4. Chicken / Egg Issues

How can the server send an IP datagram to the client, if the client doesnt know its own IP address (yet)? Whenever a bootreply is being sent, the transmitting machine performs the following operations:

1. If the client knows its own IP address ('ciaddr' field is nonzero), then the IP can be sent 'as normal', since the client will respond to ARPs [5].

2. If the client does not yet know its IP address (ciaddr zero), then the client cannot respond to ARPs sent by the transmitter of the bootreply. There are two options:
   1. If the transmitter has the necessary kernel or driver hooks to 'manually' construct an ARP address cache entry, then it can fill in an entry using the 'chaddr' and 'yiaddr' fields. Of course, this entry should have a timeout on it, just like any other entry made by the normal ARP code itself. The transmitter of the bootreply can then simply send the bootreply to the client's IP address. UNIX (4.2 BSD) has this capability.

   2. If the transmitter lacks these kernel hooks, it can simply send the bootreply to the IP broadcast address on the appropriate interface. This is only one additional broadcast over the previous case.