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DHCP Address Assignment and Allocation Mechanisms
(Page 1 of 4)
The two main functions of the Dynamic
Host Configuration Protocol are to provide a mechanism for assigning
addresses to hosts, and a method by which clients can request addresses
and other configuration data from servers. Both functions are based
on the ones implemented in DHCP's predecessor, BOOTP, but the changes
are much more significant in the area of address assignment than they
are in communication. It makes sense to start our look at DHCP here,
since this will naturally lead us into a detailed discussion of defining
characteristic of DHCP: dynamic addressing.
DHCP Address Allocation Mechanisms
Providing an IP address to a client
is the most fundamental configuration task performed by a host configuration
protocol. To provide flexibility for configuring addresses on different
types of clients, the DHCP standard includes three different address
- Manual Allocation: A particular IP address
is pre-allocated to a single device by an administrator. DHCP only communicates
the IP address to the device.
- Automatic Allocation: DHCP automatically
assigns an IP address permanently to a device, selecting it from a pool
of available addresses.
- Dynamic Allocation: DHCP assigns an IP
address from a pool of addresses for a limited period of time chosen
by the server, or until the client tells the DHCP server that it no
longer needs the address.
I don't really care for the names
automatic and dynamic allocation, because they
don't do a good job of clearly conveying the differences between these
methods. Both can be considered automatic because in each
the DHCP server assigns an address with no administrator intervention
required. The real difference between them is only in how long the IP
address is retained, and therefore, whether a host's address varies
over time. I think better names would be static/permanent automatic
allocation and dynamic/temporary automatic allocation.
But then, nobody really cares much what I think. J
Regardless of what you call them,
all three of these methods exist for configuring IP hosts using DHCP.
It is not necessary for an administrator to choose one over the others.
Instead, he or she will normally combine the methods, using each for
the devices where it makes the most sense.
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Version 3.0 - Version Date: September 20, 2005
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