Proceedings of the 16th Australian Information Warfare Conference (pp. 14-23), held on the 30 November - 2 December, 2015 Cyber resilience is characterised by an ability to understand and adapt to changing network conditions, including cyber attacks. Cyber resilience may be characterised by an effects-based approach to missions or processes. One of the fundamental preconditions underpinning cyber resilience is an accurate representation of current network and machine states and what missions they are supporting. This research outlines the need for an ontological network representation, drawing on existing literature and implementations in the domain. This work then introduces an open-source ontological representation for modelling cyber assets for the purposes of Computer Network Defence. This representation encompasses computers, network connectivity, users, software, vulnerabilities and exploits and aims for interoperability with related representations in common use. The utility of this work is highlighted against a functional use-case depicting a realistic operational network and mission. Finally, a future research direction is defined.