LANs, WANs, and LAN components
LAN
A network of endpoints and components close together in a limited area: a campus, a building, an office suite, a home. Size varies wildly, from two computers in a home office to hundreds across multiple buildings.
The organization typically builds and owns the whole thing, down to the physical cabling.
Ethernet won the LAN transport war in the early 90s. Bandwidth has scaled from shared-media 10 Mbps to 400 Gbps (Nexus 9000 data center switches).
WAN
A WAN provides access to other networks across a large geographic area, using facilities from an ISP or carrier (the phone or cable company). You lease, you don’t own.
The provider connects your sites to each other, to other organizations, to external services, and to remote users. It carries voice, data, and video.
LAN vs. WAN in one line: LANs have higher data rates, a smaller area, and no leased lines. WANs are the opposite on all three.
What lives in a LAN
- Endpoints: PCs, printers, VoIP phones, mobile devices.
- Access points (wireless devices aggregate here, then into the switch).
- L2 access switches: where endpoints plug in.
- Core: access switches aggregate up to core switches, firewalls, and routers, which lead out to the WAN.
- NICs on every IP-enabled device, plus the protocols and cabling tying it together.
A small LAN might be one switch. A large office LAN is multiple interconnected switches forming an L2 topology.
Access ports and the voice VLAN trick
A switch port connecting to an end-user device is an access port, usually assigned to a VLAN. PC traffic on its VLAN (say VLAN 50) goes untagged.
The VoIP rollout trick: instead of recabling the building, the phone plugs into the existing wall port and the PC plugs into the phone. One cable, two devices.
Traffic is separated by putting the phone on a voice VLAN (say 150), which is tagged. The phone knows how to tag; the PC’s untagged traffic stays on VLAN 50.
Why switches beat hubs
A hub was a Layer 1 device with zero intelligence. Every connected device shared the same wire and bandwidth: constant collisions, no privacy, inefficient.
On a switch, every port is its own segment, its own collision domain.
Switch characteristics
- Dedicated communication per port (that single collision domain means privacy).
- Simultaneous conversations: multiple host pairs talking at once.
- Full duplex: transmit and receive at the same time, like a phone call. Half duplex is a walkie-talkie, one side at a time.
- Media rate adaptation: ports auto-detect and adapt speed (a 1 Gb port accommodates a 100 Mb host).
Catalyst switch roles
Single rack-unit switches (the 2000/3000 series) are typically access layer. The Catalyst 9000 series is typically core: high port density, large frame buffers, speed variety (10/40 Gb), fast internal switching, low per-port cost.