CompTIA Network+ cheat sheet
CompTIA Network+ · Keep the key ideas within reach.
Networking Concepts
OSI layers: 1 Physical (bits), 2 Data Link (frames, MAC), 3 Network (packets, IP), 4 Transport (segments, TCP/UDP), 5 Session, 6 Presentation, 7 Application.
TCP is connection-oriented and uses SYN, SYN-ACK, ACK; UDP is connectionless with no delivery guarantee.
Common ports: 22 SSH, 23 Telnet, 25 SMTP, 53 DNS, 80 HTTP, 443 HTTPS, 67/68 DHCP, 161/162 SNMP, 3389 RDP.
Private IPv4: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; APIPA: 169.254.0.0/16; loopback: 127.0.0.0/8.
Usable hosts per subnet = 2^h - 2, where h is the number of host bits; number of subnets = 2^n, where n is borrowed bits.
IPv6 is 128 bits; no broadcast; link-local fe80::/10; loopback ::1; multicast ff00::/8.
DNS records: A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), TXT (text/policy), PTR (reverse), NS (name server).
Cloud service models: IaaS provides infrastructure, PaaS provides development platform, SaaS provides applications.
Star topology uses a central switch; mesh provides redundant paths; hub-and-spoke centralizes traffic through a hub site.
Network Implementation
Cat5e supports 1 Gbps to 100 m; Cat6 supports 10 Gbps to 55 m; Cat6a supports 10 Gbps to 100 m.
Fiber: single-mode (SMF) has a small core and long reach; multimode (MMF) has a larger core and shorter reach.
DHCP DORA: Discover, Offer, Request, Acknowledge.
VLANs divide a switch into separate broadcast domains; 802.1Q trunk tags carry multiple VLANs between switches.
STP/RSTP prevents Layer 2 loops by blocking redundant paths; RSTP converges faster than STP.
802.11 standards: a 5 GHz 54 Mbps, b 2.4 GHz 11 Mbps, g 2.4 GHz 54 Mbps, n 2.4/5 GHz MIMO, ac 5 GHz, ax Wi-Fi 6.
NAT translates private to public addresses; PAT/overload uses source ports to share one public IP.
Routing: OSPF is link-state, BGP is path-vector for internet routes, EIGRP is advanced distance-vector.
SDN separates the control plane from the data plane; SD-WAN manages multiple WAN links centrally.
Network Operations
SNMP uses UDP 161 for queries and UDP 162 for traps; syslog commonly uses UDP 514.
RTO is maximum acceptable downtime; RPO is maximum acceptable data loss.
3-2-1 backup: 3 copies, 2 media types, 1 offsite or offline.
Full backup copies all data; incremental copies changes since last backup of any type; differential copies changes since last full.
Restore chain: full + all incrementals since that full, or full + latest differential.
Monitoring metrics: latency, jitter, packet loss, bandwidth, throughput, utilization, and error rate.
Documentation set includes network diagrams, IP address management, asset inventory, baselines, and change records.
Configuration management uses versioned golden configurations and rollback plans.
Patch management cycles: identify, test, approve, deploy, verify.
Network Security
CIA triad: confidentiality, integrity, availability.
AAA: authentication verifies identity, authorization grants permissions, accounting logs activity.
RADIUS uses UDP 1812/1813; TACACS+ uses TCP 49 and separates AAA functions.
Symmetric encryption (AES) uses one shared key; asymmetric encryption (RSA/ECC) uses a public/private key pair.
Stateful firewalls track connections; stateless firewalls filter per packet; WAF protects web applications.
IDS detects and alerts; IPS is inline and can block.
IPsec: AH provides integrity/authentication without encryption; ESP provides confidentiality and integrity.
802.1X uses supplicant, authenticator, and authentication server for port-based access control.
Common attacks: DoS/DDoS, MITM/on-path, spoofing, phishing, ARP poisoning, VLAN hopping.
Network Troubleshooting
Troubleshooting sequence: identify problem, establish theory, test theory, plan action, implement, verify, document.
169.254.x.x indicates DHCP failure; 0.0.0.0 or no address indicates link/configuration issue.
Ping tests ICMP reachability; traceroute/tracert maps path and per-hop latency.
nslookup/dig query DNS directly; ipconfig /flushdns clears local DNS cache.
ipconfig/ifconfig shows IP, mask, gateway; netstat shows connections/listening ports; arp shows IP-to-MAC mappings.
Duplex mismatch causes late collisions and poor throughput; verify both ends match full/half duplex.
Wireless issues: interference, channel overlap, low SNR, roaming misconfiguration, and wrong security settings.
High jitter and packet loss degrade VoIP/video more than average latency alone.
Escalate with documented symptoms, tests performed, and recent changes; use change control for fixes.