Ad-Hoc Networking

Microsoft Windows is called a Network Operating System. Computers running an operating system like Microsoft Windows (any of the many versions) were designed to be networked. As I've said elsewhere, if you have one computer, you have the beginning of a network.

The minimum complement of equipment, that you need for a computer network, is 2 computers and the appropriate networking components. The simplest networking component set would be two Ethernet adapters (one in each computer), connected by a bit of Ethernet cable, generally (but not always) a cross-over cable.

That's an ad-hoc Ethernet network. It's similar to hub (router / switch) based Ethernet networking, but without a hub (router / switch).

You can also have a network without any Ethernet cable, if you replace the Ethernet adapters with WiFi adapters. That's called an ad-hoc WiFi network.

An Ethernet based ad-hoc network is frequently limited to 2 computers. An Ethernet cable has just 2 ends - to get any more, you need a hub (router / switch). With a WiFi based ad-hoc network, you can have any number of computers connected, with minimal effort.

But there are several disadvantages to ad-hoc WiFi networking.

• One of the biggest is security. The minimum acceptable standard for WiFi security is WPA. Unfortunately, WPA requires a WiFi Access Point, to manage authentication / encryption. With no WAP, you're limited to using WEP to protect yourself, and WEP just isn't adequate security.
• With a router "in charge" of the network, you'll generally get more throughput. Client - server (with the server in charge) is more efficient than peer - peer (with no one in charge).
• Most WiFi equipment, in ad-hoc mode, will only operate in 802.11b mode, and get up to 11M of bandwidth total.
• Without a router, and a DHCP server built-in, you'll have to use ICS (if you're sharing Internet service), or pre-assign fixed IP addresses to each computer.
• You'll have to pre-assign channel number and SSID on each computer, as the normal WiFi Client won't find your ad-hoc network by scanning. Nor will it give you a signal strength indicator.
• You won't be able to disable SSID broadcast (not that this is a bad thing). In ad-hoc mode, SSID broadcast is forceably enabled.

Remaining aware of the limitations of ad-hoc WiFi, see specific details of the setup process

• About.Com How To Set Up an Ad Hoc (Peer) WiFi Network
• Microsoft Experts: Making the Wireless Home Network Connection in Windows XP Without a Router
• Microsoft Windows Vista Help: Set up a computer-to-computer (ad hoc) network
• Microsoft Windows XP Help: Set up a wireless network without a router

For a quick LAN, ad-hoc WiFi is OK. In an otherwise secure environment (maybe a single conference room deep within your office complex) it's perfect for a quick conference, and application sharing. For long term, really secure networking, though, you can't beat a properly setup, router (WAP) based network.

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Connecting Two Computers With A Crossover Cable

Most of my articles in this website are about Windows Networking / File Sharing, or about Internet Connectivity, and start by assuming that you have several computers, and a hub (router / switch) connecting them. But what if you have just 2 computers, and just want to quickly move files between the two? Or maybe you want to setup Internet service, for the 2 computers, without using a router to share the service?

You don't always require a hub (router / switch) to connect just 2 computers.

• Make The Right Decision
• Use The Proper Equipment
• Setup The Network - No Internet
• Setup The Network - With Internet
• Verify The Network
• Troubleshoot The Network, If Necessary

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Make The Right Decision
Start by asking yourself - what do you want to do - both now, and in the future? If you just want to immediately connect just these two computers, and quickly move files between the two, without Internet service, then this is the right start.

If your future might include Internet service, or if you might end up with a third computer, then you would really be better off using a hub (router / switch).

If you want to connect the two computers, and share Internet service, you can do that using a crossover cable. But know the issues before you start.

• If the computer with Internet service has it thru a dedicated modem, either:
  • Internally installed.
  • Connected externally, but thru a serial cable.
  • Connected externally, but thru a USB cable.
then using a crossover cable is a valid solution.
• If the computer with Internet service has it thru an Ethernet connection, or thru WiFi, then this is not a valid solution. If the Ethernet or WiFi connection is on subnet 192.168.0/24, this will not work at all. In the latter case, you will have to connect both computers directly to the LAN with subnet 192.168.0/24. If the Ethernet or WiFi connection has a private LAN address (10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/24), you're behind a NAT router, and you should setup a bridge, rather than use ICS.

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Use The Proper Equipment
The simplest solution, for networking just 2 computers, is to get an Ethernet crossover cable, and connect the two directly. A single crossover cable is the equivalent to getting a hub (router / switch), and a pair of straight-thru patch cables.

Please use Ethernet, not Firewire or USB, for connecting your computers. Firewire / USB networking requires extra drivers, and extra work. Ethernet drivers are native in all modern operating systems.

Please use a Cross-over Ethernet cable. A Straight-Thru, aka Patch, cable may work for some newer systems, which can automatically sense the need for a cross-over. But a cross-over cable will work all of the time, when you need to connect two computers directly.

Patch cables may come in many colours and lengths; some computer stores will have dozens of choices to suit your cabling needs. Cross-Over cables, when you find them in the store, will be explicitly labeled "Crossover", and will come in one colour (probably orange), and one length (probably 3 or 5 foot).

Please buy a properly made cross-over cable. If you're a masochist, or extremely desperate, you may make your own from a pair of patch cables, properly spliced. But Ethernet cables, that support modern high speed networks, require precision in their construction. I'm a fan of do-it-yourself activity (as in the reason for this website), when it's properly planned. I don't recommend do-it-yourself Ethernet cabling, when you're setting up a network. Buy a cable, unless you're very experienced with networking and can easily recognise the possible problems.

If you replace the two Ethernet adapters, one on each computer, with two WiFi adapters, you may be able to eliminate the Ethernet cable and setup an ad-hoc WiFi network. A WiFi based ad-hoc network isn't terribly different from an Ethernet based ad-hoc network, once you get the WiFi connectivity working.

Use the Device Manager in Windows, and test the network adapter in each computer. Connect your cross-over cable to the two network adapters. Observe the lights on the network adapters, and / or the status indicated by the Local Area Connection icon in the tooltray - do both computers indicate successful electrical connection?

Now, will you be setting up your network to Share Internet Service? Or just to share files, with No Internet Service?


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Setup The Network - No Internet
If you just have two computers, and no Internet service to either, run the Network Setup Wizard on each computer. Select the last option

This computer belongs to a network that does not have an Internet connection.

Having connected the two computers physically, and checked that you have no physical problems, you need to make the logical (TCP/IP) settings. If you have Windows XP / Vista, or other current operating systems on your computers, you're in luck. Modern operating systems use a system called APIPA, and should be able to provide ip settings automatically, so the two computers will connect to each other. If you allow the two computers to dynamically assign addresses, APIPA should take care of this for you.

Be prepared to get an error message - Limited Or No Connectivity - if you use APIPA configuration.

NOTE: if any of your computers are NOT running Windows XP / Vista, you'll have to set the IP address and subnet mask manually, on those computers. Remember IP addresses have to be unique for IP addressing to work.

• Run "ipconfig /all", from a command window, on each APIPA compliant computer first.
• Make a list of which addresses are automatically assigned.
• Manually configure each non-APIPA compliant computer.
  • Set each computer up with a unique IP address, in the 169.254.x.x subnet (written as 169.254/16 in many cases).
  • Each computer gets a subnet mask of "255.255.0.0".
  • Each computer gets an IP address of "169.254.x.x", where the "x.x" MUST be different for each computer. Check your list of addresses assigned by APIPA!
    
  • Each value of "x" must fall between 1 and 255 (not including either 1 or 255).
  • You assign IP addresses using the TCP/IP Properties wizard. Select "Use the following IP address". Only worry about IP address and Subnet mask - the other settings are only useful if you have an outside connection. With locally connected computers, just IP address and Subnet mask are essential, and should be assigned as described above.

Having connected the two computers physically, and checked that you have no physical problems, next Verify The Network - make sure that it works properly.


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Setup The Network - With Internet
If you have two or more computers, with Internet service to one, and wish to share the service to the others, run the Network Setup Wizard, first, on the computer that has Internet service. Select the first option

This computer connects directly to the Internet. The other computers on my network connect to the Internet through this computer.

You'll probably be running ICS on this computer. Note the disadvantages and requirements of ICS, and a possible alternative. You'll have to have two separate network connections (one might be a modem, directly or internally connected). You'll indicate which connection provides the Internet service, and then which connection(s) are to be used for sharing the Internet service.

If, per the ICS alternative, you decide to use a bridge, remember that both of the network adapters, on the bridged computer, will have the same IP address. Plan your testing accordingly.

If you decide to use ICS, run the Network Setup Wizard on the other computers, and select the second option

This computer connects to the Internet through another computer on my network or through a residential gateway.

If you can't run the Network Setup Wizard on any computers, use the TCP/IP Properties wizard, and select automatic configuration.

If the second computer will be running as a server, and providing data to the Internet, remember that the ICS server will have to be online constantly. Having dealt with that requirement, setup the ICS server to (KB231162): forward the right ports to the Internet server.

Having connected the two computers physically, and checked that you have no physical problems, verify that the network works properly.


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Verify The Network
Please verify that you have connectivity between the computers first.

• Run "ipconfig /all" on each computer, from a command window. Note the IP address and subnet mask for each network connection.
• Make sure that you don't have a bridge, on any computer, unintentionally.
• Verify that all computers are on the same subnet, and that each has a unique address.
  • If this is two computers without Internet service, each computer should have an address of 169.254.n.n, and a subnet mask of 255.255.0.0. This will indicate that each is on subnet 169.254/16.
  • If this is two, or more, computers sharing Internet service, the first computer (thru which the others will be getting Internet service) must have an address of 192.168.0.1, and a subnet mask of 255.255.255.0 (on the connection used for sharing the Internet service). All of the computers getting Internet service thru the first computer must have an address of 192.168.n.n, and a subnet mask of 255.255.255.0, and show both DHCP and Autoconfiguration Enabled = Yes.
• From each computer, again in a command window, ping the other. If, for instance, the address on Computer B is "169.254.1.2", open a command window on Computer A, and enter:
  

  ping 169.254.1.2

  
  If you get back a series of responses like
  

  Reply from 169.254.1.2: bytes=32 time<1ms TTL=128
  Reply from 169.254.1.2: bytes=32 time<1ms TTL=128
  Reply from 169.254.1.2: bytes=32 time<1ms TTL=128
  Reply from 169.254.1.2: bytes=32 time<1ms TTL=128

  
  then you are ready.

When you open Windows Explorer on each computer, and look in Network Neighborhood, you should see both computers. And when you open (doubleclick on) one entry, you should see the folders and files.

And, if the computer directly connected to Internet service can access the Internet, the other computers should be able to do so also.


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Troubleshoot The Network, If Necessary
So what if it doesn't work, per the basic testing above? Well, now you start troubleshooting, and methodically, in the right sequence.

• Verify physical connectivity again.
• Troubleshoot Windows Networking.
• Troubleshoot File Sharing.
• And, having done ALL of the above, post in a forum where the experts give serious help, and Ask For Help, with details provided.

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Internet Connectivity Problems Caused By The MTU Setting

The messages sent and received between your computer, and the Internet web servers that you're accessing, may go thru dozens of networks. The Internet is, by design, dynamic. The networks that you use, to access any server, may change within seconds.

Any one of those networks might have a restriction on the maximum message (packet) size that it will accept. Each computer has a setting, called the Maximum Transmission Unit (MTU), which controls how large it may make any packet. The larger your packets, the fewer packets required for sending or receiving a web page, but the greater chance any network have a problem with your packet size.

Setting the MTU on your computer can be a double edged sword.

• If you make the MTU too large, some networks will split (fragment) your packets. Some servers may have a problem with fragmented packets, causing the dreaded "Server not available..." error, or other symptoms.
  
• If you make the MTU too small, your computer will send and receive small packets. You'll be able to access any server, thru any network, but a web page will require too many packets. The speed that your web pages download will make you think you're not connected at all.
  

If you have a problem accessing some websites (or running some programs like email or IM), but not others, or if this problem seems to come and go, you may have an MTU setting problem. The best known examples of this problem are those with dial-up or PPPoE sevice, or those using ICS. An MTU issue can affect anybody, though, and different people (computers) will, almost certainly, be affected differently, with different web sites being unreachable at different times.

Here are several articles, of differing technical detail and level, discussing the issue further, and offering ways to diagnose and correct it. Read through each one, until you find one that explains the issue to your liking. This list, like each of the ones that I create, is alphabetised.

• Allen Smith: ICS Web-browsing Problem: the Real Solution.
• Annoyances: Can't access some websites while using a shared PPPoE connection.
• Microsoft: (KB319661): Connectivity Problems on ICS Clients When You Use a PPPoE Connection on a Windows XP ICS Host.
• Microsoft: (KB314825): How to Troubleshoot Black Hole Router Issues.
• NetHeaven: Sites With Broken / Working Path MTU Discovery
• Nitecruzr Dot Net: The MTU Setting Problem - Why Is It So Obscure?

When you get ready to adjust the MTU setting, make it easy on yourself. Download DrTCP, from DSLReports, and use it to make the changes for you. Simply copy the downloaded file into any convenient folder, and run it from there.

You'll be changing the "MaxMTU" or "MTU" value under Adapter Settings. If you have multiple network adapters, be sure to choose the one that provides the Internet service. For instructions about what values to change MTU to, see the articles linked above. Read all 6, and pick the one that you're most comfortable with.

Note: An MTU problem can be confused with, or masked by, a DNS problem, or LSP / Winsock Corruption. If you're here after trying the above procedures, unsuccessfully, consider each of the latter possibilities.

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Using The Network Setup Wizard in Windows XP

There are a lot of network settings in any Windows operating system. The many settings can affect how your computers connect to each other, and to the Internet. The Network Setup Wizard is provided as a part of Windows XP, to make the more common settings, for you. In Windows Vista, you'll use the Network and Sharing Center wizard.

The Network Setup Wizard is most useful when run on a properly designed, setup, and prepared network.

• Make sure the following network components are installed, in the network items list in (Name of connection) Connection - Properties.
  • Client for Microsoft Networks.
  • File and Printer Sharing for Microsoft Networks.
  • Internet Protocol (TCP/IP)
• Make sure that the DHCP Client service is running - Started and Automatic.
• Make sure that the Internet Connection Firewall / Internet Connection Sharing (ICS) (pre-SP2), or Windows Firewall / Internet Connection Sharing (ICS) (SP2) service is running - Started and Automatic.
• Configure your firewall setup, including installing any third party firewalls, after you run the wizard.

I know of several ways to start the wizard.

• From (Start - Programs - Accessories - Communications - ) Network Setup Wizard.
• From Windows Explorer, with Common tasks enabled.
  • In Network Connections, look under Network Tasks.
  • Select
    

    Setup a home or small office network.

• From the Help and Support Center.
  • Start Help and Support.
  • Search on "Network Setup Wizard".
  • Under "Pick a task", select "Start the Network Setup Wizard".
    

    To start the Network Setup Wizard
    You must be logged on to this computer as an administrator to complete this procedure.
    

    1. Start the Network Setup Wizard.
    2. Follow the instructions on your screen.

  • Select the shortcut in "Network Setup Wizard".

The Network Setup Wizard will run on your computer only if

1. You are logged in with administrative authority.
2. Your computer is not joined to a domain.

When you run the Network Setup Wizard, you are given a total of five choices, on two screens, which identify how you connect your home or small office network to each other, and / or to the Internet. See Practicallynetworked XP ICS - Starting the Network Setup Wizard for more graphical instructions.

• This computer connects directly to the Internet. The other computers on my network connect to the Internet through this computer.
• This computer connects to the Internet through another computer on my network or through a residential gateway.
• Other. Selecting "Other" gives you three additional choices.
• This computer connects to the Internet directly or through a network hub. Other computers on my network also connect to the Internet directly or through a hub.
• This computer connects directly to the Internet. I do not have a network yet.
• This computer belongs to a network that does not have an Internet connection.

Alternatives
The Network Setup Wizard is a Windows XP component. If you have a computer running another version of Windows, like Windows 98 or ME, you can run NetSetup. Copy NetSetup.exe from "C:\Windows\System32" to a CD or a USB flash drive, carry the CD or flash drive to the target computer, and run it from there. You are under no obligation to copy anything to any device or drive; any settings that you make on this computer, using the Network Setup Wizard, you can make on another computer, manually. The Network Setup Wizard is simply a convenience tool.

You can do just what NetSetup.exe, or the Wizard, does, on any computer running Windows XP, 2000, 98, or ME.

• Install any missing network components, from Network Neighbourhood - Properties - Configuration.
  • Client for Microsoft Networks.
  • File and Printer Sharing for Microsoft Networks.
  • TCP/IP.
• Set the Computer Name, and Network Name, from My Computer - Properties - Computer Name, or Network Neighbourhood - Properties - Identification.
• Set TCP/IP to obtain IP and DNS server addresses automatically, from Network Neighbourhood - Properties - Configuration - TCP/IP Properties.

Note that the Network Setup Wizard only makes very basic system settings, and doesn't check for the presence and configuration of other computers on the network. If you run the Wizard, and you don't get the results that you expect, you'll need to read either Troubleshooting Network Neighborhood Problems (for local access problems), or Troubleshooting Internet Service Problems (for Internet access problems). You may also benefit from reading Solving Network Problems. Be persistent.

If you cannot run either NetSetup.exe or the Network Setup Wizard, on any computer, you can still setup your network. On a computer with Windows 2000, or in any other case where neither netsetup nor the wizard can be used, you can make all settings manually.

ICS Host

This computer connects directly to the Internet. The other computers on my network connect to the Internet through this computer.

This network configuration uses (KB306126): Internet Connection Sharing (ICS) to share this computer's Internet connection with the rest of the computers on your network. Communication to and from the Internet to all the computers on your network are sent through this computer, called the ICS host computer.

If you're going to use an ICS host to provide Internet service for your network, please use (KB283673): Internet Connection Firewall (Windows pre-SP2), Windows Firewall (Windows SP2), or a third party personal firewall at all times. Please don't ever connect an unprotected computer to the Internet.

Note the other disadvantages and requirements of ICS. You'll have to have two separate network connections (one might be a modem, directly connected). You'll indicate which connection is to be used to connect to the Internet. Other connections on the computer will then be used for sharing the service.

As an ICS Host, the wizard will perform steps 1 - 11, from the list of Actions below.

Gateway (ICS or NAT Router) Client

This computer connects to the Internet through another computer on my network or through a residential gateway.

This computer is part of a home or small office network that connects to the Internet through another computer on the network or using a residential gateway (i.e., a NAT router). If you have another computer on your network that shares its Internet connection, called the ICS host computer, this computer will be able to send and receive e-mail and access the Web, as if it were connected directly to the Internet.

A residential gateway is a hardware device that works similarly to a host computer. Typically, a DSL or cable modem connects to the Internet service, and the residential gateway connects to the modem. Internet communication is shared by the residential gateway to all of the computers on your network.

As a NAT Client, the wizard will perform steps 1-5, then 10-11, from the list of Actions below.

Multiple Direct Internet Connections

This computer connects to the Internet directly or through a network hub. Other computers on my network also connect to the Internet directly or through a hub.

This network configuration typically has an external DSL or cable modem connected to an Ethernet network hub. The other computers on your network are also connected to the network hub. Each computer on the network has a direct connection to the Internet by means of the network hub and DSL or cable modem.

If you are using this configuration for your home or small office network, I highly recommend that you disable file and print sharing on the TCP/IP protocol and enable it on the IPX/SPX protocol. If you share files and folders on your computers using the TCP/IP protocol, they could be seen on the Internet. Only enable IPX/SPX for file and printer sharing if you are using this network configuration for your home or small office.

I do not recommend this network configuration. It exposes all computers on the network directly to the Internet, creating potential security problems. I highly recommend that you use a secure host device, such as a computer running Windows XP with ICS and Windows Firewall enabled, or using a residential gateway.

As a Directly Connected Client, the wizard will perform steps 1-5, then 10, from the list of Actions below.

Single Direct Connection To The Internet

This computer connects directly to the Internet. I do not have a network yet.

Select this option if you only have one computer and it has an Internet connection. The Network Setup Wizard configures this computer to use Windows Firewall, to protect your computer from intrusions from the Internet.

Networked Locally But Not To The Internet

This computer belongs to a network that does not have an Internet connection.

Select this option if you have two or more computers networked together, but don't have an Internet connection. You can have a home or small office network, using Ethernet, a home phoneline network adapter (HPNA), or wireless adapters. If you have different network adapter types, such as Ethernet, HPNA, or wireless devices, installed in your Windows XP computer, the Network Setup Wizard can create a network bridge to allow all of the computers in your network to communicate.

If you're lucky and have Ethernet on both computers, you can use a hub and Ethernet cables to connect your computers. This is, by far, the best choice in your case. If you're connecting just 2 computers, you can even use a cross-over cable, instead of a hub.

Actions Taken By The Wizard

Depending upon the Option selected, the wizard will do any of the following:

1. Set the computer name, computer description, and workgroup name that you specify.
2. Install these network components if they're not already present:
   
   • Client for Microsoft Networks.
   • File and Printer Sharing for Microsoft Networks.
   • TCP/IP.
3. Share any printers connected to the computer.
4. Create the "Shared Documents" folder, if it doesn't exist.
5. Share the "Shared Documents" folder.
6. Enable Internet Connection Sharing, on the network connection that you specify.
7. Enable the Internet Connection Firewall (pre-SP2) / Windows Firewall (SP2), on the shared network connection.
8. Create a Network Bridge, if more than one local area network connection exists.
9. Configure the local area network connection (or Network Bridge), using IP address 192.168.0.1/24.
10. Configure the local area network connection, to obtain an IP address automatically.
11. Install software, to allow the client to control the host's Internet connection.

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Sharing Your Dialup Internet Service Doesn't Have To Mean ICS

If your Internet service is configured under Dialup Networking / Remote Access Services, you have PPP compatible dialup Internet service. If this is true, you can still surf the internet with the convenience and safety of an NAT router. ICS is OK, but you can do better.

There are a number of NAT routers - both wired and wireless - that will provide a dialup Internet connection. Here's a list of examples, currently available.

• Netgear FVS338 (wired)
  
• Netgear FWG114P (wireless)
  
• SMC SMC7004ABR (wired)
  
• SMC SMC7004AWBR (wireless)
  
• USR USR8000A (wired)
  
• USR USR8022 (wireless)
  

You connect the router to a full featured, external, serial dialup modem of your choice. As some have discovered, neither a USB modem, nor a WinModem, will work with these routers.

The Creative Modem Blaster is one possible product, which you may be able to buy in either CompUSA or in Walmart (depending upon what you have available), that may work for you. Use this as an example - I'm sure the local computer store in your neighborhood has something just as good.

If you are, or are thinking about, using ICS to share your connection, you have to have an Ethernet card in the computer.

If you have a standalone computer, you still can, and you should, use a router for protection. You will have to install an Ethernet card, if you don't have one already, to connect your computer to the router. You can get a quality card for $15 at Walmart, for instance.

You connect the Ethernet card to the router (using an Ethernet patch cable), you connect the router to the modem (using a RS-232 serial cable), and you connect the modem to your phone service (using an RJ-11 phone cable). It's a 5 minute job, when you're used to it.

If you have broadband, and are setting up dialup service as a backup, you connect a second Ethernet patch cable from the router WAN port to the broadband modem. That's another 30 seconds.

And if you want a WiFi LAN, but don't care for the WiFi features of either of the 3 choices above, get one of the wired routers listed above, and get any standard (Ethernet WAN) WiFi router, or access point, that you like. You can setup any WiFi router as an access point, with either one of the three above wired routers providing the dial-up Internet service.

The bottom line? Whether you have one computer, two computers, or more, and you have PPP-compatible dialup Internet service, or Ethernet compatible broadband Internet service, you can (should) connect thru a router, for layered security.

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ICS Is OK - But You Can Do Better

A few years back, when routers cost $200 or more, Microsoft added a feature in Windows that lets you share your internet connection with other computers, and save the cost of a router.

All you have to do is to add a network adapter to your computer. Internet Connection Sharing, as they call it, is a NAT router in your computer. It provides a DHCP server, and a routed internet connection, to any client computers connected to the added network card, thru a simple cross-over cable or hub, or through an ad-hoc WiFi LAN.

If the internet connection is thru a modem (as in dialup internet access), ICS can even share that service to any client computers. It's quite simple to setup.

ICS is not without cost, though.

• The added network adapter gets a forced (and (KB230148): not easily configured) ip address of 192.168.0.1. If the primary network adapter (where the internet service originates) is on the 192.168.0.0/24 subnet, you have a problem. If the primary network adapter is using a private LAN address (10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/24), you're behind a NAT router, and you should setup a bridge, rather than use ICS.
  
• The ICS server has to be on, whenever you wish to access the internet from any ICS client.
  
• Any time you load software on the ICS server, and / or restart it, you will affect internet service to the ICS clients.
  
• Occasionally, the internet traffic from the ICS clients may strain the resources of the ICS server.
  
• Depending upon its configuration, a personal firewall running on the ICS server may also strain the resources of the server. This is generally a problem with malicious incoming traffic, which the firewall must log and / or research.
  
• If the ICS server is also used for Internet access (as a desktop computer), the ICS server may be vulnerable to any malware acquired thru the Internet.
  
• If service is thru a dialup modem, the modem management software (as in Remote Access Services) may affect stability of the server. Activity on the server may, conversely, affect stability of RAS.
  
• If service is thru a dialup modem, malware can hijack the modem, and make unsolicited extremely expensive dialup connections, that can result in a phone bill in the thousands of dollars.
  

And note this Microsoft advisory

Internet Connection Sharing, Internet Connection Firewall, and Network Bridge are features designed for home and small office networks. These features are offered in some of the Microsoft Windows Server 2003 family operating systems. Information about these features is presented here so you as an IT administrator can be aware of these potential capabilities within your organization's network.

If you have a domain, with a DHCP server, ICS isn't a good idea at all. You can only have one DHCP server on any LAN. The DHCP server in ICS will cause problems.

(Edit 10/31/2006): This week, ICS is being used as an attack vector, to disable Windows Firewall. If you're using ICS / WF to share your Internet service, and to protect your LAN, you may want to reconsider your network design. Internet Connection Sharing / Windows Firewall simply isn't good protection.

A far better solution for sharing internet service is to use a NAT router, and to connect the primary computer (otherwise the ICS server), and all secondary computers (otherwise the ICS clients), as peers, to the router.

• The ip address on all interfaces of a router is configurable. You can avoid using subnet 192.168.0.0/24, if desired.
  
• Any computer can be turned off, without affecting internet service to the others.
  
• The former ICS server can have software installed, and can be restarted, at any time, without affecting the other clients.
  
• The router, which will be used only for routing packets, will handle the processing of those packets without straining the resources on any of the client computers.
  
• The router, by blocking malicious web traffic, will lessen the load on the personal firewalls on the client computers.
  
• The router will provide an additional layer of security, which will be unaffected by any web browsing done by the clients.
  
• The router, which will be used only for routing packets, won't be vulnerable, as an ICS server also used for Internet access (as a desktop computer).
  
• Routers for external dialup modems, which will manage the modem without any effect on the former host, are available. Activity on the former server won't affect the dialup service either.
  
• By moving a dialup modem from the computer to the router, the possibility of a modem hijack is eliminated.
  

When you think about it, if you have Ethernet based internet service, or PPP dialup internet service, using a NAT router to manage and share the service makes a lot of sense. If your internet service isn't of either type, for instance a USB connected broadband modem, or non-PPP dialup service, you should consider getting better service.

If you can afford broadband internet service, you can afford a NAT router - you can get a broadband NAT router for as little as $40 at Walmart. If you have dialup service, a dialup router / modem is still affordable. Especially considering the convenience, performance, and security gains.

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