Monoculture in a computer network is defined as executing the same protocols and software on all the networking devices. Multiple, correlated failures may be triggered in a monoculture network by attacks that exploit existing vulnerabilities in either protocols, software, or hardware. In this paper, optimal multiculture network design is used to improve the network reliability in the presence of correlated attacks. The multiculture network design is formalized using two sequential optimization problems: The first one for optimally selecting the number of network devices from each available vendor at the design stage, and the second one for optimally placing the devices in the network topology. Results show the capability of multiculture design in improving network reliability of eight actual topologies. Keywords-reliability, exploit, correlated failures, diversity.FONDECYTFONDECYT