SSC CGL 2025 Tier-II · Paper-I, 19 January 2026 · Computer Knowledge Test · Question 3 of 150

We transcribed this text from the pictures of a candidate's response sheet for this paper; the answer is the key SSC marked with a green tick on that sheet. Solution last reviewed 11 September 2026.

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SSC does not publish CGL question papers or a public answer key. This paper reaches us as a candidate's response sheet saved as pictures, so we transcribed the text from those pictures, and a second reader checked every question against them; where the layout carries meaning, we show the picture too. Each answer is the key SSC marked with a green tick on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. UnlockIAS wrote the solutions.

In the OSI model, which layer is responsible for routing packets across different networks using logical addressing?

(a)Data Link Layer
(b)Session Layer
(c)Network Layer
(d)Transport Layer

Answer

Why

Correct — C. Two phrases in the stem point to the same layer.

Routing: ITU-T X.200, the OSI reference model, lists routing and relaying as Network Layer functions, and routing finds a route between network-addresses.

Logical addressing: those network-addresses are provided by the Network Layer and can be independent of the addresses the lower layers use.

Routing across networks by logical address → Network Layer, option (c).

Why the others are wrong

(a)
Data Link Layer — The Data Link Layer sends frames between physical addresses and detects errors from the Physical Layer. Routing by logical (network) address belongs to the layer above it.
(b)
Session Layer — The Session Layer organizes and synchronizes the dialogue between two communicating ends: opening a session, keeping the exchange orderly, closing it. It plays no part in choosing a packet's path.
(d)
Transport Layer — The Transport Layer provides end-to-end transfer between the two end systems. X.200 says it is relieved of any concern with routing and relaying, because the Network Layer below handles them.

Concept

The OSI reference model (ISO/IEC 7498-1, also published as ITU-T X.200) divides networking into seven layers: Physical, Data Link, Network, Transport, Session, Presentation, Application. Each layer offers services to the layer above and hides how it provides them.

The Network Layer carries data from one end system to another, even across several subnetworks in tandem. It identifies each end system by a network-address, and its routing functions choose a route between those addresses, so the layers above never deal with routing.

X.200 also allows routing and relaying inside the Data Link Layer in some local-network setups, so "routing" alone does not decide the item. Logical addressing does: network-addresses belong to the Network Layer.

Key facts

OSI layers, bottom to top: Physical, Data Link, Network, Transport, Session, Presentation, Application.
Network Layer: routing and relaying between network-addresses.
Data Link Layer: frames between physical addresses, with error detection.
Transport Layer: end-to-end transfer between end systems, leaving routing to the Network Layer.

Study next

Physical (MAC) addresses against logical (IP) addressesWhere IP and TCP sit when the TCP/IP stack is mapped onto OSIEach OSI layer named by its function rather than its number

Common traps

Choosing the Transport Layer because it works end to end, when X.200 says it leaves routing to the Network Layer.
Choosing the Data Link Layer because it also uses addresses, when those are physical, not logical.

The item describes a layer by its job rather than its number, so learn each OSI layer as a function: routing plus logical addressing means the Network Layer.

Related PYQs

No directly related past PYQ was found.

Topic: Basics of networking and cyber security — all 6 SSC CGL questions on it, every solved shift.