Firewalls are used to provide network security throughout an enterprise and to control what information can be accessed--and to a certain extent…

A traditional network firewall operates at Layers 3 (Network) and 4 (Transport) of the OSI model. Its primary function is to filter traffic based on information in the packet headers. This includes the source and destination IP addresses (Layer 3), the protocol being used such as TCP or UDP (Layer 4), and the source and destination port numbers (Layer 4). A standard firewall is not concerned with the payload of the packet. Therefore, it does not inspect or process encrypted data; it simply sees an encrypted packet and makes a pass/deny decision based on its header information.

A. IP address: Firewalls fundamentally rely on source and destination IP addresses to create access control rules.

C. Port: Port numbers (e.g., 80 for HTTP, 443 for HTTPS) are a critical component of firewall rules for filtering application traffic.

D. Protocol: Firewalls filter traffic based on the protocol type, such as TCP, UDP, or ICMP, as defined in the packet header.

1. National Institute of Standards and Technology (NIST). (2009). NIST Special Publication 800-41 Rev. 1: Guidelines on Firewalls and Firewall Policy. Section 2.1, "Firewall Technologies". (This document details that packet filter firewalls, the most basic type, examine the protocol, source/destination IP addresses, and source/destination ports).

2. Kent, S., & Seo, K. (2005). Security Architecture for the Internet Protocol. RFC 4301. Section 1.1. (This IETF standard, while focused on IPsec, implicitly distinguishes the role of firewalls in managing traffic by header information, separate from the encryption provided by protocols like IPsec).

3. Cheswick, W. R., Bellovin, S. M., & Rubin, A. D. (2003). Firewalls and Internet Security: Repelling the Wily Hacker (2nd ed.). Addison-Wesley Professional. Chapter 4, "Packet Filtering". (This foundational academic text explains that packet filters make decisions based on Layer 3 and 4 headers, not the data/payload within).