Master VLANs, Routing, Gateways, DNS, MAC & Subnet Masks

1. What is VLAN

VLAN (Virtual Local Area Network) is a LAN that is segmented by a router, creating separate broadcast domains.

Without VLANs, broadcast traffic like ARP requests would be sent to all devices, wasting bandwidth and CPU resources.

Analogy: a high school with 800 students divided into 10 classes, each class assigned a number, similar to VLANs dividing a network into smaller segments.

Devices in the same VLAN can communicate freely, while devices in different VLANs cannot communicate without additional configuration.

2. Single‑Arm Routing and Layer‑3 Switches

What is Single‑Arm Routing?

Single‑arm routing uses a basic Layer‑2 switch combined with a router to enable communication between different VLANs.

What is a Layer‑3 Switch?

When traffic between VLANs grows, a router can become a bottleneck. A Layer‑3 switch integrates routing functions into a switch, providing high‑speed routing with ASIC hardware and large internal bandwidth.

3. What is a Gateway

Definition

A gateway (also called a network interconnector) operates at the transport layer to connect networks that use different high‑level protocols.

Understanding the Concept

Just as a door connects rooms, a gateway connects separate networks, allowing packets to pass between them.

Gateway IP Address

The gateway is an IP address within a subnet, typically the first or last usable address, used to forward traffic to other networks.

Example: Network A (192.168.1.0/24) may use 192.168.1.1 as its gateway; Network B (192.168.2.0/24) may use 192.168.2.1.

How Gateways Enable Communication

When a host determines that a destination is outside its local subnet, it sends the packet to its gateway, which then forwards it toward the destination network.

Default Gateway

If a host has multiple gateways, the default gateway is used when no specific route is known.

4. What is DNS

DNS (Domain Name System) translates human‑readable domain names into IP addresses, enabling devices to locate servers on the Internet.

For example, querying DNS for www.baidu.com returns the IP address 61.135.169.105, which the client then uses to communicate.

5. MAC Address

IP vs. MAC

IP addresses identify a device on a network and can change, while the MAC address is a fixed physical identifier assigned by the hardware manufacturer.

MAC addresses are unique and do not change, unlike IP addresses which may vary.

6. Subnet Mask

A subnet mask separates the network portion and host portion of an IP address, allowing devices to determine whether another address is on the same network.

Analogy: the subnet mask is like a list that tells which people share the same surname (network) and can therefore communicate directly.