At which Layer would you check ACLs for TCP ports being blocked?

The correct answer is D. Layer 4. ACLs filtering TCP ports operate at Layer 4 (Transport Layer), because TCP is a Transport Layer protocol - port numbers like 80 (HTTP), 443 (HTTPS), and 22 (SSH) only exist at this layer, so any port-based blocking must be evaluated here. Layer 1 (Physical) deals with…

Implement Industrial Cyber Security

Question

Options

ALayer 1
BLayer 2
CLayer 3
DLayer 4

How the community answered

(27 responses)

A
11% (3)
B
4% (1)
C
4% (1)
D
81% (22)

Explanation

ACLs filtering TCP ports operate at Layer 4 (Transport Layer), because TCP is a Transport Layer protocol - port numbers like 80 (HTTP), 443 (HTTPS), and 22 (SSH) only exist at this layer, so any port-based blocking must be evaluated here.

Layer 1 (Physical) deals with electrical signals and cables - it has no concept of addresses, protocols, or ports.
Layer 2 (Data Link) handles MAC addresses and frame switching - still no awareness of IP or TCP.
Layer 3 (Network) works with IP addresses, so ACLs here can block by source/destination IP but cannot inspect TCP port numbers.

Memory tip: "TCP lives at Layer 4ourteen" - or simply remember that the T in TCP stands for Transport, and Transport = Layer 4. If the ACL rule mentions a port number, you're at Layer 4.

Topics

#OSI Model#ACLs#Transport Layer#TCP Ports

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