CompTIA Tech+ (FC0-U71): Virtualization, Cloud, Internet Services, Networking and Wi-Fi

Virtualization lets one physical computer run several isolated virtual machines (VMs) at once, and the software that makes it possible is the hypervisor. The hypervisor sits between the hardware and the VMs and gives each VM its own share of the CPU, memory, storage and network, so each one behaves like a separate computer with its own operating system.

Two operating-system roles are easy to mix up:

• Host OS — the operating system installed directly on the physical hardware. When you install a virtualization app on a Windows laptop, Windows is the host.
• Guest OS — the operating system installed inside a virtual machine. A Linux VM on that Windows laptop runs Linux as the guest.

The rule of thumb: the host is always the one on the real hardware; the guest always lives inside a VM. The hypervisor itself is neither — it is the layer that creates and runs the VMs. A hypervisor that runs directly on the hardware with no host OS underneath (common on servers) is called Type 1 or bare metal; one that runs as an app on top of a host OS (common on desktops and laptops) is called Type 2 or hosted.

Why virtualize? One powerful server can replace several underused ones (less hardware, power and space), a VM can be copied, snapshotted or moved easily, and testing a different operating system no longer needs a second computer. Virtualization is also the foundation that cloud providers build their services on.

A deployment model describes where systems run and who owns the equipment. Tech+ names three:

The test for hybrid is simple: if some systems stay in the organization's own building and others run at a provider, it is hybrid — neither on premises nor cloud alone describes the whole setup. A common example is a retailer that keeps its point-of-sale database in its own server room while running its public website and customer-support tools at a provider.

You may also see public cloud (a provider's infrastructure shared by many customers) and private cloud (cloud-style infrastructure dedicated to one organization). Public and private describe who shares the infrastructure; hybrid describes combining environments.

The deployment model and the service model (next section) are two separate choices. A requirement about where data must stay decides the deployment model; a requirement about what you are willing to manage decides the service model. A real scenario often needs both, such as a hybrid deployment that also uses a platform service for a new web app.

Cloud service models differ in how much the provider manages and how much is left to the customer. The more the provider manages, the less control — and the less work — the customer has.

Two questions place any scenario: Is the customer bringing its own code? If not and they just use an app, it is SaaS. Must the customer install or patch an operating system? If yes, it is IaaS; if the customer brings code but refuses to manage servers or operating systems, it is PaaS.

Other "as a Service" names follow the same pattern. Desktop as a Service (DaaS), for example, streams a whole virtual desktop to the user to the user, with the provider running the desktop machines.

An internet service type is the medium that carries a home's or office's connection to the provider, and each one trades speed, availability and cost differently.

Each type has a prerequisite and a signature strength. DSL and cable reuse wiring already in the building (telephone copper and TV coax), fiber needs its own new line, satellite and fixed wireless need only an antenna at the site, and cellular needs only tower coverage — which is why it is the one that moves with the user. A mobile hotspot shares a phone's cellular connection with other devices over Wi-Fi. Upload speed is where the wired types differ most: fiber plans are commonly symmetrical, while cable and DSL are built for downloading.

Satellite latency depends on the satellites' orbit: traditional services use satellites far from Earth, while newer low-Earth-orbit services have much lower latency than those, so avoid treating one latency figure as a rule.

Devices on a network are identified two ways: a MAC address identifies a network adapter's hardware, and an IP address identifies the device's logical location on a network.

A switch learns which MAC address is connected to which of its ports and sends each frame only to the right port, all within one LAN. A router reads the destination IP address, which includes the network part, to decide where to forward traffic between networks — for example between your home LAN and the internet. MAC addresses are meaningless beyond the local network, which is why routers need IP addresses.

Two related terms work at different levels: a hostname is a readable name given to a computer (DNS maps names to IP addresses), and a port number picks out a service on a device, as the next section explains. Note that modern phones and computers can use a randomized, per-network MAC address on Wi-Fi for privacy, but the adapter's original hardware address still comes from the manufacturer.

A port is a number that tells a device which service should receive incoming traffic, and each common service uses a well-known default port. The IP address finds the device; the port finds the service on it.

The tested distinctions are near-twins. HTTP vs HTTPS: only HTTPS encrypts, so it is the one to look for whenever a site asks for a password or personal details. IMAP vs POP3: both retrieve mail, but only IMAP leaves the mailbox on the server, so every device works from the same copy. FTP vs SFTP: both move files, but only SFTP encrypts the password and the data.

A networking model describes which computers provide resources and which use them. In client/server, a central server provides files, accounts and services that client computers request; in peer-to-peer, every computer can both share its own resources and use the others', with no central server.

The trade-off in one line: client/server spends money on a server to gain central control, while peer-to-peer saves that money and gives up central control.

Each networking device has one main job, and a small network combines them so every job is covered.

A home or small-office wireless router usually combines a router, a small switch (a handful of ports), an access point and a basic firewall in one box, and providers often supply a modem built in too. The roles are still separate jobs: converting the line signal is the modem's, and sharing the connection is the router's. When a site needs more wired ports than its router has, a switch connected to the router supplies them.

LAN vs WAN. A local area network (LAN) covers one building or site, such as a home or a single office. A wide area network (WAN) links networks across large distances, such as a company's offices in several cities joined over a provider's connections; the internet is the largest WAN. Smaller local types also exist: a WLAN is a LAN using Wi-Fi, and a PAN (personal area network) links the devices around one person, like a phone and Bluetooth earbuds.

The 802.11 standards define how Wi-Fi works, and newer standards are faster and more efficient while staying backward compatible with older devices.

Older vs newer. A newer router accepts older clients, but each link runs at the best standard both ends support — a new router does not upgrade an old adapter. An 802.11n phone on an 802.11ac router therefore connects using 802.11n on 5 GHz, at 802.11n speeds. A band works only when both ends support it, which is why a 6 GHz network needs a Wi-Fi 6E (or newer) adapter as well as a 6E router.

Speed considerations. The speed printed on the box is a theoretical maximum shared by all the devices on that access point. Real speed is lower and depends on the slowest standard in use on the link, the band, distance and walls, interference, how many devices are active, and the internet service itself — internet speed cannot exceed the plan coming through the modem, although transfers between devices on the LAN are not limited by the plan.

Wi-Fi uses three bands, and the basic trade-off is range versus speed: lower frequencies travel farther and pass through walls better, higher frequencies carry more data over shorter distances.

In practice, devices close to the router benefit most from the higher bands, while devices far away or behind several walls hold a steadier connection on the lower band.

Two different problems weaken Wi-Fi:

• Attenuation is the signal getting weaker with distance and as it passes through materials — walls, floors, metal, concrete, water. It is fairly constant for a given spot. Fixes: move closer, relocate the router to a central spot, add an access point or mesh node, or use a lower band.
• Interference is other radio energy on the same frequencies disrupting the signal — neighboring Wi-Fi networks, microwave ovens, cordless phones and Bluetooth on 2.4 GHz. It comes and goes with the source, so problems that appear only at certain times point to interference. Fixes: change channel or move to 5 GHz or 6 GHz.