Switched Local Area Networks

A switched local area network connects hosts within a limited area through link-layer switches that forward frames based on MAC addresses, learning the network topology automatically and segmenting it into separate collision domains.

Definition

A switched local area network is a local-area network in which link-layer switches interconnect hosts and forward frames selectively based on destination MAC addresses, automatically learning which addresses are reachable through which ports.

Scope

This topic covers the operation of switched LANs: link-layer (MAC) addressing, the Address Resolution Protocol that maps IP addresses to MAC addresses on a local link, how learning switches build forwarding tables by observing frames, how they segment collision domains while preserving a single broadcast domain, and virtual LANs (VLANs) that partition a physical switch into logical networks. It excludes the specific Ethernet frame format and switching hardware, treated in the dedicated Ethernet topic, and the cross-network routing of the network layer.

Core questions

• How are link-layer (MAC) addresses structured, and how do they differ from IP addresses?
• How does the Address Resolution Protocol map an IP address to a MAC address on a local link?
• How does a switch learn the location of hosts and build its forwarding table?
• How do switches segment collision domains while keeping a single broadcast domain?
• What are virtual LANs (VLANs), and why are they used?

Key concepts

• MAC (link-layer) addresses
• Address Resolution Protocol (ARP)
• link-layer switches
• switch forwarding tables
• self-learning
• collision domains and broadcast domains
• virtual LANs (VLANs)
• spanning tree protocol

Key theories

Self-learning, transparent switching

An Ethernet switch records the source MAC address and incoming port of each frame, building a forwarding table that lets it send subsequent frames only out the port toward the destination; this learning is automatic and transparent to hosts.

Address resolution

Because hosts know each other's IP addresses but link delivery needs MAC addresses, the Address Resolution Protocol broadcasts a query for the MAC address corresponding to a local IP address and caches the reply, bridging the network and link layers.

Collision-domain segmentation and VLANs

Switches confine collisions to individual links and can be logically partitioned into virtual LANs, isolating broadcast traffic and grouping hosts independently of physical wiring for security and manageability.

Clinical relevance

Switched LANs are the everyday building block of enterprise, campus, and data-center networks. Self-learning switches make plug-and-play local networking possible, VLANs let one physical infrastructure serve many logical networks for isolation and security, and understanding ARP and broadcast domains is essential for diagnosing connectivity problems, broadcast storms, and address conflicts.

History

Early LANs used shared media where all hosts shared one collision domain. Transparent bridges, and Radia Perlman's spanning tree protocol to prevent loops among them, allowed multiple segments to be joined safely. The Address Resolution Protocol (RFC 826, 1982) linked IP to Ethernet addressing. Bridges evolved into high-port-density switches, turning LANs into fully switched, full-duplex networks with VLAN support.

Key figures

• Radia Perlman
• David C. Plummer
• Larry L. Peterson

Seminal works

• kurose2021
• rfc826

Frequently asked questions

How does a switch know where to send a frame?

A switch learns by watching traffic: when a frame arrives, it records the source MAC address and the port it came in on. Over time it builds a table mapping addresses to ports, so it can forward a frame only out the port toward its destination, flooding to all ports only when the destination is still unknown.

What is a VLAN?

A virtual LAN partitions a physical switch (or set of switches) into multiple logical broadcast domains. Hosts in different VLANs behave as if they were on separate switches, which improves security and isolation and lets administrators group devices logically rather than by where they are physically plugged in.