Introduction to LAN

Start with a local conversation

H1 needs to exchange files with H2 in the same office. Their Ethernet interfaces attach to SW1, and both hosts use the same IPv4 subnet. The switch joins those local links. The Internet connection is not needed for this local exchange: a failed provider link need not prevent two healthy local hosts from communicating.

Ethernet switchingIllustrative topology

Separate the device roles

One physical home gateway can combine switching, routing and wireless functions. Those remain different roles even when the hardware is a single box. A managed switch may have an IP address for administration; hosts do not need the switch to have an IP address merely to forward ordinary Layer 2 frames.

Scale from an office to a campus

Ethernet switchingIllustrative topology

A LAN may cover one room, a building or a nearby campus. Access switches provide edge ports; an aggregation or distribution layer joins access blocks. Larger designs can add a separate core. These names describe roles, not a mandatory number of boxes.

Redundant links need a deliberate loop-prevention or aggregation design. Do not add an extra cable between switches and assume Ethernet will safely choose a path. Spanning tree and link aggregation are later topics.

Build one isolated access segment

Use a fresh lab switch with three suitable Ethernet access ports and no production connections. Connect H1–H3 as below. Assign these addresses and the listed locally administered MAC addresses in the host or hypervisor settings if supported; otherwise record the actual MAC values and use those in observations. No default gateway is required for this same-subnet exercise.

configure terminal
vlan 10
 name LAB
interface range GigabitEthernet1/0/1 - 3
 switchport mode access
 switchport access vlan 10
 no shutdown
end

The commands make the intended VLAN explicit instead of relying on an existing port assignment. Other switch state is assumed to be a clean lab baseline; do not paste this recipe into a live network.

Verify links, membership and local reachability

SW1#show interfaces status
Port      Name  Status       Vlan  Duplex  Speed  Type
Gi1/0/1         connected    10    a-full  a-1000 10/100/1000BaseTX
Gi1/0/2         connected    10    a-full  a-1000 10/100/1000BaseTX
Gi1/0/3         connected    10    a-full  a-1000 10/100/1000BaseTX

Each intended port is connected and assigned to VLAN 10. A link being up does not prove the host address or application is correct.

H1$ ping -c 3 10.10.10.12
3 packets transmitted, 3 received, 0% packet loss

This Linux-host example tests H1–H2 IP reachability without crossing R1. If it fails, check host addressing and firewall policy as well as cables, port state and VLAN membership. Ping success does not measure every application or forwarding condition.