When to Use DHCP

Chapter 19's description of DHCP advised using that protocol for configuring computers' network settings if your network uses DHCP—that is, if a DHCP server is available. In turn, the question of whether to run a DHCP server is answered by whether you want to use DHCP to configure most of your computers' networking features. This logic is somewhat circular, though, and the way to break out of the cycle is to consider the network as a whole. Which is better, configuring each computer's IP address and related information individually or setting up an extra server to handle the job?

One of the considerations in determining DHCP's value is the amount of effort invested in administering systems. All other things being equal, the break-even point for setting up a DHCP server is somewhere between half a dozen and a dozen computers. Below that number, it's generally simpler to use static IP addresses. Above that number, the effort invested in DHCP configuration is less than the extra effort of maintaining static IP addresses. Of course, other issues can intervene. Factors that tend to favor DHCP include ordinary users maintaining their computers' network settings, high turnover rates in computers or the OSs installed on them (such as networks with lots of laptops or simply OS reinstallations), the presence of multiboot systems, a network with odd or tricky configurations, and a network dominated by clients that don't need fixed IP addresses. Static IP address assignment is most useful when your network includes many servers that operate best on fixed IP addresses. Some factors can swing either way. For instance, consider a network with a diverse population of OSs. Maintaining such a system with static IP addresses can be tricky because you must know how to assign static IP addresses to each OS, including any quirks each OS has in this respect. DHCP can help simplify this configuration, although perhaps not dramatically—you must still know how to tell each OS to use DHCP, after all. On the down side, specific DHCP clients and servers may have interactive quirks, so you might run into problems configuring some of the more exotic OSs using DHCP. (In my experience, though, Linux's standard DHCP server works without problems with Linux, FreeBSD, Solaris, MacOS X, MacOS Classic, Windows 9x/Me, Windows NT/2000/XP, OS/2, and BeOS DHCP clients. It should work with additional OSs, too, but I haven't tested them.)

Tip If your network includes some systems that must operate with fixed IP addresses and some that don't need fixed addresses, you have three choices. You can assign all IP addresses statically, you can assign some addresses via DHCP and assign others statically, or you can use DHCP for all computers and configure the DHCP server to provide fixed addresses to at least some clients. Mixing DHCP and static IP addresses isn't a problem; you must simply use a range of addresses for fixed IP addresses that DHCP won't try to assign. In fact, the DHCP server itself is likely to be assigned a fixed IP address.

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