Planning a Large-scale Strategy for DHCP Servers

The network administrator can use relay agents implementing RFC 1541 (usually IP routers) so that DHCP servers located on one node of the internetwork can respond to TCP/IP configuration requests from remote nodes. The relay agent forwards requests from local DHCP clients to the DHCP server and subsequently relays responses back to the clients.

Figure 13.2 An Internetwork Using Automatic TCP/IP Configuration with DHCP

The additional planning issues for a large enterprise network includes:

• Compatibility of hardware and software routers with DHCP, as described at the beginning of this chapter.
• Planning the physical subnetting of the network and relative placement of DHCP servers. This includes planning for placement of DHCP (and WINS servers) among subnets in a way that reduces b-node broadcasts across routers.
• Specifying the DHCP option types and their values to be predefined per scope for the DHCP clients. This might include planning for scopes based on the needs of particular groups of users. For example, for a marketing group that uses portable computers docked at different stations, or for a unit that frequently moves computers to different locations, shorter lease durations can be defined for the related scopes. This way, frequently changed IP addresses can be freed for reuse.

As one example, the segmenting of the WAN into logical subnets could match the physical structure of the internetwork. Then one IP subnet can serve as the backbone, and off this backbone each physical subnet would maintain a separate IP subnet address.

In this case, for each subnet a single computer running Windows NT Server could be configured as both the DHCP and WINS server. Each server would administer a defined number of IP addresses with a specific subnet mask, and would also be defined as the default gateway. Because the server is also acting as the WINS server, it can respond to name resolution requests from all systems on its subnet.

These DHCP and WINS servers can in turn be backup servers for each other. The administrator can partition the address pool for each server to provide addresses to remote clients.

There is no limit to the maximum number of clients that can be served by a single DHCP server. However, your network can have practical constraints based on the IP address class and server configuration issues such as disk capacity and CPU speed.