Nginx as network edge HTTP cache?

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http://serverfault.com – Looking to implement an office network with peak # users of 40-50 people and clients and I'm thinking that nginx could be used as a transparent proxy to cache requests to any HTTP-based site at the edge of the network. Is this possible? And if so, how? I have tried googling and searching but it seems that all resources seem to point at hosting your own servers and caching requests to those. However, we will also be hosting servers in this network, so nginx will be used to route requests to those as well. So: how do I set up nginx as a transparent proxy for requests to internet servers, in a (HowTos)