| route add 0.0.0.0 MASK 0.0.0.0
Thus, packets destined for non-local subnets are transmitted directly onto the local media instead of being sent to a router. In essence, the local interface card can be designated as the default gateway. This can be useful where several class C networks are used on one physical network with no router to the outside world, or in a proxy-ARP environment.
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 ARP requests to send is controlled by the ArpRetryCount registry parameter described in Appendix A, which defaults to 3. If another host replies to any of these ARP requests, the IP address is already in use. When this happens for an interface with a single manually-configured address, 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 update the ARP caches on other computers, the offending computer re-broadcasts another ARP request, spoofing the MAC address of the defending computer, to restore the proper 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 computer running a version of Windows with the Windows NT codebase 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 10.199.40.123 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 with a DHCPDecline message when an IP address conflict is detected and, instead of disabling the TCP/IP protocol, they request a new address from the DHCP server and request that the server mark the conflicting address as bad.
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