A LAN (local area network) is a computer network that connects devices within a limited area, such as a home, an office floor, a building or a campus, under the control of a single owner or organization. In networking fundamentals, the LAN is the basic building block: it delivers high bandwidth and sub-millisecond latency between nearby devices and connects to wider networks through a router.
Today, almost every LAN is built on Ethernet (IEEE 802.3) for wired connections and Wi-Fi (IEEE 802.11) for wireless access, both of which identify devices by MAC address. What makes a network local is not its technology alone, but the combination of limited geographic scope, private ownership and a shared Layer 2 domain.
How a LAN forwards traffic
A LAN delivers frames between devices at Layer 2 of the OSI model, using MAC addresses rather than IP addresses to find the destination within the network. The central device is an Ethernet switch, which learns which MAC addresses are reachable through each of its ports.
When a frame arrives, the switch records the sender’s MAC address and port in its MAC address table. If the destination is known, the frame goes out only on the matching port; if it is unknown, the switch floods it to all ports except the incoming one. Broadcast frames always reach every device in the same broadcast domain.
For IP communication, a device uses ARP (or Neighbor Discovery in IPv6) to translate the IP address of a local destination into its MAC address. Traffic for any address outside the LAN is sent to the default gateway, the router that connects the LAN to other networks.
The table below shows the main components of a typical LAN and the layer at which each works.
| Component | OSI layer | Function |
|---|---|---|
| Network interface | Layers 1 and 2 | Connects a device and carries its MAC address |
| Switch | Layer 2 | Forwards frames between ports based on MAC addresses |
| Wireless access point | Layer 2 | Bridges Wi-Fi clients into the wired LAN |
| Router or Layer 3 switch | Layer 3 | Routes traffic between VLANs and to the WAN |
| DHCP server | Application layer | Assigns IP addresses, gateway and DNS settings |
Media, speeds and distance limits
LAN performance is defined by the Ethernet standard and the cabling in use, and each combination has a maximum distance. For twisted-pair copper, the classic limit is 100 meters per link, including patch cords.
| Standard | Speed | Medium | Maximum link length |
|---|---|---|---|
| 1000BASE-T | 1 Gbit/s | Cat 5e or better twisted pair | 100 m |
| 2.5GBASE-T | 2.5 Gbit/s | Cat 5e or better twisted pair | 100 m |
| 10GBASE-T | 10 Gbit/s | Cat 6A twisted pair | 100 m (about 55 m on Cat 6) |
| 10GBASE-SR | 10 Gbit/s | OM3 multimode fiber | About 300 m |
| Wi-Fi 6 (802.11ax) | Shared, real throughput often several hundred Mbit/s | Radio, 2.4, 5 and 6 GHz bands | Tens of meters indoors |
Wired links are full duplex and dedicated to each port, whereas Wi-Fi capacity is shared among all clients of an access point and depends on signal strength and interference. This is why bandwidth-critical devices such as servers, storage and desktop workstations are usually wired.
Many LAN switches also supply Power over Ethernet (PoE), which feeds phones, cameras and access points through the data cable. IEEE 802.3af provides up to 15.4 W per port, 802.3at up to 30 W and 802.3bt up to 90 W at the switch.
LAN compared with WLAN, VLAN and WAN
The LAN is easiest to understand when set against its neighbors. A WLAN is a LAN whose access layer is wireless, a VLAN is a logical LAN carved out of shared switches, and a WAN connects LANs across large distances.
| Network type | Scope | Typical ownership | Typical latency |
|---|---|---|---|
| LAN | Room to campus | The organization or household | Well under 1 ms wired |
| WLAN | Coverage area of access points | Same as the LAN it extends | A few milliseconds |
| VLAN | Logical segment within switches | Defined by the network administrator | Same as the underlying LAN |
| WAN | City, country or global | Carriers and service providers | Tens to hundreds of milliseconds |
VLANs, defined in IEEE 802.1Q, add a tag to each frame so that one physical switch can carry several separate broadcast domains. Traffic between VLANs must pass through a router or Layer 3 switch, where access rules can be enforced.
Addressing and segmenting a LAN
Most LANs use private IPv4 address ranges reserved for internal use: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. The router applies network address translation (NAT) so that all internal devices can share one or a few public addresses, while IPv6 LANs typically receive a /64 prefix per segment.
A single segment with a /24 prefix offers 254 usable host addresses. Larger organizations divide the LAN into several subnets and VLANs, for example by department or device type, to limit broadcast traffic and contain security incidents.
- Staff devices in one segment with access to internal servers.
- Voice and video devices in a separate VLAN with quality-of-service priority.
- Guest Wi-Fi isolated from internal systems and allowed only Internet access.
- Printers, cameras and IoT devices in a restricted segment, because they are often poorly patched.
Common LAN problems and their causes
Most LAN failures come from physical faults, loops and configuration errors, not from exotic attacks. A small set of symptoms covers the majority of support cases.
| Symptom | Likely cause | Remedy |
|---|---|---|
| Whole network slows to a halt | Switching loop causing a broadcast storm | Enable Rapid Spanning Tree Protocol and loop protection |
| Link stuck at 100 Mbit/s | Damaged cable or a pair not terminated correctly | Test and replace the cable |
| Devices get 169.254.x.x addresses | No reply from the DHCP server | Check the DHCP service and VLAN relay settings |
| Intermittent connectivity for one device | Duplicate IP address | Use DHCP reservations instead of manual addresses |
The most consequential design mistake is a flat network, in which every device shares one segment. It works at first, but a single compromised device can then reach every other system, so segmentation should be planned from the start.
Put simply
A LAN is the network inside one home, office or building. It links computers, phones, printers and other devices that are close to each other, so they can share files, printers and an Internet connection.
Think of it as the internal corridors of a building. People inside can move between rooms quickly and freely, and the front door, which is the router, leads out to the wider world of the Internet.
In practice
An accounting firm with 30 employees moves into a new office. The network plan includes 30 wired workstations, 10 IP phones, 4 Wi-Fi 6 access points, 3 printers and a small server, all connected to two 48-port gigabit PoE switches linked to each other by a 10 Gbit/s fiber uplink.
The administrator creates four VLANs: staff, voice, printers and guests. The power budget is calculated in advance: 10 phones at about 6 W each need roughly 60 W, and 4 access points at about 20 W each need roughly 80 W, so about 140 W in total, well within the 370 W PoE budget of one switch. All cable runs stay under 90 meters of permanent cabling, leaving room for patch cords within the 100-meter limit.
In the first month, a visitor plugs a small switch into two wall outlets and creates a loop. Rapid Spanning Tree blocks the redundant path within seconds, and the rest of the office notices nothing.