CISSP-ISSAP · Question #176
The simplest form of a firewall is a packet filtering firewall. Typically a router works as a packet- filtering firewall and has the capability to filter on some of the contents of packets. On which…
The correct answer is A. Transport layer D. Network layer. Packet-filtering routers operate at the Network layer (Layer 3) and Transport layer (Layer 4) of the OSI model. At Layer 3, they inspect IP addresses (source and destination) to make routing and filtering decisions. At Layer 4, they examine port numbers and protocol types…
Question
The simplest form of a firewall is a packet filtering firewall. Typically a router works as a packet- filtering firewall and has the capability to filter on some of the contents of packets. On which of the following layers of the OSI reference model do these routers filter information? Each correct answer represents a complete solution. Choose all that apply.
Options
- ATransport layer
- BPhysical layer
- CData Link layer
- DNetwork layer
How the community answered
(43 responses)- A84% (36)
- B12% (5)
- C5% (2)
Explanation
Packet-filtering routers operate at the Network layer (Layer 3) and Transport layer (Layer 4) of the OSI model. At Layer 3, they inspect IP addresses (source and destination) to make routing and filtering decisions. At Layer 4, they examine port numbers and protocol types (TCP/UDP) - giving them the ability to block specific services like HTTP (port 80) or SSH (port 22).
Why B and C are wrong: The Physical layer (Layer 1) deals only with raw bit transmission over physical media - no logical addressing exists there to filter on. The Data Link layer (Layer 2) handles MAC addresses and frame delivery within a local network segment; routers don't filter based on MAC addresses since those change at each hop.
Memory tip: Think "Network + Transport = IP address + Port number." A router's job is routing (Layer 3 = Network), and filtering by port number moves up one layer to Transport (Layer 4). If a firewall can block 192.168.1.1:80, it's working at both layers - N and T = No Traffic without rules.
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