What is the difference between a switch and a router?

A link-layer switch forwards frames within a local-area network based on MAC addresses and is transparent to the network layer, while a router forwards packets between networks based on network-layer (IP) addresses and runs routing protocols. Switches build a single LAN; routers connect different networks together.

Why do we need MAC addresses if we already have IP addresses?

MAC addresses identify a network interface on a local link and are used for delivery over that single hop, while IP addresses identify a host within the global addressing structure and are used for end-to-end routing. The two operate at different layers, and address resolution maps an IP address to the MAC address of the next hop.

Link Layer and Medium Access

The link layer moves frames between directly connected nodes over a single physical link, and the medium access control sublayer governs how multiple nodes share a broadcast channel without their transmissions destructively colliding.

Definition

The link layer is the protocol layer responsible for transferring frames between two nodes connected by a single link, including framing, link access via a medium access control protocol, and error detection on the link.

Scope

This area covers the second layer of the network stack: framing of data into link-layer frames, error detection and correction over noisy links, and the medium access control (MAC) protocols that coordinate access to a shared communication channel. It includes channel-partitioning, random-access, and taking-turns protocols, switched local-area networks, and the Ethernet family with its switches and addressing. It excludes the physical signaling details below it and the network-layer routing that operates across multiple links above it.

Sub-topics

Core questions

How is a stream of bits delimited into frames, and how are transmission errors detected or corrected?
How can many nodes share a single broadcast channel efficiently and fairly?
What are the trade-offs among channel partitioning, random access, and taking-turns MAC protocols?
How do learning switches forward frames in a local-area network, and how do they differ from routers?
Why are link-layer (MAC) addresses needed in addition to network-layer addresses?

Key concepts

framing
error detection (parity, checksum, CRC)
medium access control (MAC)
channel partitioning (TDMA, FDMA)
random access (ALOHA, CSMA/CD)
MAC addresses
Ethernet
link-layer switches
collision and broadcast domains
address resolution (ARP)

Key theories

Random multiple access and CSMA/CD: Random-access protocols let nodes transmit whenever they have data and recover from collisions; carrier-sense multiple access with collision detection (CSMA/CD), used in classic Ethernet, listens before sending and aborts on detecting a collision, then backs off randomly.
Error detection via redundancy: Appending check bits computed from the data — parity, checksums, or cyclic redundancy checks — lets a receiver detect (and sometimes correct) bit errors introduced on a link, trading a small overhead for reliability.
Self-learning switched LANs: Ethernet switches build forwarding tables automatically by observing the source addresses of incoming frames, forwarding selectively rather than broadcasting, which segments collision domains and scales local-area networks.

Clinical relevance

Link-layer technology is what physically connects devices: Ethernet switches form the backbone of enterprise and data-center networks, Wi-Fi's MAC protocol governs every wireless LAN, and error-detection codes protect data on every link from copper to fiber to radio. Designing low-latency data centers, diagnosing collision and broadcast-storm problems, and segmenting networks with VLANs all rest on link-layer concepts.

History

The ALOHA system at the University of Hawaii (Abramson, early 1970s) pioneered random multiple access over a shared radio channel. Metcalfe and Boggs adapted these ideas into Ethernet at Xerox PARC in 1976, using CSMA/CD over a coaxial cable. Ethernet was later standardized as IEEE 802.3 and evolved from shared coax to switched twisted-pair and fiber, displacing collision-based contention with full-duplex switching.

Key figures

Robert Metcalfe
David Boggs
Norman Abramson
Andrew S. Tanenbaum

Seminal works

metcalfe1976
kurose2021
tanenbaum2010

Frequently asked questions

What is the difference between a switch and a router?: A link-layer switch forwards frames within a local-area network based on MAC addresses and is transparent to the network layer, while a router forwards packets between networks based on network-layer (IP) addresses and runs routing protocols. Switches build a single LAN; routers connect different networks together.
Why do we need MAC addresses if we already have IP addresses?: MAC addresses identify a network interface on a local link and are used for delivery over that single hop, while IP addresses identify a host within the global addressing structure and are used for end-to-end routing. The two operate at different layers, and address resolution maps an IP address to the MAC address of the next hop.