Introduction

A firewall is a part of a computer system or network that is designed to block unauthorized access while permitting authorized communications. It is a device or set of devices configured to permit, deny, encrypt, decrypt, or proxy all (in and out) computer traffic between different security domains based upon a set of rules and other criteria.

References

1. http://en.wikipedia.org/wiki/Firewall
2. https://wiki.cse.buffalo.edu/services/content/firewall-systems