What a network actually is

A network is two or more devices that can exchange data. That's it. The complexity comes from how many devices, how far apart, and what guarantees they need about the exchange. Everything else in this course — switches, routers, MPLS, OSPF, DWDM — is just plumbing to make that exchange happen reliably at scale.

The minimum cast

| Term | Meaning | |---|---| | Host | Any device that sends or receives data: a laptop, a server, a printer, a phone, a smart thermostat. The endpoints of a network. | | NIC | Network Interface Card. The hardware that connects a host to the network — these days usually just a chip on the motherboard plus a port. | | MAC address | A 48-bit hardware address burned into a NIC. Like 00:1A:2B:3C:4D:5E. Used to identify devices on the same physical link. | | IP address | A logical address assigned to a NIC, like 192.168.1.10. Used to route between networks. | | Link | The cable (or air, for Wi-Fi) connecting two devices. | | Frame / Packet | The data, wrapped in headers, that moves across links. |

A laptop with one Ethernet port is a host with one NIC, one MAC, and (usually) one IP. A server with two Ethernet ports has two NICs, two MACs, and two IPs.

"Local" vs "remote"

Two hosts are on the same network (technically: the same broadcast domain) if they can talk to each other without a router in between. Two hosts on different networks need a router to relay traffic between them. This distinction — local vs remote — is one of the most important ideas in networking. You'll see it again and again: a switch handles local traffic; a router handles inter-network traffic.

Why we need addresses at all

Imagine a room of ten people. If A says "Hi" out loud, everyone hears it — but only B is supposed to respond. So A starts with "Hey, B…" That's an address.

Now imagine a hundred buildings, each with ten rooms, each with ten people. To find someone specific you need a building address (city, street, number), a room address inside the building, and a name inside the room. Networking does the same thing in layers: IP addresses identify the building, MAC addresses identify the person in the room.

What to remember

• A network is just devices that can exchange data. Everything else is detail.
• MAC is local (one link); IP is global (between networks).
• Same broadcast domain = "can reach each other without a router."
• We'll spend the next 80 lessons making the exchange reliable, scalable, secure, and fast.