Due to the rising importance of visualization for teaching purposes, several network protocol visualization applications and applets are flowing on the Internet. So far, no taxonomy has been developed to classify this rather broad material. In this paper, we propose a taxonomy of network protocol visualization resources, to be used either by software visualization researchers to classify their tools or by educators to easily determine if a particular visualization actually satisfies their teaching needs. The taxonomy (which has been obtained by analyzing stand-alone applications and applets available on the Internet) is mainly intended to integrate previously existing taxonomies of algorithm visualization tools by mostly considering some characteristic features which are specific of the considered subject of network protocols.