Last-minute reference for N10-009. Troubleshooting is the heaviest domain, and most troubleshooting questions hand you the clue that eliminates three options.
Exam facts
| |
|---|
| Duration | 90 minutes |
| Questions | Max 90 |
| Passing score | 720 / 900 |
| Version | V9, superseding N10-008 |
| Price | Not published by CompTIA; varies by region |
Domain weights
| Domain | Weight |
|---|
| Network troubleshooting | 24% |
| Networking concepts | 23% |
| Network implementation | 20% |
| Network operations | 19% |
| Network security | 14% |
Subnetting quick table
| CIDR | Block size | Usable hosts |
|---|
| /30 | 4 | 2 |
| /29 | 8 | 6 |
| /28 | 16 | 14 |
| /27 | 32 | 30 |
| /26 | 64 | 62 |
| /25 | 128 | 126 |
| /24 | 256 | 254 |
| /23 | 512 | 510 |
| /22 | 1024 | 1022 |
Network address = the block boundary below the host. 10.20.5.130/26 → blocks at .0 .64 .128 .192 → network is 10.20.5.128.
Ports worth knowing cold
| Service | Port |
|---|
| DHCP | UDP 67/68 |
| DNS | 53 |
| NTP | UDP 123 |
| SNMP | 161 |
| HTTP / HTTPS | 80 / 443 |
| SSH | 22 |
| RDP | 3389 |
Symptom to cause
| Symptom | Cause |
|---|
| Works by IP, fails by name | DNS |
| Late collisions, poor throughput, link up | Duplex mismatch |
| One remote subnet unreachable | Routing — use traceroute to localise |
| Copper run over 100 m failing | Distance limit; use fibre |
| Intermittent, never during tests | Set up continuous monitoring |
Security quick answers
- Guest isolation → separate VLAN with an internet-only ACL
- Rogue switch sending superior BPDUs → BPDU guard on access ports
- Rogue DHCP server → DHCP snooping with trusted uplinks
- 2.4 GHz channel plan → 1, 6, 11
Traps
- RTO is time to restore; RPO is acceptable data loss.
- A trunk carries many VLANs; an access port carries one.
- IPv6 link-local is fe80::/10 and is never routed off the link.
- No paper allowed in the exam — drill subnetting mentally.
Night-before checklist
- Subnet boundaries without writing anything down
- The five symptom-to-cause pairs above
- Ports table
- Check ID and proctoring rules — see exam day
Take the 20-question practice test.