Why can't machines in different CIDR blocks access the DNS server ???

Why can't machines in different CIDR blocks access the DNS server ??? -- Linux Enterprise Application-Linux server application information. For details, refer to the following section. [I = s] This post was last edited by snowaveg

I. Environment
1. UBUNTU SERVER8.10, server address: 172.16.10.100
2. The entire network is divided into different network segments through layer-3 switching: 172.16.10.xxx \ 172.16.20.xxx \ 172.16.30.xxx \~~~~~~.
3. bind9 is configured as a pure forwarding mode.
Ii. symptom
1. Under the same network segment (172.16.10.xxx), set the DNS server address to 172.16.10.100 to access the webpage;
2. Different network segments, such as 172.16.20.xxx and 172.16.30.xxx, cannot be resolved normally. You can ping 172.16.10.100 to 172.16.10.100.
Iii. Related Configuration
1. name. conf
Include "/etc/bind/named. conf. options ";
// Prime the server with knowledge of the root servers
Zone "."{
Type hint;
File "/etc/bind/db. root ";
};
// Be authoritative for the localhost forward and reverse zones, and
// Broadcast zones as per RFC 1912.
Zone "localhost "{
Type master;
File "/etc/bind/db. local ";
};
Zone "127. in-addr.arpa "{
Type master;
File "/etc/bind/db.127 ";
};
Zone "0. in-addr.arpa "{
Type master;
File "/etc/bind/db.0 ";
};
Zone "255. in-addr.arpa "{
Type master;
File "/etc/bind/db.255 ";
};
Include "/etc/bind/named. conf. local ";

2. named. conf. local
//
// Do any local configuration here
//
// Consider adding the 1918 zones here, if they are not used in your
// Organization
// Include "/etc/bind/zones. rfc1918 ";

3. named. conf. options
Options {
Directory "/var/cache/bind ";
// If there is a firewall between you and nameservers you want
// To talk to, you might need to uncomment the query-source
// Directive below. Previous versions of BIND always asked
// Questions using port 53, but BIND 8.1 and later use an unprivileged
// Port by default.
// Query-source address * port 53;
// If your ISP provided one or more IP addresses for stable
// Nameservers, you probably want to use them as forwarders.
// Uncomment the following block, and insert the addresses replacing
// The all-0's placeholder.
Forwarders {
168.95.1.1;
139.175.10.20;
221.136.0.1;
};

Auth-nxdomain no; # conform to RFC1035
Listen-on-v6 {any ;};
Listen-on port 53 {127.0.0.1; 172.16.10.100 ;}