The First Step: Create an Ad hoc Network and share your files instantly without using a router, Read here

If already been setup, proceed with the steps below:

Windows XP as the Gateway Configuration

In this configuration, I recommend to put/allocate the IP address of the Wireless Adapter of Computer with Internet Connection Fixed or Static.

Why? One time in Windows XP, I don't know if this is a random bug, because of old updates or not; When other computers boot and connect first to Ad hoc Network, It will automatically configured as a Gateway (192.168.0.1). So, when you boot Windows XP that will be your gateway to the Internet, IP address conflict message will appear.

Right-Click the Wireless Network Icon > Open Network Connections > Right-Click the Correct Wireless Network Connection > Properties

Click the "Internet Protocol (TCP/IP)" > Properties

Choose "Use the following IP address:" and enter the following

IP address: 192.168.0.1

Subnet mask: 255.255.255.0

Windows Vista as a Gateway Configuration

So far, I have not yet experienced IP Conflict in using the Automatic IP configuration in Vista. If yes, Please let me know by posting a comment below.

Enabling ICS (Internet Connection Sharing) in Windows Vista/XP For Ad hoc Networks

1. From my previous Tip: Open the Network Connection Window by simply launching the command: ncpa.cpl

or Control ncpa.cpl

2. Click the local area network (LAN) for DSL/Cable Internet Connection, WAN Miniport (PPPOE) for iBurst, USB Modem for GPRS, 3G, HSDPA or any type Dial-up Networking that provides you a primary Internet Connection > Right-Click Properties

3. In the Sharing Tab (Advanced Tab in Windows XP), Click the option "Allow other network users to connect though this computer's Internet connection"

In Windows Vista, Choose the Wireless Network Connection

Now Start Connecting other laptops/PC/devices and test the Internet Connection.

To Connect the other Computers: Right-Click the Wireless Icon > Select "Connect to a Network" For Vista or "View Network Wireless Networks" for XP > Click the name of the ad hoc Network and Again, Click the Connect Button > Input the Passphrase or Security Key and finally, Click the Connect Button again.

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UPDATE: NOT WORKING IN WINDOWS VISTA (SP1 with Oct 2008 updates)

In some GPRS/3G/HSDPA Modems, It doesn't work because the settings cannot be saved and it resets next time you connect.

Whereas in Windows XP configured as a Gateway, It works! How?

Solution: Trick? Setup Windows XP as a Gateway, ICS and Don't reconnect after setting the option above because it randomly resets.

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Windows Vista Note: If you are using a Broadband Dial-up Networking to connect and the Username and Password was saved for the current logon user only. It will automatically dial and connect to the Internet if the current user are logged on.

So, if the gateway computer is not yet connected and the other computers across the network (request) browses the Internet, it will automatically dial and connect to the Internet. Cool Right?

Just make sure that the Wireless Adapter in the Gateway Computer is connected to the adhoc Network. That's it!

Enjoy surfing the net!!! :)

Network Related Tips:

NEW Simple Way to Monitor Opened Files in a Shared Folder Across the Network

[Help] How to Setup Broadband DSL Internet Connection in Windows Vista for a Single PC

Simple Ways to Tweak your Network and Internet Connections- Part 1

Simple Ways to Tweak your Network and Internet Connections- Part 2