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DHCP for Windows 2000 by Neall Alcott

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The DHCP Relay Agent

As noted earlier, the entire DHCP conversation takes place using broadcast messages. When a DHCP client requests an IP address, it sends a broadcast DHCPDISCOVER message to the local subnet. DHCP servers, in turn, respond to the request by broadcasting DHCPOFFER messages. The client receives these messages and accepts one of them. The client then responds with a broadcast DHCPREQUEST message, in which the DHCP server that was selected is identified. The DHCP server, upon receiving the DHCPREQUEST message, allocates the IP address in its database and responds with a DHCPACK message to the client.

So what happens if the network environment is not a single subnet, but a routed environment containing multiple subnets? In a routed environment, separate segments are connected via routers. A router is a hardware component that contains two or more interfaces that connect the router to the multiple physical segments. The router directs traffic between the subnets by examining the destination IP addresses in the packet headers. If a packet is destined for another subnet, the router examines its routing table to determine if a route to that subnet is available. If one is available, the router sends the packet out the interface that is the next hop to the subnet. If a route is not available, the router drops the packet and sends an ICMP message to the sending computer stating that the destination is not available.

Besides routing packets, another role a router plays ...

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