Determine domain and IP address space
Domain Names are the human readable names used to represent one or more IP addresses. IP addresses are the unique identifier of computing devices on a network. Both pieces of information are valuable to an adversary who is looking to understand the structure of a network. 
Detectable by Common Defenses (Yes/No/Partial): No
Explanation: Public or easily obtainable information by design.
Difficulty for the Adversary
Easy for the Adversary (Yes/No): Yes
Explanation: AS and IANA data are easily available, existing research tools.
- Rotem Kerner. (2015, October). RECONNAISSANCE: A Walkthrough of the “APT” Intelligence Gathering Process. Retrieved March 1, 2017.