but you should make sure that DHCP in the router is set up to not hand out the static address you will be using or you may have an IP clash on the network. I always set up DHCP in the router to use addresses with the final octect of 100. For example 192.168.2.100. This way DHCP will use address 100 and up and I can uses addresses below 100 (i.e. 192.168.2.20) for static IP's.
You can change you dynamic Ip address by the following two methods:
Go to start>Run>cmd
Type ipconfig/release they reset your IP address as 0.0.0.0.
Then you can set your desire IP address between your IP range by using the command line ipconfig/renew.
Reset your modem.
Unplug and plug it again.
Switch off the modem and on again.
Before and after changing your dynamic IP address in Windows 7 check your Iphone address using the site Ip-Details.com ,whether the Ip get changed or not.