A Windows XP-based Laptop for the Office and Home: Working with Fixed IP Addresses
Published: June 17, 2002
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I've been talking about how to make an easy transition between your office and home networks in Managing One Laptop for the Office and Home and Troubleshooting Home Network Issues. This column continues that discussion and is prompted by a question from Adrian Arambul, a reader in Anaheim, California:
Could you discuss how to manage one laptop for the office and home when that laptop uses a fixed IP address and belongs to a domain?
Your friendly IT department configures your laptop to work successfully with their network. When you connect the laptop to your home network, your job is to make sure you don't interfere with what they've done. And if you do make changes to your network connections settings, make sure that you know how to reverse them. This is important when you start making manual changes to the IP address. One way to ensure enforcement of non-personal use requirements for that laptop is to make lots of calls to the Help desk to fix settings you've changed.
I covered how to deal with a domain-based computer on a home network in Troubleshooting Home Network Issues. Simply give your home workgroup the same name as the work domain and they should see each other fine. And even when they don't, they can connect to each other using the IP address of the other computer. But how do you deal with a fixed IP address, when you can't use the new Alternate Connection in Windows XP and that IP address isn't in the same range of addresses as your home network? There are really only two choices:
| • | Change your home network to be in the same range of addresses as your laptop. |
| • | Change your laptop, temporarily, to be in the range of your home network. |
Changing Your Home Network IP Range
Changing the range of addresses your home network uses is a nuisance, but has the advantage of being something you do once and don't have to do again. Unfortunately, you can only do this if your work laptop's IP address is in one of the private ranges, and you can configure your Internet connection (cable modem, DSL router, or an Internet gateway) to use the same set of addresses for its internal connection. The chances are this isn't an option for a cable modem or a DSL router, but if it is, this is the best way to go.
If you have the sort of home network that uses a full firewall/proxy server, such as Microsoft Internet Security and Acceleration Server, or an Internet gateway that lets you control the set of ranges to use, you can set the internal address range of your gateway or firewall to line up with the range your laptop is already on, and use that same range for your home network. See the documentation for your gateway or firewall for instructions on how to change the IP address range.
The private ranges are:
10.0.0.0 — 10.255.255.255
169.254.0.0 — 169.254.255.255
172.16.0.0 — 172.31.255.255
192.168.0.0 — 192.168.255.255
If your work laptop falls in one of these ranges, and you've got full control over your cable modem, DSL router, or use a firewall or gateway that gives you control over IP addresses, you can happily change your home network to be in the same range as your laptop. The easiest way to do this is to pay attention to the last section of digits in the laptop IP address, and increase it by one for each additional machine on your home network. For example, if your laptop has an IP address of 192.168.0.10, then set your two home machines to 192.168.0.11 and 192.168.0.12, and set your cable modem, DSL router, or Internet gateway to have the IP address that your laptop uses for a default gateway.
Configuring the Laptop to Match Your Home Network
Unfortunately, many of us don't have the sort of Internet account that allows us to completely control the internal IP addresses of our home network. The range of addresses used internally is controlled by the settings on your cable modem or by a DSL modem/router. However, if you have an Internet gateways, such as the Linksys BEFSR41, D-Link DI-704, or a whole raft of others, you can make the transition from work to home relatively painlessly. These gateways act as DHCP servers, handing out private IP addresses to all the machines on your home network, while hiding the ugly details from your cable or DSL provider. Some include a built-in four to eight port hub or switch, giving you all you need to connect your home network except for the cables. They can be found with four switched ports at a variety of outlets for under $100. Having one of these can make life much easier on a home network.
Since the gateways can act as a DHCP server, it's fairly easy to reconfigure your laptop to work with them—just change it to use DHCP when you're at home. Then specify your current fixed IP address and settings using the Alternate Configuration feature. (See Managing One Laptop for the Office and Home.) Now when you're on the network at work where you don't have DHCP, it will use the alternate address. When you come back home, it will pick up a DHCP address from the Internet gateway and automatically be in the same IP address range as the other computers on your home network.
Note: Before you make any changes to your laptop's TCP/IP settings, write the current ones down. Then you'll have all the settings you need to reconfigure your TCP/IP settings without having to call the help desk when it's time to change them back. Your IT team will appreciate your consideration, and you'll save yourself time and annoyance getting reconfigured for work. I like to write them down and keep one set at home, another in my desk at work, and a third in a text file on my desktop.
To change the IP address on a computer running Windows XP
1. | Open Network Connections. |
2. | Right-click the network card you want to change, and then click Properties. |
3. | On the Networking tab, click Internet Protocol (TCP/IP). |
4. | Click Properties to open the Internet Protocol (TCP/IP) Properties dialog box.  Figure 1 |
5. | To use DHCP to automatically assign an address, click Obtain an IP address automatically. Also set the DNS to obtain an address automatically. |
6. | If you're setting up to use a manual address, click Use the following IP address and type the necessary information. You'll also need to know the correct DNS server address for your Internet service provider (or work network, if you're reconfiguring it to run at work). |
That's really all there is to it—you can manage your laptop between work and home with the minimum of pain by making careful configuration changes. You can't change the settings at work, so you need to make your home network as comfortable with those work settings as you can, by setting up your home workgroup to have the same name as your work domain. Adding a low cost Internet gateway to your home network gives you the ability use DHCP at home, getting around the fixed IP address of work and giving you much more flexibility in your home network as well. Adding another machine to the home network is now easy and doesn't require any additional settings to your Internet connection—just plug it in to the port on your gateway and set it to obtain its IP address automatically and you're done.
Send in Your Questions
If you use Windows XP Professional at work, and you have a topic you'd like to see me cover in one of my columns, feel free to write me at Charlie@mvps.org. Please understand that I can not answer individual e-mail messages or even acknowledge them. But I will be happy to consider your topic request for a future column.
