Duplicate IP Address Detection
Duplicate address detection is an important feature. When the stack is first initialized or when a new IP address is added, gratuitous ARP requests are broadcast for the IP addresses of the local host. The number of ARPs to send is controlled by the ArpRetryCount registry parameter, which defaults to 3. If another host replies to any of these ARPs, the IP address is already in use. When this happens, the Windows-based computer still boots; however, the interface containing the offending address is disabled, a system log entry is generated, and an error message is displayed. If the host that is defending the address is also a Windows-based computer, a system log entry is generated, and an error message is displayed on that computer. In order to repair the damage possibly done to the ARP caches on other computers, the offending computer re-broadcasts another ARP, restoring the original values in the ARP caches of the other computers.
A computer using a duplicate IP address can be started when it is not attached to the network, in which case no conflict would be detected. However, if it is then plugged into the network, the first time that it sends an ARP request for another IP address, any Windows NT–based computer with a conflicting address detects the conflict. The computer detecting the conflict displays an error message and logs a detailed event in the system log. A sample event log entry is shown below:
The system detected an address conflict for IP address 220.127.116.11 with the system having network hardware address 00:DD:01:0F:7A:B5. Network operations on this system may be disrupted as a result.
DHCP-enabled clients inform the DHCP server when an IP address conflict is detected and, instead of invalidating the stack, they request a new address from the DHCP server and request that the server flag the conflicting address as bad. This capability is commonly known as DHCP Decline support.
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