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DHCP Address Assignment and Allocation Mechanisms
(Page 3 of 4)
While manual allocation is possible
in DHCP, dynamic allocation is its real raison d'être.
An administrator sets up a pool (usually a range or set of ranges)
of IP addresses that are available for use. Each client that is configured
to use DHCP contacts the server when it needs an IP address. The server
keeps track of which IP addresses are already assigned, and leases
one of the free addresses from the pool to the client. The server decides
the amount of time that the lease will last. When the time expires,
the client must either request permission to keep using the address
(renewing the lease) or must get a new one. This matter of leases and
how they are handled will be the subject of most of the rest of this
Dynamic allocation is the method
used for most client machines in modern DHCP-enabled IP internetworks.
It offers numerous benefits, including the following:
- Automation: Each client can be automatically
assigned an IP address when it is needed with no intervention and no
need for an administrator to manually decide which address goes with
- Centralized Management: All the IP addresses
are managed by the DHCP server. An administrator can easily look to
see which devices are using which addresses and perform other network-wide
- Address Reuse and Sharing: By limiting
the amount of time that each device holds an IP address, the DHCP server
can ensure that the pool of IP addresses is only used by devices actively
using the network. After a period of time, addresses no longer being
used are returned to the pool, allowing other devices to use them. This
allows an internetwork to support a total number of devices larger than
the number of IP addresses available, as long as not all the devices
connect to the internetwork at the same time.
- Portability and Universality: BOOTP (and
DHCP manual allocation) both require that the DHCP server know
the identity of each client that connects to it, so the server can find
the client's assigned address. With dynamic allocation, there are no
predefined allocations, so any client can request an IP address. This
inherently makes dynamic allocation the ideal choice for supporting
mobile devices that travel between networks.
- Conflict Avoidance: Since IP addresses
are all assigned from a pool that is managed by the DHCP server, IP
address conflicts are avoided.
The last point, of course, assumes
that all the clients use DHCP. The administrator must ensure that the
address pool is not used by non-DHCP devices. More about this in a
later topic on DHCP address ranges and address management.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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