Did you try the usual stuff ipconfig /flushdns, /release, /renew?
This are the first steps on Windows but you can try:
1. Find out your IP address e.g. go to a website like
http://www.whatismyip.com/ it will show you your IP.
2. Copy It.
3. Open Network & Sharing Center
4. Open the Properties of the network you what your IP to change. In my case it is a EVDO modem.
5. On the Networking tab double-click TCP/IPv4 which will open a window to edit IP and DNS IP address.
6. On this window Click on "Use the following IP address" and paste that IP you copied at Step 2.
7. Change the last digit of the IP to any value above it e.g. if its 192.168.23.123 edit the last digit to something like 122 so that the IP becomes 192.168.23.122.
8. Click OK until all the windows close.
9. Disconnect from that network.
10. Reconnect to that network and if it goes as planned the ISP should force a Dynamic IP. Check
http://www.whatismyip.com/ if it has changed. If you cannot browse go to Step 6 and select "Obtain an IP address automatically" then Disconnect -> Reconnect the network.
This must solve conf but sometimes having the network disconnected for 48 hours can be needed also, some of the IP problems are tricky and the solution always depend on the OS. Try this first and say something.