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.