DHCP Dynamic Host Configuration Protocol
Resource: Windows 2000 Server Resource kit TCP/IP Core Networking Guide

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Preventing Address Conflicts

برای جلوگیری کردن از دادن IP یکسان به دو Client توسط DHCP Server می توان در بخش Conflict detection attempts بجای عدد 0 مقدار 1 و یا بیشتر داد.

اگر این مقدار را برای مثال 2 قرار دهید یک DHCP Server قبل از دادن یک IP Address به یک Client آن آدرس را در شبکه Ping می کند جستجو می کند که آن IP در شبکه نباشد

و تا 2 بار منتظر جواب Ping می باشد که مطمئن شود که IP در شبکه فعال نمی باشد سپس آن را Lease می کند .تذکر مقدار بیش از 2 می تواند عمل IP Lease را در شبکه های

که بزرگ می باشد بسیار زمان بر کند در نتیجه مقدار بیش از 2 نباشد بهتر می باشد .

 Figure 1: DHCP Server Properties - Advanced Tab

  Figure 1: DHCP Server Properties - Advanced Tab


Windows 2000 has both server-side and client-side conflict detection to prevent duplicate IP addresses on your network.

Server Conflict Detection
The DHCP server detects conflicts by pinging an IP address before offering that address to clients. If the ping is successful (a response is received from a computer), a conflict is registered and that address is not offered to clients requesting a lease from the server. The DHCP server pings only addresses that have not been successfully and previously leased. If a client receives a lease on an IP address that it already had or is requesting a renewal, the DHCP server does not send a ping.

If conflict detection is enabled, an administrator-defined number of pings are sent. The server waits 1 second for a reply. Because the time required for a client to obtain a lease is equal to the number of pings selected, choose this value carefully as it directly impacts the overall performance of the server. In general, one ping should be sufficient.

A DHCP server receiving a reply to any of the pings (meaning there is a conflict) attaches a BAD_ADDRESS value to that IP address in the scope, and will try to lease the next available address. If the duplicate address is removed from the network, the BAD_ADDRESS value attached to the IP address can be deleted from the scope's list of active leases, and the address returned to the pool. Addresses are marked as BAD_ADDRESS for the length of the lease for which the scope is configured.

If your network includes legacy DHCP clients, enable conflict detection on the DHCP server. By default, the DHCP service does not perform any conflict detection. In general, conflict detection should be used only as a troubleshooting aid when you suspect there are duplicate IP addresses in use on your network. The reason for this is that, for each additional conflict detection attempt that the DHCP service performs, additional seconds are added to time needed to negotiate leases for DHCP clients.

Client Conflict Detection
Windows 2000 or 98-based client computers also check to determine if an address is already in use before completing address configuration with the DHCP server. If the client detects a conflict, it sends a DHCPDecline message to the DHCP server. The DHCP server attaches a BAD_ADDRESS value to the IP address in the scope, as detailed in "Server-Side Conflict Detection." The client begins the lease process again, and is offered the next available address in the scope.

For networks that include clients that are not running Windows 2000 or 98, server-side conflict detection should be enabled.

DHCP Dynamic Host Configuration Protocol


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