How DHCP Works
DHCP is an open standard that is based partly on the BOOTP protocol specified in RFC 951
and RFC 1541. DHCP can be used by Unix, Macintosh, and Windows-based systems. However,
the protocol did not attain mainstream corporate recognition until the service was incorporated
into Windows NT.
Windows 2000 dial-up networking
DHCP enables a host to learn its IP address dynamically. This process is termed a lease
because the address assigned belongs to the host for an administratively defined time. On Windows
implementations, this assignment is set for 72 hours by default.
NOTE:DHCP leases are discussed in the following section.
From a router perspective, DHCP requires one of two components: a DHCP server on the
local subnet or a method of forwarding the broadcast across the router. DHCP lease requests
are broadcasts, so the network designer would need a DHCP server present on each segment in
the network. This clearly would not scale well and is impractical in most network designs, but
it would provide addressing information to the clients.
The alternative is to provide a little help to DHCP. This is accomplished with the IP
helper address, a statically defined address on each router interface that is connected to the
local segment that needs the help. This segment, with the help of the helper address, will be
able to get to the DHCP server. Broadcast requests for addresses are sent to the helper
address as unicasts, thus significantly reducing overall broadcast traffic.
Most DHCP implementations, including Microsoft’s, can provide a great deal of information
to the client as well, including time servers, default gateways, and other addressbased
When using the router as a DHCP server, there is generally less of a motivation to provide
redundancy; whenever more than a handful of networks require addressing services, it is generally
better to add a dedicated server. If the router is unavailable, it is unlikely that users will
be concerned about the loss of a DHCP lease. If there are multiple networks, the likelihood of a single router point of failure is reduced, but there is also an increased load on the router from
the number of leases that must be managed. When designing for DHCP, most architects and
administrators consider the DHCP lease length.
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