  Security metrics present the security level of a system or a network in both
qualitative and quantitative ways. In general, security metrics are used to
assess the security level of a system and to achieve security goals. There are
a lot of security metrics for security analysis, but there is no systematic
classification of security metrics that are based on network reachability
information. To address this, we propose a systematic classification of
existing security metrics based on network reachability information. Mainly, we
classify the security metrics into host-based and network-based metrics. The
host-based metrics are classified into metrics ``without probability" and "with
probability", while the network-based metrics are classified into "path-based"
and "non-path based". Finally, we present and describe an approach to develop
composite security metrics and it's calculations using a Hierarchical Attack
Representation Model (HARM) via an example network. Our novel classification of
security metrics provides a new methodology to assess the security of a system.
