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The data link layer is Layer 2 of the OSI model. It packages network-layer data into frames and manages how those frames are addressed and sent across a directly connected link. Ethernet is one familiar example; frame formats and link rules vary by protocol.
What does the data link layer do?
Layer 2 sits between the network layer above it and the physical layer below it. It provides the link-specific rules for transferring data between devices on the same link or LAN segment. It does not decide how to route traffic across an entire internetwork; that is a network-layer function.
The network layer passes data down to the data link layer, which encapsulates it in a link-layer protocol data unit, commonly called a frame. The physical layer then transmits the frame’s bits over the medium. A frame’s exact fields and behavior depend on the link protocol, so Ethernet is an example rather than a universal format.
How are frames, MAC addresses, and IP addresses different?
In Ethernet, a frame includes source and destination MAC addresses. Those addresses support delivery on the local Ethernet link. An IP address is a network-layer address used for communication across networks; it is not interchangeable with a MAC address.
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| Comparison | Data link layer | Network layer |
|---|---|---|
| OSI position | Layer 2 | Layer 3 |
| Common data unit | Frame | Packet |
| Address example | MAC address | IP address |
| Typical scope | A directly connected link or local segment | Forwarding between networks |
These are useful distinctions, not a claim that every protocol uses identical terminology or formats. For Ethernet specifically, see Cisco’s overview of Ethernet and IEEE’s 802.3-2022 catalog description.
What are the LLC and MAC sublayers?
In IEEE 802 LAN architecture, the data link layer is described as two sublayers:
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- Logical Link Control (LLC): The upper sublayer, providing a logical interface toward the network layer and identifying or supporting network-layer protocols.
- Media Access Control (MAC): The sublayer associated with the particular access method, including medium-specific access functions.
This LLC/MAC division describes IEEE 802 architecture; it is not a universal naming scheme for every data-link protocol. IEEE’s catalog describes LLC as the upper part of the IEEE 802 data link layer. The IEEE 802.2 catalog entries for the 1985 and 1989 editions are historical: the former is marked superseded, and the latter withdrawn. The IEEE 802.2 working-group page says the standard was withdrawn in 2010 and points to ISO/IEC 8802-2 as stabilized.
Cisco’s explanation of Logical Link Control describes different LLC service types. Their functions depend on the protocol and service chosen.
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Does Layer 2 guarantee reliable delivery?
No—not in every network. Depending on the link protocol and service, data-link functions may include acknowledgements, sequencing, flow control, or recovery. Other services provide best-effort, connectionless transfer. It is therefore more accurate to say that Layer 2 handles link-specific delivery rules than to claim it always guarantees delivery.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do switches and routers relate to Layer 2?
Bridges and Layer 2 switches use link-layer information to forward frames within a LAN. Routers make network-layer forwarding decisions to move traffic between networks. A switch’s role in forwarding local frames is different from a router’s role in forwarding between IP networks; both may be present in the same network.
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For a practical treatment of Ethernet frames and Layer 2 switching, Cisco Press offers a sample chapter from Introduction to Networks Companion Guide: “Ethernet Frames”.
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