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Chapter 8

Planning for DHCP Service (Task)

You can use DHCP services in a network you are creating or in a network that exists. If you are setting up a network, see Chapter 3, Planning Your TCP/IP Network (Task) before you attempt to set up DHCP services. If the network exists, continue in this chapter.

This chapter describes what you need to do before you set up DHCP service on your network. The information is intended for use with DHCP Manager, although you can also use the command-line utility dhcpconfig to set up DHCP service.

This chapter contains the following information:

Preparing Your Network for DHCP Service (Task Map)

Before you set up your network to use DHCP, you must first collect information and make decisions about how you will configure the server(s). Use the following task map to identify the tasks for preparing your network for DHCP.




Map your network topology

Determine what services are available on the network and where they are located.

"Mapping Your Network Topology"

Determine the number of DHCP servers you need

Use the expected number of DHCP clients as a basis for determining the number of DHCP servers you need.

"Determining the Number of DHCP Servers"

Update system files and netmasks table

Reflect the network topology accurately.

"Updating System Files and Netmask Tables"

Mapping Your Network Topology

If you have not already done so, you should map the physical structure or layout 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 DHCP services, and what configuration information the DHCP server can provide to clients.

See "Planning Your TCP/IP Network" in System Administration Guide, Volume 3 for more information about planning your network.

The DHCP configuration process can look up 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 are some examples of network services you may have on your network that the DHCP configuration does not discover:

  • Time server

  • Log server

  • Print server

  • Install server

  • Boot server

  • Swap server

  • X Window font server

  • TFTP server

Network Topology to Avoid

DHCP does not work well in network environments where more than one IP network shares the same network hardware media, either through the use of multiple network hardware interfaces or multiple logical interfaces. When multiple IP networks run across the same physical LAN, a DHCP client's request arrives on all network hardware interfaces. This 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 and cannot assign an IP address.

You can use DHCP on one of the networks, but not more than one. If this does not suit your needs, you must reconfigure the networks. Suggestions for reconfiguration include:

  • Use variable length subnet masks (VLSM) to make better use of the IP address space you have, so you do not need to run multiple LANs on the same physical network. See RFC-1519 for more information on VLSM and Classless Inter-Domain Routing (CDIR).

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

Determining the Number of DHCP Servers

The data store option 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/BOOTP clients that can be supported by one DHCP server for each data store.

Table 8-1 Estimated Maximum Number of Clients

Data Store

Maximum Number of Clients

Text files




Binary files


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

The section "Choosing the Data Store" compares data store options.

Updating System Files and Netmask Tables

During the configuration process, DHCP Manager or the dhcpconfig utility scans various system files on your server for information it can use 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 it. If you make changes to the system files after you configure the server, you should reconfigure the service to pick up the changes.

Table 8-2 Information for DHCP Configuration




Time zone

System date, time zone settings

The date and time zone are initially set during the Solaris installation. You can change the date by using the date command and change the time zone by editing the /etc/TIMEZONE file, which sets the TZ variable.

DNS parameters


The DHCP server uses the /etc/resolv.conf file to look up DNS parameters such as DNS domain name and DNS server addresses. See System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP) for more information about resolv.conf.

NIS or NIS+ parameters

System domain name, nsswitch.conf, NIS, NIS+

The DHCP server uses the domainname command to obtain the domain name of the server system, and the nsswitch.conf file to determine where to look for domain-based information. If the server system is a NIS or NIS+ client, the DHCP server queries NIS or NIS+ services to get NIS/NIS+ server IP addresses.

Default router

System routing tables, user prompt

The DHCP server searches the network routing tables to find the default router for clients attached to the local network. For clients not on the same network, the DHCP server must prompt the administrator for the information.

Subnet mask

Network interface, netmasks table

The DHCP server looks to its own network interfaces to determine the netmask and broadcast address for local clients. If the request had been forwarded by a relay agent, the server looks up the subnet mask in the netmasks table on the relay agent's network.

Broadcast address

Network interface, netmasks table

For the local network, the DHCP server obtains the broadcast address by querying the network interface. For remote networks, the server uses the BOOTP relay agent's IP address and the remote network's netmask to calculate the broadcast address for the network.

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