?– Manual allocation: A client IP address is assigned by the network administrator, and
DHCP is used simply to convey the assigned address to the client.
Dynamic allocation is the only one of the three mechanisms that allows automatic reuse of
an address that is no longer needed by the client to which it was assigned. Dynamic
allocation is particularly useful for assigning an address to a client that will be connected
to the network only temporarily, or for sharing a limited pool of IP addresses among a group
of clients that do not need permanent IP addresses. Dynamic allocation can also be a good
choice for assigning an IP address to a new client being permanently connected to a
network in which IP addresses are suf?¬?ciently scarce that it is important to reclaim them
when old clients are retired.
DHCPDISCOVER
When a DHCP client boots up for the ?¬?rst time, it transmits a DHCPDISCOVER message
on its local physical subnet. Because the client has no way of knowing the subnet to which
it belongs, the DHCPDISCOVER is an all-subnets (all-hosts) broadcast (destination IP
address of 255.255.255.255). The client does not have a con?¬?gured IP address; therefore,
the source IP address of 0.0.0.0 is used.
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