DHCP server: how can I guarantee the reserved ip even it's also inside the range

[Glenn Satchell]

>Date: Tue, 18 Apr 2006 22:03:12 -0700 (PDT)
>From: Tian Tian <tiantianml at yahoo.com>
>Subject: DHCP server: how can I guarantee the reserved ip even it's also inside 
the range 
>To: dhcp-users at isc.org
>Cc: tiantianml at yahoo.com
>
>Dear Sir/Madam,
>
>I am the ISC DHCP server user. How can I guarantee the
>reserved ip that can be assigned to the specified host
>anytime even the reserved ip is inside the dynamical
>range?
>
>For example,
>
>=========================================
>subnet 10.0.1.0 netmask 255.255.255.0 {
>    default-lease-time 28800;
>    max-lease-time 28800;
>    option subnet-mask 255.255.255.0;
>    option broadcast-address 10.0.1.255;
>    option routers 10.0.1.1;
>    option domain-name-servers 10.0.1.1;
>    option domain-name "";
>    option netbios-node-type 8;
>    option netbios-name-servers 10.0.1.1;
>    range 10.0.1.10 10.0.1.20;
>
>    host test {
>        hardware ethernet 00:12:3F:1F:2C:31;
>        fixed-address     10.0.1.10;
>    }
>}
>=========================================
>
>The fixed-address (10.0.1.10) is reserved for host
>"test" and it is also inside the dynamic range
>(10.0.1.10 - 10.0.1.20). In this case (I need this
>case), the ip 10.0.1.10 could be assigned to a host
>"somebodyelse" before the host "test" come to request
>a ip.
>
>Plese help me! How can I guarantee the reserved ip
>that will not be assigned to "somebodyelse"?
>
>Thanks!
>
>TT

You assumptions about other hosts getting the address are 100%
correct.

You *must* change your range to exclude the reserved IP address.

In your example the range would need to become 

	range 10.0.1.11 10.0.1.20;

If you were reserving say 10.0.1.15, then the range could become

	range 10.0.1.10 10.0.1.14;
	range 10.0.1.16 10.0.1.20;

Sorry but there's no other way.

And on another note, host statements are global in scope so there is no
benefit in putting them inside the subnet declaration. Doing so only
causes possible confusion. The host statements should be moved to th
eglobal scope, eithe rbefore or after the subnet declaration.

regards,
-glenn