Conserve IP addresses on your network
The Internet has grown exponentially throughout the 1990s, doubling in size nearly every 10 months. A result of this growth is that address registration authorities, such as InterNIC Registration Services (http://www.internic.net), have allocated IP address space at an astounding rate. Because IP addresses are 32 bits long, the potential exists for 4 billion unique addresses. Although the Internet hasn't reached that size, address depletion is occurring. A factor in address depletion is IP's use of hierarchical addressing, which means InterNIC allocates addresses per network rather than per host. Thus, InterNIC allocates one Class C address, which as many as 254 hosts can use, to networks that might contain only a few hosts.
One innovation to conserve IP addresses is DHCP (for more information about other IP address conservation methods, see the sidebar "Alternative Address Conservation Schemes," page 118). DHCP is a widely used protocol that systems administrators can use to temporarily assign IP addresses to hosts. This functionality lets systems administrators control address proliferation by allocating an address only to a host that is connected to the local network. Although DHCP offers other capabilities, the protocol is an important tool for conserving network addresses.
Do You Need DHCP?
Many sites avoid DHCP because their administrators see it as another service to run and manage. However, depending on a site's environment, the effort you spend managing DHCP might be much less than the alternative. For example, our company uses one Class C address split between two physical locations. All hosts have a static IP address, and the DNS server lists all the hosts. When our company first connected to the Internet in 1994, many anonymous FTP sites did a reverse lookup on the host IP address to ensure that the host IP address matched the user's email address domain. In time, our company had to allocate an IP address on both of its subnetworks for every portable host, because tracking who was using which address in which office was too much of a hassle. Also, users had to reconfigure their systems when they moved to a different subnetwork. As our company added dial-up and VPN capabilities, it found that the server might allocate three or four addresses to a user (one on each subnet, plus one or two for the VPN connection), even when the user was using only one or two. After determining that running the server was less costly than running out of IP addresses, our company decided to employ DHCP.
An important advantage of DHCP is that host renumbering becomes less of a headache—DHCP reduces the amount of hands-on configuration a systems administrator must perform on host systems; this headache is particularly a problem at small remote offices. In addition, most companies change their ISP every year or two, which requires a company to change its IP network address even if its domain name remains the same. In a DHCP environment, changing IP host addresses is as simple as changing the address parameters on the DHCP server (and changing the host addresses of any systems with static addresses).
So, do you need DHCP? Consider implementing DHCP if your environment includes many remote dial-up users who move from network to network, a large network in which address management at the desktop level is difficult or inconvenient, more potential users than you have addresses, or frequent network changes. However, DHCP isn't the best choice for routers, servers, printers, and other systems that need static addresses; sites that have plenty of available addresses; or sites in which addresses don't frequently change.
DHCP Defined
As defined in Request for Comments (RFC) 2131, DHCP is an Internet standards track protocol that lets a TCP/IP network server provide clients with configuration information such as the default gateway (i.e., router), cookie, time server, DNS server, and WINS addresses; domain name; and subnet mask. Although this article provides a Windows NT spin on this protocol, DHCP is hardware and software platform-independent. DHCP server software is available for most OSs, and most TCP/IP client software implementations support DHCP; furthermore, any DHCP client can communicate with any DHCP server.
Also, DHCP defines a mechanism for IP host address allocation. DHCP supports three allocation mechanisms, and any site will use one or more of these mechanisms depending on its local network administration policies:
- Automatic allocation—The server assigns a permanent IP address to a client.
- Dynamic allocation—The server assigns an IP address to a client for a limited time (i.e., the lease period) or until the client explicitly relinquishes the address.
- Manual allocation—The server conveys to the client an address that the network administrator specifically assigned.
Dynamic allocation is the only mechanism that supports automatic reuse of addresses. This capability is the factor that makes DHCP a useful tool for temporarily assigning addresses to client systems that aren't always connected to a network. Because dynamic allocation is the most commonly employed mode of DHCP, it's the mode we'll discuss in detail.
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