The field of network security is a very dynamic, and highly technical field dealing with all aspects of scanning, hacking and securing systems against intrusions. There are many positions related to this, however the most common would be that of a Network Security Engineer, a Security Research Analyst, a Penetration tester, and an Incident Investigator. All four of these jobs have salaries starting in the forty to fifty thousand annually range, and can grow well past the one-hundred thousand mark, depending on skill and experience.
A Network Security Engineer is responsible for end-user support for general security related issues, as well as enforcement of the local security policy. They are also typically responsible for performing ongoing security scans to ensure that the System Administrators are keeping their systems up-to-date and secure.
A Security Research Analyst is someone who performs research in the field of network security to devise new weaknesses and vulnerabilities, as well as new methods of intercepting and preventing system compromises. They will also typically develop applications to employ the data they have gathered.
Another position, the Penetration Tester, is someone who scans networks and/or hosts for vulnerabilities, and then reports on these vulnerabilities to show how they could break into a computer system or network. This information helps the Network Security Engineers to keep their systems secured.
The last position outlined, is that of the Incident Investigator. This is someone who investigates a system forensically, or after an intrusion, to help trace the attacker, and isolate how the system was compromised. They will also work with the ISP of the attacker to help further track their source. Also, they will report on this information to help find the most commonly exploited weaknesses, so that System Administrators can patch the most critical holes.