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DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA)
(Page 2 of 3)
APIPA Operation
APIPA is really so simple that it's
surprising it took so long for someone to come up with the idea. It
takes over at the point where any DHCP lease process fails. Instead
of just halting with an error message, APIPA randomly chooses an address
within the aforementioned private addressing block. It then performs
a test very similar to the one in step #13 in the
DHCP allocation process: it uses ARP to
generate a request on the local network to see if any other client responds
using the address it has chosen. If there is a reply, APIPA tries another
random address and repeats the test. When the APIPA software finds an
address that is not in use, it is given to the client as a default address.
The client will then use default values for other configuration parameters
that it would normally receive from the DHCP server. This process is
illustrated in Figure 269.
Figure 269: DHCP Automatic Private IP Addressing (APIPA) In this example, Client #1 is trying to get an IP address from its DHCP server, but the server is out of addresses, so it does not respond to the client’s requests. The client is configured to use APIPA, so it randomly selects an address from the APIPA address block. It sends an ARP Request on the local network to see if any other device is using that address; in this case, Usually there will be no conflict, but here Client #2 is in fact using the address, so it responds. Client #1 chooses a different address and this time gets no reply. It begins using that address, while continuing to check regularly for a DHCP server to come online.
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A client using an autoconfigured
address will continue to try to contact a DHCP server periodically.
By default, this check is performed every five minutes. If and when
it finds one, it will obtain a lease and replace the autoconfigured
address with the proper leased address.
APIPA is ideally suited to small
networks, where all devices are on a single physical link. Conceivably,
with 20 APIPA-enabled DHCP clients on a network with a single DHCP server,
you could take the server down for maintenance and still have all the
clients work properly, using 169.254.x.x addresses.
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