Making Decisions for IP Address Management (Task Map)

As part of the DHCP service setup, you determine several aspects of the IP addresses that the server is to manage. If your network needs more than one DHCP server, you can assign responsibility for some IP addresses to each server. You must decide how to divide responsibility for the addresses. The following table is a task map that describes tasks to manage IP addresses when you use DHCP on the network. The table also contains links to the appropriate sections that detail how to perform each task.

Number and Ranges of IP Addresses

During the initial server configuration, DHCP Manager allows you to add one block, or range, of IP addresses under DHCP management by specifying the total number of addresses and the first address in the block. DHCP Manager adds a list of contiguous addresses from this information. If you have several blocks of noncontiguous addresses, you can add the others by running DHCP Manager's Address Wizard again, after the initial configuration.

Before you configure your IP addresses, know how many addresses are in the initial block of addresses you want to add and the IP address of the first address in the range.

Client Host Name Generation

The dynamic nature of DHCP means that an IP address is not permanently associated with the host name of the system that is using it. The DHCP management tools can generate a client name to associate with each IP address if you select this option. The client names consist of a prefix, or root name, plus a dash and a number assigned by the server. For example, if the root name is charlie, the client names are charlie-1, charlie-2, charlie-3, and so on.

By default, generated client names begin with the name of the DHCP server that manages them. This strategy is useful in environments that have more than one DHCP server because you can quickly see in the DHCP network tables which clients any given DHCP server manages. However, you can change the root name to any name you choose.

Before you configure your IP addresses, decide if you want the DHCP management tools to generate client names, and if so, what root name to use for the names.

The generated client names can be mapped to IP addresses in /etc/inet/hosts, DNS, or NIS+ if you specify to register host names during DHCP configuration. See Client Host Name Registration for more information.

Default Client Configuration Macros

In DHCP, a macro is a collection of network configuration options and their assigned values. The DHCP server uses macros to determine what network configuration information to send to a DHCP client.

When you configure the DHCP server, the management tools gather information from system files and directly from you through prompts or command-line options you specify. With this information, the management tools create the following macros:

• Network address macro – The network address macro is named to match the IP address of the client network. For example, if the network is 192.68.0.0, the network address macro is also named 192.68.0.0. The macro contains information needed by any client that is part of the network, such as subnet mask, network broadcast address, default router or router discovery token, and NIS/NIS+ domain and server if the server uses NIS/NIS+. Other options that are applicable to your network might be included. The network address macro is automatically processed for all clients located on that network, as described in Order of Macro Processing.

• Locale macro – The locale macro is named Locale. The macro contains the offset (in seconds) from Coordinated Universal Time (UTC) to specify the time zone. The locale macro is not automatically processed, but is included in the server macro.

• Server macro – The server macro is named to match the server's host name. For example, if the server is named pineola, the server macro is also named pineola. The server macro contains information about the lease policy, time server, DNS domain, and DNS server, and possibly other information that the configuration program was able to obtain from system files. The server macro includes the locale macro, so the DHCP server processes the locale macro as part of the server macro.

  When you configure IP addresses for the first network, you must select a client configuration macro to be used for all DHCP clients that use the addresses you are configuring. The macro that you select is mapped to the IP addresses. By default, the server macro is selected because the macro contains information needed by all clients that use this server.

Clients receive the options contained in the network address macro before the options in the macro that is mapped to IP addresses. This processing order causes the options in the server macro to take precedence over any conflicting options in the network address macro. See Order of Macro Processing for more information about the order in which macros are processed.

Dynamic and Permanent Lease Types

The lease type determines whether the lease policy applies to the IP addresses you are configuring. During initial server configuration, DHCP Manager allows you to select either dynamic or permanent leases for the addresses you are adding. If you configure the DHCP server with the dhcpconfig command, leases are dynamic.

When an IP address has a dynamic lease, the DHCP server can manage the address. The DHCP server can allocate the IP address to a client, extend the lease time, detect when the address is no longer in use, and reclaim the address. When an IP address has a permanent lease, the DHCP server can only allocate the address. The client then owns the address until explicitly releasing the address. When the address is released, the server can assign the address to another client. The address is not subject to the lease policy as long as the address is configured with a permanent lease type.

When you configure a range of IP addresses, the lease type you select applies to all the addresses in the range. To get the most benefit from DHCP, you should use dynamic leases for most of the addresses. You can later modify individual addresses to make them permanent, if necessary. However, the total number of permanent leases should be kept to a minimum.

Reserved IP Addresses and Lease Type

IP addresses can be reserved by manually assigning them to particular clients. A reserved address can be associated with a permanent lease or a dynamic lease. When a reserved address is assigned a permanent lease, the following statements are true:

• The address can be allocated only to the client that is bound to the address.

• The DHCP server cannot allocate the address to another client.

• The address cannot be reclaimed by the DHCP server.

If a reserved address is assigned a dynamic lease, the address can be allocated only to the client that is bound to the address. However, the client must track lease time and negotiate for a lease extension as if the address were not reserved. This strategy enables you to track when the client is using the address by looking at the network table.

You cannot create reserved addresses for all the IP addresses during the initial configuration. Reserved addresses are intended to be used sparingly for individual addresses.