update different subnet in different zone

[Simon Hobson]

kalyanasundaram S wrote:
>yah well thanks,
>  that works  fine.
>  I was confusing that while allocating the address how the dhcp 
>server updates the corresponding zone.
>So just a clarrification
>
>   if server allocates IP from one subnet it looks for 
>"ddns-domainname" and update the corresponding zone
>   if not found go to higher level (or global) and again look for 
>ddns-domainname option.
>   If none found no update will be made?..
>
>am i correct?
>where all i can have this ddns-domainname option (inside => 
>subnet,pool,group,sharednetwork,host) in all these places  are 
>possible to have ddns-domainname option?

Recommended reading : "The DHCP Handbook" by Ralph Droms and Ted 
Lemon, Macmillan Technical Publishing, ISBN 1-57870-137-6.
All this (and much more) is well explained.

But in short, and omitting a few details for clarity, all options 
follow a scoping heirarchy. You can define an option in several 
scopes :

Global
Shared Network
Subnet
Pool

Group
Host

The server evaluates any declarations from global downwards and will 
use the most specific value found. So you can define a global value 
that applies to everything unless overridden, then you can redefine 
the value for a specific Subnet, and redefine it for a specific pool, 
and for a specific host.

ddns-domainname is a good example. You may well define a common value 
(eg mydomain.com) as a global option. Then in one subnet you could 
redefine it (eg sales.mydomain.com). You could create a group of host 
declarations and have a different value for those hosts (eg 
finance.mydomain.com).

Simon