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The TCP/IP Guide

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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 Client/Server Implementation, Features and Issues

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DHCP Message Relaying and BOOTP Relay Agents
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Pages in Current Topic/Section
12
3
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DHCP Server Conflict Detection
Next Topic/Section

DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA)
(Page 3 of 3)

APIPA Limitations

Bear in mind that APIPA is not a proper replacement for full DHCP. The 169.254.0.0/16 block is a private IP range and comes with all the limitations of private IP addresses, including inability to use these addresses on the Internet. Also, APIPA cannot provide the other configuration parameters that a client may need to get from a DHCP server. Finally, APIPA will not work properly in conjunction with proxy ARP, because the proxy will respond for any of the private addresses, so they will all appear to be used.

Since it uses ARP to check for address conflicts, APIPA is not well-suited for large internetworks. To use it on an internetwork with multiple subnets, you would require software that allows each subnet to use a different portion of the full 169.254.0.0/16 blocks, to avoid conflicts. In practice, APIPA is a solution for small networks; large internetworks deal with the problem of not being able to contact a DHCP server by making sure that a client can always contact a DHCP server. J

Key Concept: An optional DHCP feature called Automatic Private IP Addressing (APIPA) was developed to allow clients to still be able to communicate in the event that they are unable to obtain an IP address from a DHCP server. When enabled, the client chooses a random address from a special reserved block of private IP addresses, and checks to make sure the address is not already in use by another device. It continues to check for a DHCP server periodically until it is able to find one.



Previous Topic/Section
DHCP Message Relaying and BOOTP Relay Agents
Previous Page
Pages in Current Topic/Section
12
3
Next Page
DHCP Server Conflict Detection
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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