Network devices

Why so many devices?

A network is built from devices that each do one job well. Knowing exactly what each one does is the base of all troubleshooting: if two PCs on the same switch can talk but cannot reach the internet, you look at the router, not the switch.

Endpoints and the NIC

Endpoints (also called hosts or end devices) are where data starts and ends: laptops, phones, printers, IP cameras, and servers. A server is just a computer whose job is to offer a service, such as a website, email or files. Every endpoint has a NIC (Network Interface Card), the hardware that connects it to the network. A NIC can be a port on the motherboard or a Wi-Fi chip. Each NIC has a MAC address burned in by the maker, which you will meet in the next module.

Hub: the old way

A hub is a simple box with many ports. Whatever arrives on one port, it copies out of every other port, without looking at it. It works at Layer 1, the physical layer. Because every device shares the same wire, only one device can send at a time. If two send together, their signals crash into each other, which is called a collision.

Hubs are obsolete because they waste bandwidth (everyone receives everything), they are insecure (any PC can see others' traffic), and they force half duplex, meaning a device can send or receive but not both at once. You will still meet them in exam questions and in Packet Tracer.

Switch: the heart of the LAN

A switch also has many ports, but it is smart. It learns which device (MAC address) lives on which port and sends each frame only out of the port where the destination is. It works at Layer 2, the data link layer. Every switch port can run full duplex, sending and receiving at the same time, so collisions disappear. In an office, every desk socket ends at a switch port.

Router: joining networks

A router connects different networks and decides the best path between them using IP addresses. It works at Layer 3, the network layer. Your office LAN is one network and the internet is another, so a router sits between them. The router address that PCs send outside traffic to is called the default gateway.

Wireless devices

An access point (AP) connects Wi-Fi devices to the wired network. In a small office each AP is configured on its own (an autonomous AP). In a campus with 200 APs, a wireless LAN controller (WLC) manages them centrally: it pushes settings, handles roaming between APs and shows which ones are down. Such APs are called lightweight APs because the controller does much of the thinking.

Firewall

A firewall inspects traffic and allows or blocks it based on rules, for example "allow web traffic out, block everything coming in unless it is a reply". Next-generation firewalls (NGFW) can also recognise applications and block known attacks. Firewalls usually sit at the edge between the LAN and the internet, and also between internal zones, such as the hospital billing network and the guest Wi-Fi.

Modem and ONT

Your provider's line does not carry Ethernet directly. A modem converts between the provider's signal (on a telephone or cable line) and Ethernet. For FTTH (Fibre To The Home), such as JioFiber or Airtel Xstream Fiber, the device is an ONT (Optical Network Terminal), which converts light on the fibre into Ethernet.

Collision domains and broadcast domains

A collision domain is a group of devices whose signals can collide with each other. All ports of a hub are one collision domain. Each port of a switch is its own collision domain. A broadcast domain is a group of devices that all receive a broadcast, a message sent to everyone. All ports of a switch (in one VLAN) are one broadcast domain. A router does not forward broadcasts, so each router interface is its own broadcast domain.

The home router: several devices in one box

The small box at home called a "router" or "Wi-Fi router" is really a router, a small switch (usually 4 LAN ports), an access point, a basic firewall, a DHCP server that gives out IP addresses, and sometimes the ONT or modem too. In a business these are separate devices, because each needs to be faster, more reliable and separately managed.