Isc-dhcp option82

I have a question about correct isc-dhcp configuration.
I want to lease ip addresses to users based on switch port. For this I use DLink DES-3200 series switches. Everything works well, but recently I've decided to lease particular subnet to all unknown user, i.e. not explicitly specified in dhcpd.conf file.
Here is a config example:
# dhcpd.conf

default-lease-time 30;
max-lease-time 60;
authoritative;
log-facility local7;
option domain-name-servers 8.8.8.8;

include "/usr/local/etc/dhcpd/dhcpd.classes";

shared-network "clients"
{
    subnet 10.5.20.0 netmask 255.255.255.0 {}
    include "/usr/local/etc/dhcpd/dhcpd.networks";
}

dhcpd.classes

class "10.5.20.4_2" { match if ( substring(option agent.remote-id,2,15)="10.5.20.4" and binary-to-ascii(10, 16, "",  substring(option agent.circuit-id, 4, 2)) = "2" ); }
class "10.5.20.4_1" { match if ( substring(option agent.remote-id,2,15)="10.5.20.4" and binary-to-ascii(10, 16, "",  substring(option agent.circuit-id, 4, 2)) = "1" ); }
class "10.5.20.2_1" { match if ( substring(option agent.remote-id,2,15)="10.5.20.2" and     binary-to-ascii(10, 16, "",  substring(option agent.circuit-id, 4, 2)) = "1" ); }
class "10.5.20.2_3" { match if ( substring(option agent.remote-id,2,15)="10.5.20.2" and binary-to-ascii(10, 16, "",  substring(option agent.circuit-id, 4, 2)) = "3" ); }
class "10.5.20.2_2" { match if ( substring(option agent.remote-id,2,15)="10.5.20.2" and binary-to-ascii(10, 16, "",  substring(option agent.circuit-id, 4, 2)) = "2" ); }
class "10.5.20.2_4" { match if ( substring(option agent.remote-id,2,15)="10.5.20.2" and binary-to-ascii(10, 16, "",  substring(option agent.circuit-id, 4, 2)) = "4" ); }

dhcpd.networks

subnet 172.30.20.0 netmask 255.255.255.0
{
   option subnet-mask 255.255.255.0;
   option routers 172.30.20.1;
    pool {range 172.30.20.3; allow members of "10.5.20.4_2"; }
    pool {range 172.30.20.2; allow members of "10.5.20.4_1"; }
}
subnet 172.30.160.0 netmask 255.255.255.0
{
   option subnet-mask 255.255.255.0;
   option routers 172.30.160.1;
    pool {range 172.30.160.3; allow members of "10.5.20.2_1"; }
    pool {range 172.30.160.4; allow members of "10.5.20.2_3"; }
    pool {range 172.30.160.10; allow members of "10.5.20.2_2"; }
    pool {range 172.30.160.12; allow members of "10.5.20.2_4"; }
}

So if add add let's say:

subnet 172.20.111.0  netmask 255.255.255.0 {
                        option routers 172.20.111.1;
                        max-lease-time 60;
                        min-lease-time 30;
                        range 172.20.111.10  172.20.111.20 ;
                        }

at the end of dhcpd.networks file (which I include into shared-network 'clients' clause, see above), all my clients start getting ip addresses from 172.20.111.0 range, regardless if they have a class specified for their port.

Is there a way to make dhcpd server first look at class declarations and then subnet ?