Why DHCP?

For a networked installation, the workstation needs network connectivity. With normal Jumpstart, it broadcasts a RARP request and the boot server responds with an IP address. Unfortunately, RARP requests cannot be sent across a routed network. They are isolated within the broadcast domain of a network segment (for more information, see the excellent book by W. Richard Stevens, TCP/IP Illustrated, Volume 1).

Some routers can be configured to act as RARP servers, and therefore give the client an IP address. However, this will not work either, because the client to be jumpstarted will request the kernel image from the router.

DHCP is a protocol that clients can use to request an IP address, as well as other network-related information. With OpenBootProm (OBP) version 3.25, Sun added DHCP support for booting over the network.

Like RARP, the DHCP protocol uses LAN broadcasts and normally wouldn't be able to cross a subnet boundary. However, modern routing equipment usually allows the systems administrator to specifically configure the router to pass DHCP requests from one subnet to a DHCP server on some other network (for Cisco IOS routers, "ip helper <addr>" where <addr> is the IP address of the remote DHCP server). In this way, you can limit the number of DHCP servers you need to deploy to a reasonably small and easy-to-manage group, and still take advantage of Sun's Jumpstart facility.