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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)
                9  TCP/IP Dynamic Host Configuration Protocol (DHCP)
                     9  DHCP Address Assignment and Dynamic Address Allocation and Management

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DHCP Leases, Lease Length Policies and Management
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DHCP Address Assignment and Allocation Mechanisms
(Page 4 of 4)

Automatic Allocation

The third option, automatic allocation, can be used in cases where there are enough IP addresses for each device that may connect to the network, but where devices don't really care what IP address they use. Once an address is assigned to a client, that device will keep using it. Automatic allocation can be considered a “special case” of dynamic allocation: it is essentially dynamic allocation where the time limit on the use of the IP address by a client (the lease length) is “forever”.

In practice, automatic allocation is not used nearly as much as dynamic allocation, for a simple reason: automatically assigning an IP address to a device permanently is a risky move. Most administrators feel it is better to use manual allocation for the limited number of machines that really need a permanent IP address assignment, and dynamic addressing for others. We'll discuss this more in the next topic.

Key Concept: DHCP defines three basic mechanisms for address assignment. Dynamic allocation is the method most often used, and works by having each client lease an address from a DHCP server for a period of time; the server chooses the address dynamically from a shared address pool. Automatic allocation is like dynamic allocation but the address is assigned permanently instead of being leased. Manual allocation pre-assigns an address to a specific device, just as BOOTP does, and is normally used only for servers and other permanent, important hosts.



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