dhcp classes help

[Tom Greaser]

Im wanting to setup a match for the vendor info
to allow voip phones and ata to work but deny any pc that
may get into those networks from picking up an ip

Im new to classes / subclasses / matching so i thought i would shoot this 
to the group and see if anyone would direct me into a better way of doing thing
(not afraid to pickup a book / man dhcp* )


group {

## voip phones

option domain-name-servers ;
authoritative;
option ntp-servers ;
option log-servers ;
option option-150 ;
option tftp-server-name "";
dhcp-renewal-time 86400; ## 1day
default-lease-time 2592000;  ##1 month
max-lease-time 2592000;   ##1 month
ddns-updates off;
boot-unknown-clients false;

class "voip-phones" {

match if (substring (option vendor-class-identifier, 0, 31) = "Cisco Systems, Inc. IP Phone CP" ) or
	 (substring (option vendor-class-identifier, 0, 10) = "ATA188-H17");


subnet 10.183.50.0 netmask 255.255.255.0 {
   allow members of "voip-phones";
        range 10.183.50.4 10.183.50.254;
        option routers 10.183.50.1;
   }  ## end subnet

50 other subnet statements later


    } ## end class
} ## end group