Network forensics is the part of Network security. Moreover, network forensics is the post activity of network attacks.

Network security is a state or property of ensuring confidentiality, integrity, availability and so on of information and all network resources and services. Network forensics is an activity of collecting, storing, analyzing and so on of evidence of a network attack.

As described in the Wikipedia:

Forensic science is the scientific method of gathering and examining information about the past. Especially this is important in law enforcement where forensics is done in relation to criminal or civil law, [1] but are forensics Also Carried Out in other fields, such as astronomy, archeology, biology and geology to Investigate ancient times.

Network security maintenance (as continuos ensuring, supporting network security) includes information security management and thus network forensics activity as its important part.

Network forensics deals with the digital evidence of network resources breaking-in

Network Security is to defence the networks and when it will be failed to secure their networks then Network Forensics comes to finds the details of failure security and collect the all evidences in this regards which will presents in Law court .