The client subnet data can include the representative CIDR block 240 which is used to make queries on behalf of all clients associated with the equivalence class 4. The representative CIDR block 240 includes a source address and either a netmask or a prefix length. As already mentioned above, the representative CIDR block 240 can be chosen arbitrarily, for example, the representative CIDR block 240 may be the least of client addresses in the equivalence class. Once the answer is received from the authority 250, the answer is stored to the cache 230 associated with the equivalency class 4, and served to the client.

In some example embodiments, cache 230 can be flushed if it is determined that the client subnet data have changed. Additionally, the answer in the cache 230 can expire after a predetermined period of time. This predetermined period of time can vary but, in generally, is smaller for more popular destinations.

FIG. 3 is a block diagram illustrating components of a query and an answer, according to some example embodiments. As mentioned above, if the answer is not found in cache 230, the recursive server (also referred to as a resolver) can query authority 250. As shown in the figure, a query 310 can include a domain name and EDNS0 options 320. The EDNS0 options 320 can be added by the recursive server associated with ISP 210 may. The EDNS0 options 320 can include the representative CIDR block. The representative CIDR block 240 includes a source address, and either a netmask or a prefix length. The authority 230 may respond with an answer 330. The answer may include a network address corresponding to the query.