Advantages of DHCP

DHCP servers offer a number of advantages over earlier methods of getting IP addresses. Here are the features a DHCP server can offer.

1. Automatic management of IP addresses, including the prevention of duplicate IP address problems

2. Allows support for BOOTP clients, so you can easily transition your networks from BOOTP to DHCP

3. Allows the administrator to set lease times, even on manually allocated IP addresses.

4. Allows limiting which MAC addresses are served with dynamic IP addresses

5. Allows the administrator to configure additional DHCP option types, over and above what is possible with BOOTP

6. Allows the definition of the pool or pools of IP addresses that can be allocated dynamically. A user might have a server that forces the pool to be a whole subnet or network. The server should not force such a pool to consist of contiguous IP addresses.

7. Allows the association of two or more dynamic IP address pools on separate IP networks (or subnets). This is the basic support for secondary networks. It allows a router to act as a BOOTP relay for an interface which has more than one IP network or subnet IP address.

Here are some features that are not part of the DHCP server itself, but related to the way it is administered.

1. Central administration of multiple servers

2. The ability to make changes while the server is running and leases are being tracked. For example, you can add or take away IP addresses from a pool, or you can modify parameters.

3. The ability to make global modifications (those that apply to all entries) to parameters, or to make modifications to groups of clients or pools

4. The maintenance of a lease audit trail, such as a log of the leases granted

DHCP supports four strategies for IP address allocation. These are independent features. A particular server can offer any or none of them.

• Manual. The unique client identifier-to-IP address binding has been made by an administrator. Therefore the DHCP service should not reallocate IP addresses of this type to other clients after the lease expires. This type of IP address allocation is useful when the administrator wants a host to maintain the same IP address but still wants to detect when an IP address is no longer being used. An example is a host that provides a service located by the IP address, like mail.

• Permanent. The server's administrator creates a configuration for the server that includes only IP addresses, and gives this configuration to clients. After an IP address is associated with a MAC address, the association is permanent unless the server's administrator intervenes. Allocating permanent IP addresses has the drawback that such IP addresses cannot be reclaimed automatically.

• Dynamic (through leases with limited duration). The server tracks leases and gives IP addresses to DHCP clients automatically as they become available when leases expire. No interaction is needed by the administrator. This is the preferred IP address type for non-BOOTP clients.

• BOOTP. Addresses that are reserved for use by BOOTP clients. This allows an administrator to enter a pool of IP addresses intended only for BOOTP clients.