A slave-and-forwarder configuration is useful when you do not want all the servers at a given site to interact with the rest of the Internet servers. The stations might be administratively prohibited from having Internet access. To give the stations the appearance of access to the Internet domain system, the stations could be slave servers to the forwarding server on the gateway station. The gateway server would forward the queries and interact with other name servers on the Internet to resolve each query before returning the answer. A benefit of using the forwarding feature is that the central station develops a more complete cache of information, which all the stations can take advantage of. The use of slave mode and forwarding is discussed further in "Setting Up a BIND Configuration".
There are two main reasons to use forwarders. First, if your station does not have full network access, it cannot send IP packets to the rest of the network. Therefore, it must rely on a forwarder with access to the network. Second, the forwarder can see all queries as they pass through the server and, therefore, builds up a more complete cache of data than the cache in a typical station name server. In effect, the forwarder becomes a meta-cache from which stations can benefit, thereby reducing the total number of queries from that site to the rest of the network.