Question about DHCP and Ping Packets

Hi,

I have some problems with understanding what Cisco means when said:

==========
  Configuring the Number of Ping Packets
By default, the DHCP Server pings a pool address twice before
assigning a particular address to a requesting client. If the ping is
unanswered, the DHCP Server assumes (with a high probability) that
the address is not in use and assigns the address to the requesting
client. To change the number of ping packets the DHCP Server should
send to the pool address before assigning the address, use the
following command in global configuration mode:

Router(config)# ip dhcp ping packets number
==========

How can the router send icmp packet if he doesn't know host MAC address?
So, the router just will send ARP requests?
When the router receive ARP replay he already knows, that a
particular address is in use. For knowing that he don't need to send
ICMP.

Or, maybe, this for situation when client try to release address.
Then the router knows the MAC address and can try to send icmp request.
But, guide states "assigning a particular address", not reassigning.

Why don't just say ARP?

And, as I understand, this also means that I should permit icmp
requests from and replay to my DHCP server (to router or switch).
Or, if I don't, the clients will loose their addresses.
(I know that I can disable this feature ( ip dhcp ping packets number
0))

Thanks

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Nataliya Samoylenko
Blogs and organic groups at http://www.ccie.net