Troubleshooting failed Telnet access to a remote device: Which of the following could be the cause? Consider addressing, filtering, cabling, and VTY configuration.

Introduction / Context:
Telnet connection failures can originate from multiple layers of the OSI model and from device configuration. Effective troubleshooting requires checking logical addressing, control-plane filters, physical connectivity, and line (VTY) configuration.

Given Data / Assumptions:

• Potential issues listed: (1) IP addresses are incorrect, (2) an ACL is filtering Telnet, (3) a defective serial cable, (4) the VTY password is missing.
• Remote access is attempted via Telnet.
• Standard Cisco IOS behavior for VTY lines and ACLs.

Concept / Approach:
Telnet requires IP connectivity end to end and an allowed control-plane path to VTY lines with correct authorization. Any single failure among addressing, reachability, filtering, or line authentication can prevent a session from establishing.

Step-by-Step Solution:

Verification / Alternative check:
Use 'ping' and 'traceroute' for reachability, 'show interfaces' for physical health, 'show access-lists' and interface configs for filters, and 'show run | section line vty' to review VTY settings.

Why Other Options Are Wrong:
1 only / 3 only / 2 and 4 only: Each subset ignores other equally plausible causes. Telnet failures are multi-factor; all listed can cause failure.

Common Pitfalls:
Forgetting that line-level restrictions ('transport input') or AAA misconfigurations also block Telnet; assuming L3 pings imply Telnet must work (port 23 may still be filtered).

Final Answer:
All of the above