3 minutes.
Then from my phone I disconnected it wait 4-5 minutes and connect again.
It will get new IP address.
But this is not good practical.
Smart phone support with IP address should have able to renew IP address.
Specially OS android with as we known powered by Linux.
Anyway if I use terminal by releasing IP address also denied.
It seems I should root my phone before doing SU privileges, to use function such renew IP address.
It will great someone can developed tools can do this 