Limited Or No Connectivity

With XP SP2, Microsoft wants you to be aware when your computer, although configured for automatic address assignment, does not in fact get service from a DHCP server. Your computer, and maybe one or more other computers, will have (KB220874): APIPA addresses.

This is simply a new message - it is not a new problem, and the APIPA address is a symptom of the problem, not the problem itself. You have to solve the problem, not the symptom. Manually assigning an IP address, subnet mask, etc won't solve anything.

There are 2 possible reasons for not getting DHCP service.

• No connectivity.
  
• Connectivity, but no DHCP server.
  

No Connectivity
If your computer has no connectivity, whether you leave the APIPA address, or use a manually assigned address, you will gain nothing. You will have to diagnose and fix the physical connectivity problem.

A case of LSP / Winsock corruption can cause No Connectivity, so if you can't easily find a physical connectivity problem, check that next.

Connectivity, but no DHCP server
If your computer has limited connectivity, it has an APIPA address, and it may have connectivity to other computers on the local network. APIPA addresses don't pass thru routers though, so you'll at best have connectivity only with other computers, also with APIPA addreses, and also on your local network.

There is one case where this is not at all a problem. If you have 2 or more computers - either 2 computers connected directly with a cross-over cable, or 2 or more computers connected thru a hub or switch, you may have a LAN with no DHCP server, and no gateway.

In this case, each computer will self-assign an IP address, per APIPA design. If your only need to connect the computers is to let them share files with each other, then you're fine.

If you have a gateway on your LAN, and intend for the computers to communicate outside the LAN, however, you have a problem. Manually assigning normal addresses, such as 192.168.n.n, to match the rest of the computers on the LAN, will accomplsh nothing.

If you manually assign an IP address that will communicate with the outside world, you'll have to do this for every computer on your network with an APIPA address. You'll be better off finding and fixing the problem.

Make sure that the DHCP Client service is running - Started and Automatic.

If the client computer is running Windows Vista, check for a couple connectivity issues which are unique to that operating system.

If your DHCP Client is running, and none of the above help, the problem is probably not with your computer. Either you have no device on your local network that can provide DHCP services, or the DHCP server is ignoring the requests from your computer. The latter condition can either be caused by a MAC address filter, or by an exhausted DHCP scope (all available IP addresses having been issued).

Do you have a DHCP server on the LAN, or are your computers behind a NAT router? If either is the case, check the log on the server, or the router, for clues.

More Analysis
If none of the above scenarios apply to you, or if you can't work as above, then continue by asking for help for basic Internet Connectivity. Or troubleshoot the Internet Connectivity problem yourself, methodically.

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Have Laptop Will Travel?

When you use a computer at the office, the office network provides several features that are transparent to you. When you use a portable computer there, then disconnect it from the office network, and carry it to a remote site, or maybe to your home network, there are issues which you need to consider.

• IP Configuration
  
• Name / Address Resolution
  
• Windows Networking (My Network Places / Network Neighborhood)
  
• Domain / Workgroup Login and Authentication
  
• Possible Solutions for Complex Situations
  
• Two Network Connections On One LAN
  

IP Configuration
If you carry your computer from site to site, and connect to the networks at each site, this will work quite easily, as long as there is a DHCP server at each site to issue you an IP address, and DNS settings.

• Open the Properties applet for your network connection (wired or wireless).
  
• Under TCP/IP Properties, on the General tab, select "Obtain an IP address automatically" and "Obtain DNS server addresses automatically".
  

What if one site doesn't have a DHCP server? If just one site lacks a DHCP server, then you're in luck. Windows XP has provided for this situation.

If, as above, you select "Obtain an IP address automatically", you will then have an Alternate tab, where you provide settings for your computer to use when a DHCP server isn't available. The default setting here is to use (KB220874): APIPA, a self-assigned dynamic address, which should allow you to communicate with other computers, on any LAN, that are also using APIPA assigned addresses.

You can, if you wish, override the APIPA selection, and manually assign fixed IP address and DNS settings. These settings, like the APIPA selection, will be used when you're connected without using DHCP to provide configuration.

For more information, see Microsoft (KB283676): How to use the Alternate Configuration feature for multiple network connectivity in Windows XP, and Automatic Configuration for Multiple Networks.

If your travel involves more than one site without DHCP, you may need a more complex solution.

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Name / Address Resolution
If your computer is primarily used on a domain with a WINS, or DNS, server, it will probably be setup to use that server in name / address resolution. If taken to another network that has a different WINS or DNS server, or no server at all, name resolution may be a problem.

The worst scenario, in this case, would be a Peer-Peer node type configured on the computer, on a LAN with no WINS or DNS server. Here, you will have no resolution at all. NetBT based broadcast, which is essential on a LAN with no server, won't even be tried.

Other settings, besides Peer-Peer aka P-Node, may be problematic when used in a home workgroup. A Hybrid, aka H-Node, setting will require the computer to always attempt name resolution first from a server, then by broadcast. You'll notice slow response when locating resources. And a Mixed, aka M-Node, setting will take a long time when you try a name that doesn't exist.

See Address Resolution on the LAN, for more detail about the Node Type.

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Windows Networking (My Network Places / Network Neighborhood)
When you're connected to the office domain, the domain browser sets up the contents of Network Neighborhood for you. When you're connected to another domain, or to a workgroup, you won't necessarily see other computers directly in Network Neighborhood. You'll need to look in Entire Network - Microsoft Windows Network - (name of domain or workgroup).

Don't count on providing access to your computer thru Network Neighborhood. Most likely, other computers won't be able to see your computer, at all. You'd have to start the browser on the laptop, for the laptop to be displayed anywhere in Network Neighborhood on the other computers. You could cause problems on the domain later, if you carry your computer back to the office and connect it to the domain, with the browser started.

As pointed out below, please DO NOT join your laptop to the workgroup, without knowing the consequences you will face when you get back to the office.

Also note that both Name / Address Resolution and Browsing generally require NetBIOS Over TCP (NetBT), which is not used in many large networks, for efficiency and security.

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Domain / Workgroup Login and Authentication
If you login to a domain at the office, you're using a domain controller to authenticate your login, and possibly to provide authorisation for access to various resources there. When you're away from the domain, you'll need to login locally to your computer. Be sure that you have a local account on your computer that you can use.

Resources at other locations may not be so transparently accessed when you're logged in locally on your computer. You may have to check the setup of your laptop, and the other networks, if you're going to access Windows resources elsewhere.

But please do everybody a favour and DO NOT drop your computer from the domain, hoping to casually rejoin the domain, the next time you're in the office. Joining a computer to a domain involves setting up a two way trust, between the computer, and the domain. This is a significant security issue, and can't (shouldn't) be done casually.

Rejoining the domain will involve explaining to your overworked network admin why you have to do this again. And then there will be paperwork, and extra security tasks, on the domain controller. Don't put the two (or more) of you through that.

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Possible Solutions for Complex Situations
What if you travel to multiple sites where there's no DHCP server? Or what if there are sites with domains, and you need the ability to move between domains?

In this case, you'll need extra software, that remembers the appropriate settings for each different site. Three such products are:

• Mobile Net Switch.
  
• MultiNetwork Manager.
  
• NetSwitcher.
  

You will need an experienced and authoritative support person, at each location, to help you run the above software. Joining a computer to a domain involves setting up a two way trust, between the computer, and the domain. This is a significant security issue, and can't (shouldn't) be done by just anybody. Be sure that you have the permission of onsite technical support personnel, if any are available. And be sure that you, or the person transporting the portable computer, has authority to make the changes to that computer when necessary.

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Two Connections To The Same LAN
If you have a portable computer, and you've setup a WiFi LAN in your house or office, you'll enjoy the freedom of moving around, at will, while still connected to the LAN. Even so, sometimes there will be times when the WiFi connection isn't enough. You'll never get rid of Ethernet, completely.

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