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As the number of World Wide Web users grows, so does the number of connections made to servers. This increases both network load and server load. Caching can reduce both loads by migrating copies of server files closer to the clients that use those files. Caching can either be done at a client or in the network (by a proxy server or gateway). We assess the potential of proxy servers to cache documents retrieved with the HTTP, GOPHER, FTP, and WAIS protocols using World Wide Web browsers. We monitored traffic corresponding to three types of educational workloads over a one-semester period, and used this as input to a cache simulation. Our main findings are (1) that with our workloads a proxy has a 30--50% maximum possible hit rate no matter how it is designed; (2) that when the cache is full and a document is replaced, classic least recently used (LRU) is a poor policy, but simple variations can dramatically improve hit rate and reduce cache size; (3) that a proxy server really functions as a second-level cache, and its hit rate may tend to decline with time after initial loading, given a more or less constant set of users; and (4) that certain modifications to proxy-server configuration parameters for a cache may have little benefit.