Identifying TCP/IP Model Layers — From a Mixed List Given the list Application, Session, Transport, Internet, Data Link, Physical, which items are actual layers of the TCP/IP model (considering the canonical 4-layer/5-layer views)?

Introduction / Context:
Different curricula present the TCP/IP stack as either 4 layers (Application, Transport, Internet, Network Access) or a 5-layer teaching model (Application, Transport, Internet, Data Link, Physical). Distinguishing these from the OSI model prevents layer confusion during troubleshooting and design.

Given Data / Assumptions:

• The mixed list contains OSI and TCP/IP terms: Application, Session, Transport, Internet, Data Link, Physical.
• We accept the 5-layer teaching view where Data Link and Physical appear separately.
• We must select only actual TCP/IP layers from the list.

Concept / Approach:
TCP/IP layers include Application, Transport, Internet, and (in the 4-layer model) Network Access; in a 5-layer view the bottom layer is split into Data Link and Physical. "Session" belongs to OSI and is not represented as a distinct layer in TCP/IP; its functions are handled within the TCP/IP Application layer and by protocols like TCP for sessions.

Step-by-Step Solution:
From the list, mark Application (1) as valid.Mark Transport (3) as valid.Mark Internet (4) as valid.Do not mark Session (2) because it is an OSI layer.Data Link and Physical can be TCP/IP lower layers (5-layer pedagogy), but the correct option provided here that matches the standard trio is 1, 3 and 4.

Verification / Alternative check:
Compare common textbooks: TCP/IP (Application, Transport, Internet, Network Access). In 5-layer mapping, Network Access is split into Data Link and Physical, but the answer set targets the classic trio from the provided choices.

Why Other Options Are Wrong:

• 1 and 2: Includes Session, which is not a TCP/IP layer.
• 2, 3 and 5 / 3, 4 and 5: Either include Session or omit Application.
• 4 and 6 only: Nonsensical indexing; Physical as "6" is incorrect in the context.

Common Pitfalls:
Mixing OSI Session/Presentation with TCP/IP Application; in TCP/IP those functions are collapsed.

Final Answer:
1, 3 and 4