Quick reference & IT glossary
A working database for everyday IT: cable categories, PoE, RAID, subnets, ports, Wi-Fi channels, troubleshooting commands and plain-language definitions.
Ethernet cable categoriesPoE standardsRAID levelsSubnet cheat sheet (IPv4)Common network portsWi-Fi bands and channelsCCTV storage rules of thumbBackup termsTroubleshooting commandsGlossary
Ethernet cable categories
| Category | Typical speed | Bandwidth | Notes |
|---|---|---|---|
| Cat5e | 1 Gbps | 100 MHz | Fine for 1 Gbps to 100 m; common in older installs |
| Cat6 | 1 Gbps (10 Gbps to about 55 m) | 250 MHz | Common modern default |
| Cat6A | 10 Gbps to 100 m | 500 MHz | Better against crosstalk; thicker, costlier |
| Fibre (OM3/OM4, OS2) | 10 Gbps and above | n/a | Long runs, between buildings, backbones |
Maximum copper channel length is 100 m (about 90 m permanent link plus patch cords).
PoE standards
| Standard | Type | Power at switch | Power at device |
|---|---|---|---|
| 802.3af | Type 1 | 15.4 W | 12.95 W |
| 802.3at | Type 2 (PoE+) | 30 W | 25.5 W |
| 802.3bt | Type 3 | 60 W | 51 W |
| 802.3bt | Type 4 | 90 W | 71.3 W |
Check total switch PoE budget, not just per-port power.
RAID levels
| Level | Minimum disks | Fault tolerance | Notes |
|---|---|---|---|
| RAID 0 | 2 | None | Speed only; any disk failure loses all data |
| RAID 1 | 2 | 1 disk | Mirror; 50% usable |
| RAID 5 | 3 | 1 disk | Good capacity; rebuild risk on large disks |
| RAID 6 | 4 | 2 disks | Safer for large arrays |
| RAID 10 | 4 | 1 per mirror | Fast and resilient; 50% usable |
RAID protects against disk failure, not deletion, ransomware or fire. It is not a backup.
Subnet cheat sheet (IPv4)
| CIDR | Subnet mask | Usable hosts | Typical use |
|---|---|---|---|
| /30 | 255.255.255.252 | 2 | Point-to-point links |
| /29 | 255.255.255.248 | 6 | Tiny segment |
| /28 | 255.255.255.240 | 14 | Small VLAN |
| /27 | 255.255.255.224 | 30 | Small office VLAN |
| /26 | 255.255.255.192 | 62 | Medium VLAN |
| /25 | 255.255.255.128 | 126 | Large VLAN |
| /24 | 255.255.255.0 | 254 | Standard LAN |
| /23 | 255.255.254.0 | 510 | Large flat network |
| /22 | 255.255.252.0 | 1022 | Campus segment |
Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
Common network ports
| Port | Protocol | Service |
|---|---|---|
| 22 | TCP | SSH |
| 25 / 465 / 587 | TCP | Email (SMTP) |
| 53 | TCP/UDP | DNS |
| 67 / 68 | UDP | DHCP |
| 80 / 443 | TCP | HTTP / HTTPS |
| 88 | TCP/UDP | Kerberos |
| 123 | UDP | NTP time sync |
| 161 / 162 | UDP | SNMP |
| 389 / 636 | TCP | LDAP / LDAPS |
| 445 | TCP | SMB file sharing |
| 500 / 4500 | UDP | IPsec (IKE / NAT-T) |
| 514 | UDP | Syslog |
| 554 | TCP/UDP | RTSP (IP cameras) |
| 1194 | UDP/TCP | OpenVPN |
| 3389 | TCP | Remote Desktop (never expose to internet) |
| 51820 | UDP | WireGuard (default) |
Exposing ports 445, 3389 or a camera/NVR directly to the internet is a common cause of compromise.
Wi-Fi bands and channels
| Band | Channels (20 MHz) | Notes |
|---|---|---|
| 2.4 GHz | 1–13 in India; 1, 6, 11 non-overlapping | Long range, crowded; use for IoT and legacy |
| 5 GHz | Many; some need DFS | Best default for office capacity; use 20/40 MHz in dense layouts |
| 6 GHz (Wi-Fi 6E / 7) | Lower band opened in India in 2026 (verify approvals) | Clean spectrum, shorter range, newer devices only |
Always check current local regulations and device approvals.
CCTV storage rules of thumb
| Resolution | Approx. bitrate (H.264) | Approx. per day (24×7) |
|---|---|---|
| 2 MP | About 4 Mbps | About 43 GB |
| 4 MP | About 6 Mbps | About 65 GB |
| 5 MP | About 8 Mbps | About 86 GB |
| 8 MP (4K) | About 12 Mbps | About 130 GB |
H.265 can reduce this substantially. Actual bitrate depends on scene motion, frame rate and settings. Use our estimator for a quick calculation.
Backup terms
| Term | Meaning |
|---|---|
| RPO | Recovery point objective: maximum acceptable data loss, measured in time |
| RTO | Recovery time objective: maximum acceptable downtime |
| Full / incremental / differential | Complete copy / changes since last backup / changes since last full |
| Immutable backup | A copy that cannot be altered or deleted for a set period |
| 3-2-1-1-0 | 3 copies, 2 media, 1 off-site, 1 offline or immutable, 0 errors |
Troubleshooting commands
ipconfig /all (Windows) · ip a (Linux)Show IP address, gateway, DNS and DHCP detailsping 8.8.8.8 then ping google.comTests connectivity first by IP, then by name (separates internet from DNS problems)tracert host (Windows) · traceroute host (Linux)Shows the path and where delays or breaks occurnslookup domain · dig domainChecks DNS resolutionarp -aLists IP-to-MAC mappings seen by this device (find duplicates)netstat -ano (Windows) · ss -tulpn (Linux)Lists open connections and listening portsipconfig /flushdns · ipconfig /release & /renewResets DNS cache and DHCP leasegpupdate /force · gpresult /rRefreshes and reports Group Policysfc /scannow · DISM /Online /Cleanup-Image /RestoreHealthRepairs Windows system filesTest-NetConnection host -Port 443 (PowerShell)Checks if a TCP port is reachableGlossary (68 terms)
- AD (Active Directory)
- Microsoft directory service for users, computers and policies in a domain.
- AMC
- Annual Maintenance Contract: agreed maintenance and support over a year.
- AP (Access Point)
- Device that provides Wi-Fi to clients and connects to the wired network.
- ARP
- Address Resolution Protocol: maps IP addresses to MAC addresses on a LAN.
- Asset tag
- A unique ID label attached to a physical device and its record.
- BYOD
- Bring your own device: staff using personal devices for work.
- CERT-In
- Indian Computer Emergency Response Team.
- CIDR
- Notation for IP ranges, such as 192.168.1.0/24.
- CMDB
- Configuration management database: records of IT items and their relationships.
- DHCP
- Assigns IP addresses automatically to devices.
- DHCP reservation
- A fixed IP for a device, configured on the DHCP server.
- DLP
- Data loss prevention: controls to stop sensitive data leaving.
- DMZ
- A separate network zone for public-facing servers.
- DNS
- Translates names such as example.com into IP addresses.
- DPDP Act
- India's Digital Personal Data Protection Act, 2023.
- DR
- Disaster recovery: restoring IT after a major failure.
- EDR
- Endpoint detection and response: records and detects endpoint behaviour; enables response.
- Egress fee
- A charge for data leaving a cloud provider.
- Firmware
- Software embedded in hardware such as switches and firewalls.
- GPO
- Group Policy Object: Windows policy settings applied via Active Directory.
- Hypervisor
- Software that runs virtual machines (Hyper-V, Proxmox, VMware).
- IDS / IPS
- Intrusion detection / prevention systems.
- Immutable backup
- A backup that cannot be changed or deleted for a retention period.
- IoT
- Internet of Things: smart devices such as cameras, sensors and printers.
- IPsec
- A suite of protocols for secure VPN tunnels.
- ISP
- Internet service provider.
- LACP
- Link Aggregation Control Protocol: combines links for bandwidth and resilience.
- LAN / WAN
- Local area network / wide area network.
- LDAP
- Protocol for querying directory services.
- MAC address
- Hardware address of a network interface.
- MDM
- Mobile device management.
- MDR
- Managed detection and response: a monitored service.
- MFA
- Multi-factor authentication: more than a password to sign in.
- MTTR
- Mean time to repair or recover.
- NAC
- Network access control: decides which devices may connect.
- NAS
- Network-attached storage: a file-storage appliance on the LAN.
- NAT
- Network address translation: lets private addresses share a public one.
- NGFW
- Next-generation firewall with application awareness and deeper inspection.
- NTP
- Network Time Protocol: keeps clocks in sync.
- NVR / DVR
- Network / digital video recorder for CCTV.
- OU
- Organisational unit: a container in Active Directory.
- PDU
- Power distribution unit in a rack.
- PoE
- Power over Ethernet: power and data on one cable.
- PTZ
- Pan-tilt-zoom camera.
- QoS
- Quality of service: traffic prioritisation.
- RADIUS
- Authentication service used for 802.1X Wi-Fi and VPN.
- RAID
- Redundant array of independent disks: combines disks for speed or resilience.
- RMM
- Remote monitoring and management tool used by IT providers.
- RPO / RTO
- Acceptable data loss / acceptable downtime after an incident.
- RSTP
- Rapid Spanning Tree Protocol: prevents switching loops.
- SD-WAN
- Software-defined WAN: manages multiple links across sites.
- SFP / SFP+
- Pluggable modules for fibre or copper uplinks (1 Gbps / 10 Gbps).
- SIEM
- Security information and event management: log collection and correlation.
- SLA
- Service level agreement: agreed response and resolution targets.
- SNMP
- Protocol for monitoring network devices.
- SOC
- Security operations centre: team monitoring security alerts.
- SSID
- The name of a Wi-Fi network.
- SSO
- Single sign-on.
- STP
- Spanning Tree Protocol: prevents loops (older version of RSTP).
- Syslog
- Standard for sending logs to a central server.
- UPS
- Uninterruptible power supply: battery backup.
- UTM
- Unified threat management: firewall plus several security services in one.
- VLAN
- Virtual LAN: splits one physical network into separate logical networks.
- VPN
- Virtual private network: secure tunnel over the internet.
- WAF
- Web application firewall.
- WPA3
- Latest Wi-Fi security standard for personal and enterprise networks.
- XDR
- Extended detection and response across endpoint, email, identity, network and cloud.
- Zero trust
- Security model that verifies every user and device rather than trusting the network.
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