1. Initial DHCP discovery

A basic DHCP exchange will on the local subnet appear as follows:

   UDP 0.0.0.0:68 (00:00:39:1C:0C:32) > 
       255.255.255.255:67 (FF:FF:FF:FF:FF:FF) DHCP DISCOVER

At this point the client has no IP address and so uses a source address of 0.0.0.0 and source port of 68 (often referred to as bootpc, the BOOTP client port - BOOTP being the forerunner of DHCP). The packet is sent as a UDP broadcast on port 67 (bootps). Normally this packet will be seen by any host on the local subnet.

Because the OUCS DHCP Service has two servers acting for many different subnets, the packets must be passed from the local subnet to the servers at OUCS. This is done at the router, which will forward any broadcast packet received on an interface to the central DHCP servers (163.1.2.2 and 129.67.1.2: any change to their IP addresses will be announced in advance on the itss-announce mailing list). The forwarding is performed as unicast packets from the router to each server in turn.

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