CompTIA A+ cheat sheet

Laptop memory uses SO-DIMM modules: DDR4 SO-DIMM is 260-pin and DDR5 SO-DIMM is 262-pin; the notch prevents installing the wrong generation.

OLED pixels emit their own light, so OLED panels need no backlight and can display true black by turning pixels off; LCD panels require a backlight.

Battery capacity in watt-hours equals milliamp-hours times nominal voltage divided by 1,000 (Wh = mAh × V / 1,000).

Wi-Fi calling carries voice over an IP network; it requires carrier support and a registered emergency address for E911 calls.

Bluetooth pairing requires both devices in range, one device discoverable, and confirmation of a PIN or passkey; after pairing, the devices remember each other.

MDM profiles can enforce passcodes, encryption, and screen locks, and can remotely wipe corporate data from BYOD or corporate-owned devices.

USB-C is reversible and supports data, power delivery, and alternate modes such as DisplayPort; not every USB-C port supports every feature.

Replacing a mobile screen often requires separating the digitizer from the glass or replacing a bonded assembly; check calibration and adhesive seals afterward.

Private IPv4 ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16; APIPA/link-local is 169.254.0.0/16.

A /24 subnet mask is 255.255.255.0 and provides 254 usable host addresses; /25 provides 126 usable hosts.

DHCP uses DORA: Discover, Offer, Request, Acknowledge; it supplies IP address, subnet mask, default gateway, and DNS servers.

Common ports: 80 HTTP, 443 HTTPS, 22 SSH, 25 SMTP, 53 DNS, 3389 RDP, 445 SMB.

Cat5e supports 1 Gbps at 100 m; Cat6 supports 10 Gbps up to 55 m; Cat6a supports 10 Gbps at 100 m.

Single-mode fiber carries light over long distances with a small core; multimode fiber uses a larger core for shorter distances.

In 2.4 GHz Wi-Fi, channels 1, 6, and 11 are the common non-overlapping set; 5 GHz and 6 GHz offer more non-overlapping channels.

A switch forwards frames by MAC address within a LAN; a router forwards packets by IP address between networks.

VLANs logically segment a switch into separate broadcast domains; inter-VLAN traffic requires a router or Layer 3 switch.

DDR4 and DDR5 desktop DIMMs are both 288-pin but have different notches and are not interchangeable; verify motherboard support.

M.2 SATA and M.2 NVMe SSDs use the same slot shape but different protocols; check the slot key and motherboard specification.

RAID 0 stripes with no redundancy; RAID 1 mirrors; RAID 5 uses distributed parity and tolerates one disk failure; RAID 10 mirrors and stripes.

ATX main power is a 24-pin connector; CPU power is a 4-pin or 8-pin EPS connector; PCIe power is 6-pin or 8-pin.

UEFI firmware supports GPT partitions, Secure Boot, and larger boot disks; legacy BIOS uses MBR partitions.

Laser printing steps: charging, exposing, developing, transferring, fusing, and cleaning.

Thermal paste fills microscopic gaps between the CPU and heat sink; too much paste can reduce heat transfer.

PCIe x16 slots commonly host graphics cards; physical slot length may exceed the number of wired lanes, so check the specification.

Type 1 hypervisors run directly on hardware (ESXi, Hyper-V); Type 2 hypervisors run as applications on a host OS (VirtualBox, VMware Workstation).

A virtual machine needs allocated vCPUs, RAM, virtual disk, and virtual network adapter; over-allocating host resources degrades all VMs.

IaaS provides virtual infrastructure; PaaS provides a managed development platform; SaaS provides a complete application.

Public cloud is shared provider infrastructure; private cloud is dedicated to one organization; hybrid cloud combines both.

In the shared responsibility model, the provider secures physical facilities and underlying hardware, while the customer secures data, identities, and access.

Cloud elasticity automatically scales resources with demand; pay-as-you-go billing charges for actual consumption.

Containers share the host OS kernel, while virtual machines each include a guest OS; containers are lighter but less isolated.

Object storage stores data as objects with metadata and keys; block storage presents raw volumes; file storage presents shared directories.

No power: verify outlet power, PSU switch, power cable, and front-panel header before replacing the power supply.

POST beep codes identify hardware failures during startup; consult the motherboard manual because patterns vary by vendor.

No boot device: check firmware boot order, drive data and power connections, and whether the drive has an active boot partition.

Overheating symptoms include unexpected shutdowns and fan noise; inspect dust, fan operation, airflow, and thermal paste.

Network troubleshooting order: check link lights, test cable, ping 127.0.0.1, ping default gateway, then ping an external DNS name.

An IP conflict occurs when two devices use the same address; ipconfig /release and /renew can obtain a new DHCP lease.

A stuck print queue may be cleared by restarting the Print Spooler service and removing pending jobs.

A degraded RAID array has lost redundancy; replace the failed disk and rebuild, then verify the array status.