LAN
A local area network: one broadcast domain or a few, under one administration.
Level: beginner
What is it?
A Local Area Network connects devices within a limited site or administration, such as a home, lab, or office. Ethernet and Wi-Fi are common link technologies, while switching and VLANs shape how traffic is delivered within the local infrastructure.
Why does it matter?
A LAN provides local resource access and a path toward routers, printers, servers, and internet gateways. Its design influences broadcast scope, segmentation, fault domains, and troubleshooting.
Mental model
Think of a LAN as the local delivery area. Link-layer frames reach devices within a segment or VLAN; traffic for other IP networks typically goes through a router.
How it works
- A LAN can contain multiple switches and VLANs; physical closeness does not guarantee a single broadcast domain.
- Switches forward frames according to MAC-address learning within a VLAN.
- Wireless access points bridge Wi-Fi clients into a wired network but add radio coordination and authentication concerns.
- Subnet masks and routes determine which IP destinations a host treats as local.
- Segmentation and access controls matter because devices in the same network are not automatically trusted.
Steps
- A host sends to another host in its IP subnet and resolves the peer's MAC address.
- The local switch forwards the Ethernet frame to the peer port.
- For a remote subnet, the host sends a frame to its default gateway instead.
- The router forwards the IP packet toward the remote network.
Real-world usage
- Planning home, campus, or office network topology.
- Distinguishing LAN switching from inter-network routing.
- Diagnosing DHCP, ARP, VLAN, and gateway issues.
Common mistakes
- Assuming a LAN always means one flat broadcast domain.
- Assuming devices on the same LAN are automatically authorized to communicate.
- Confusing the IP subnet with the physical wiring layout.
Interview questions
Does one LAN have to be one broadcast domain?
No. VLANs can partition a switched LAN into multiple logical broadcast domains.
When is a default gateway used?
When the destination is not reachable via the host's directly connected routes and the routing table selects the gateway as next hop.