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CompTIA Network+ (N10-009) Cheatsheet — High-Yield Tables, Diagrams & Commands

Fast recall for N10-009: OSI model, subnetting, IPv6 essentials, VLANs/trunks, Wi‑Fi, cabling/fiber, ports/protocols, DHCP/DNS, VPN/AAA, and a troubleshooting playbook with common commands.

On this page

Use this for last-mile review. Star weak rows and re-drill with practice.

1) OSI model (memorize + use for troubleshooting)

Layer Name PDU Common examples
7 Application Data HTTP/S, DNS, SMTP, DHCP (conceptual)
6 Presentation Data TLS/SSL, encoding, compression
5 Session Data Session setup/teardown (high level)
4 Transport Segment/Datagram TCP/UDP, ports
3 Network Packet IP, routing, ICMP
2 Data Link Frame Ethernet, VLANs, MAC, ARP
1 Physical Bits Cabling, RF, signaling 
    flowchart LR
	  A[App data] --> B[TCP/UDP + ports]
	  B --> C[IP + routing]
	  C --> D[Ethernet + MAC/VLAN]
	  D --> E[Bits on wire / RF]

2) IPv4 subnetting quick table

Rules

  • Hosts per subnet = 2^(host bits) - 2 (except /31 point-to-point)
  • Block size = 256 - mask_octet (in the interesting octet)
Prefix Mask Usable hosts Block size (last octet)
/24 255.255.255.0 254 1
/25 255.255.255.128 126 128
/26 255.255.255.192 62 64
/27 255.255.255.224 30 32
/28 255.255.255.240 14 16
/29 255.255.255.248 6 8
/30 255.255.255.252 2 4
/31 255.255.255.254 2 (P2P) 2

Fast sanity checks

  • Same subnet? Compare each IP to the mask (or compute the network address).
  • Default gateway must be in the same subnet as the host.

3) IPv6 essentials

  • Address types
    • Global unicast: 2000::/3 (public)
    • Unique local (ULA): fc00::/7 (private-ish)
    • Link-local: fe80::/10 (always on; neighbor discovery)
  • Compression: remove leading zeros; :: only once per address.
  • Common multicast
    • ff02::1 all nodes (local link)
    • ff02::2 all routers (local link)
    • ff02::1:ff00:0/104 solicited-node (ND)

4) VLANs, trunks, and switching basics

  • Access port: untagged, belongs to one VLAN.
  • Trunk port (802.1Q): tagged, carries multiple VLANs between switches/APs/routers.
  • Native VLAN: untagged VLAN on a trunk (avoid misuse; document it).
  • STP: prevents loops; a loop often shows as broadcast storms and MAC flapping.

5) Wi‑Fi quick table + security

Generation IEEE Bands Notes
Wi‑Fi 4 802.11n 2.4/5 MIMO
Wi‑Fi 5 802.11ac 5 MU‑MIMO, wider channels
Wi‑Fi 6/6E 802.11ax 2.4/5/6 OFDMA, dense environments

Channel planning

  • 2.4 GHz: prefer 1 / 6 / 11 to avoid overlap.

Security order (best → worst)

  • WPA3 → WPA2 (AES/CCMP) → WPA (TKIP) → WEP (avoid)
  • Enterprise auth: 802.1X with RADIUS + EAP (scenario picks often favor this).

6) Cabling, fiber, optics, and PoE

Copper

Cable Typical use Notes
Cat5e 1 Gbps @ 100 m Common baseline
Cat6 10 Gbps @ ~55 m Better noise performance
Cat6a 10 Gbps @ 100 m Best pick for long 10G copper runs

Fiber

Fiber Mode Typical range Notes
MMF Multi‑mode Short Common in buildings
SMF Single‑mode Long Metro/long distance

Transceivers

  • SFP (1G), SFP+ (10G), QSFP (40G/100G variants)

PoE

  • 802.3af (PoE) • 802.3at (PoE+) • 802.3bt (PoE++)
    Know that higher PoE classes power APs, cameras, and phones.

7) Ports & protocols (high yield)

Service Port/Proto Notes
DNS 53 UDP/TCP UDP for queries; TCP for zone/large replies
DHCP 67/68 UDP Server/client
HTTP / HTTPS 80 / 443 TCP Web; TLS on 443
SSH 22 TCP Secure remote management
Telnet 23 TCP Insecure (distractor)
RDP 3389 TCP/UDP Remote desktop
SMB 445 TCP Windows file sharing
NTP 123 UDP Time sync
SNMP 161/162 UDP Polling / traps
Syslog 514 UDP Logging (TCP variants exist)
LDAP / LDAPS 389 / 636 TCP Directory
RADIUS 1812/1813 UDP AAA (auth/accounting)
TACACS+ 49 TCP AAA (common alt)

8) DHCP & DNS quick picks

DHCP DORA Discover → Offer → Request → Acknowledge

DNS records

  • A / AAAA (name → IP)
  • CNAME (alias)
  • MX (mail)
  • TXT (verification/SPF hints)
  • PTR (reverse lookup)

9) VPN + AAA at a glance

  • Site-to-site VPN: typically IPsec tunnel between gateways.
  • Remote access VPN: user → gateway; often SSL/TLS-based or IPsec.
  • AAA: central auth and auditing; common pattern is RADIUS/TACACS+ plus MFA.

10) Troubleshooting playbook (symptom → layer → tools)

Symptom Likely layer Fast checks / tools
No link light L1 Cable, port, PoE, NIC enabled
APIPA (169.254.x.x) L3/service DHCP scope, VLAN, relay, server reachability
IP works, names fail L7/service DNS servers, nslookup/dig, firewall 53
One VLAN can’t reach gateway L2/L3 VLAN on access port, trunk allowed VLANs, SVI/gateway
High latency/packet loss L3/L4 ping, tracert/traceroute, interface errors, congestion
Intermittent drops L1/L2 Duplex/speed mismatch, bad cable, loops/STP, Wi‑Fi interference

Useful commands

 1# Windows
 2ipconfig /all
 3ping 8.8.8.8
 4tracert example.com
 5nslookup example.com
 6
 7# Linux/macOS
 8ip a
 9ip route
10traceroute example.com
11dig example.com
12sudo tcpdump -ni any port 53

Wireshark filter starters

  • dns
  • tcp.port == 443
  • ip.addr == 10.0.0.10

Next steps

  • Syllabus: objectives by domain → Open
  • Practice: drills + mocks → Start
Syllabus
Practice