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ADDRESS POOLS
The pool declaration can be used to specify a pool of addresses that
will be treated differently than another pool of addresses, even on the
same network segment or subnet. For example, you may want to provide
a large set of addresses that can be assigned to DHCP clients that are
registered to your DHCP server, while providing a smaller set of
addresses, possibly with short lease times, that are available for
unknown clients. If you have a firewall, you may be able to arrange
for addresses from one pool to be allowed access to the Internet, while
addresses in another pool are not, thus encouraging users to register
their DHCP clients. To do this, you would set up a pair of pool dec-
larations:

subnet 10.0.0.0 netmask 255.255.255.0 {
option routers 10.0.0.254;

# Unknown clients get this pool.
pool {
option domain-name-servers bogus.example.com;
max-lease-time 300;
range 10.0.0.200 10.0.0.253;
allow unknown-clients;
}

# Known clients get this pool.
pool {
option domain-name-servers ns1.example.com, ns2.example.com;
max-lease-time 28800;
range 10.0.0.5 10.0.0.199;
deny unknown-clients;
}
}

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IP ADDRESS CONFLICT PREVENTION
The DHCP server checks IP addresses to see if they are in use before
allocating them to clients. It does this by sending an ICMP Echo
request message to the IP address being allocated. If no ICMP Echo
reply is received within a second, the address is assumed to be free.
This is only done for leases that have been specified in range state-
ments, and only when the lease is thought by the DHCP server to be free
- i.e., the DHCP server or its failover peer has not listed the lease
as in use.