Preparing Your Network for the DHCP Service (Task Map)

Before you set up your network to use DHCP, you must collect information to help you make decisions for configuring one or more servers. Use the task map in the following table to identify the tasks for preparing your network for DHCP. The table lists the tasks, descriptions of what each task accomplishes, and the sections that detail the steps to perform the individual tasks.

Mapping Your Network Topology

If you have not already done so, you should map the physical structure of your network. Indicate the location of routers and clients, and the location of servers that provide network services. This map of your network topology can help you determine which server to use for the DHCP service. The map can also help you determine the configuration information that the DHCP server can provide to clients.

See Chapter 2, Planning Your TCP/IP Network (Tasks) for more information about planning your network.

The DHCP configuration process can gather some network information from the server's system and network files. Updating System Files and Netmask Tables discusses these files. However, you might want to give clients other service information, which you must enter into the server's macros. As you examine your network topology, record the IP addresses of any servers you want your clients to know about. The following servers, for example, might provide services on your network. The DHCP configuration does not discover these servers.

• Time server

• Log server

• Print server

• Install server

• Boot server

• Web proxy server

• Swap server

• X Window font server

• Trivial File Transfer Protocol (TFTP) server

Network Topology to Avoid

In some IP network environments, several local area networks (LANs) share the same network hardware media. The networks may use multiple network hardware interfaces or multiple logical interfaces. DHCP does not work well in this kind of shared media network. When multiple LANs run across the same physical network, a DHCP client's request arrives on all network hardware interfaces. This effect makes the client appear to be attached to all of the IP networks simultaneously.

DHCP must be able to determine the address of a client's network in order to assign an appropriate IP address to the client. If more than one network is present on the hardware media, the server cannot determine the client's network. The server cannot assign an IP address without knowing the network number.

You can use DHCP on only one of the networks. If one network does not suit your DHCP needs, you must reconfigure the networks. You should consider the following suggestions:

• Use a variable length subnet mask (VLSM) on your subnets to make better use of the IP address space you have. You may not need to run multiple networks on the same physical network. See the netmasks(4) man page for information about implementing variable length subnetting. For more detailed information about Classless Inter-Domain Routing (CIDR) and VLSM, see http://www.ietf.org/rfc/rfc1519.txt.

• Configure the ports on your switches to assign devices to different physical LANs. This technique preserves the mapping of one LAN to one IP network, required for DHCP. See the documentation for the switch for information about port configuration.

Determining the Number of DHCP Servers

The data store option that you choose has a direct effect on the number of servers you must have to support your DHCP clients. The following table shows the maximum number of DHCP and BOOTP clients that can be supported by one DHCP server for each data store.

Table 13-1 Estimated Maximum Number of Clients Supported by One DHCP Server

This maximum number is a general guideline, not an absolute number. A DHCP server's client capacity depends greatly on the number of transactions per second that the server must process. Lease times and usage patterns have a significant impact on the transaction rate. For example, suppose leases are set to 12 hours and users turn their systems off at night. If many users turn on their systems at the same time in the morning, the server must handle transaction peaks as many clients request leases simultaneously. The DHCP server can support fewer clients in such an environment. The DHCP server can support more clients in an environment with longer leases, or an environment that consists of constantly connected devices such as cable modems.

The section Choosing the DHCP Data Store compares the types of data stores.

Updating System Files and Netmask Tables

During DHCP configuration, the DHCP tools scan various system files on your server for information that can be used to configure the server.

You must be sure the information in the system files is current before you run DHCP Manager or dhcpconfig to configure your server. If you notice errors after you configure the server, use DHCP Manager or dhtadm to modify the macros on the server.

The following table lists some of the information gathered during DHCP server configuration, and the sources for the information. Be sure this information is set correctly on the server before you configure DHCP on the server. If you make changes to the system files after you configure the server, you should reconfigure the service to reflect these changes.

Table 13-2 Information Used for DHCP Configuration