N10-009 networking concepts explained
Networking concepts is worth 23% of CompTIA Network+ N10-009 — around 21 of the 90 questions and the second largest domain. It is also the domain that underpins the other four: addressing, the OSI model and protocols appear throughout implementation, operations, security and troubleshooting.
Weakness here costs far more than 23%.
The OSI model
The framework the whole exam thinks in.
| Layer | Name | Lives here |
|---|---|---|
| 7 | Application | HTTP, DNS, SMTP |
| 6 | Presentation | Encoding, encryption |
| 5 | Session | Session establishment |
| 4 | Transport | TCP, UDP, ports |
| 3 | Network | IP, routers, routing |
| 2 | Data Link | MAC addresses, switches, VLANs |
| 1 | Physical | Cables, connectors, signals |
Its practical value is diagnostic. Given a symptom, ask which layer it points at: no link light is layer 1, a MAC or VLAN problem is layer 2, wrong subnet or no route is layer 3, a blocked port is layer 4, name resolution is layer 7.
Learn which device operates at which layer — hub at 1, switch at 2, router at 3 — because questions test that directly.
Addressing and subnetting
Subnetting is the single highest-value skill on this exam. It appears here and in implementation and troubleshooting.
What you need fluently:
- CIDR notation and the mask each prefix implies
- Network address, broadcast address, and usable host range for a given address and prefix
- How many subnets and hosts a given prefix yields
- Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
- APIPA (169.254.x.x) and what its appearance means — usually DHCP failure
Also: public versus private addressing, NAT, and IPv6 basics including address format and why it exists.
Ports and protocols
Memorisation, and worth doing properly because these recur everywhere:
| Port | Protocol | Port | Protocol |
|---|---|---|---|
| 20/21 | FTP | 443 | HTTPS |
| 22 | SSH / SFTP | 445 | SMB |
| 23 | Telnet | 514 | Syslog |
| 25 | SMTP | 587 | SMTP (submission) |
| 53 | DNS | 993 | IMAPS |
| 67/68 | DHCP | 3389 | RDP |
| 80 | HTTP | 161/162 | SNMP |
Know the secure alternative to each insecure protocol: SSH over Telnet, HTTPS over HTTP, SFTP over FTP. That pairing appears in the security domain as well.
TCP versus UDP — connection-oriented and reliable versus connectionless and fast — and which services use which.
Topologies, network types and cabling
- Star, mesh, hybrid; LAN, WAN, WLAN, PAN, SAN.
- Cloud models and service models at a conceptual level.
- Copper and fibre: categories, distance limits, and when fibre is required.
- Connector types, and straight-through versus crossover.
Cabling questions are usually answerable from distance and speed limits, so learn those rather than every connector’s appearance.
Sample questions
Question 1. A workstation has obtained the address 169.254.12.44. What does this indicate?
- A. The host has a valid public address
- B. The host is correctly configured with a static address
- C. The host could not reach a DHCP server and self-assigned an APIPA address
- D. The host is using IPv6 addressing
Show answer
Answer: C
An address in the 169.254.x.x range is APIPA, assigned automatically when a host cannot reach a DHCP server. It is not a valid public or private routable assignment, does not indicate correct configuration, and is unrelated to IPv6.
Question 2. At which OSI layer does a switch primarily operate, and what does it use to forward frames?
- A. Layer 1, using electrical signals
- B. Layer 2, using MAC addresses
- C. Layer 3, using IP addresses
- D. Layer 4, using port numbers
Show answer
Answer: B
A switch operates at layer 2, the data link layer, forwarding frames based on MAC addresses. Layer 1 devices such as hubs do not examine addresses, layer 3 devices such as routers forward using IP addresses, and layer 4 concerns ports.
Question 3. How many usable host addresses are available in a /28 subnet?
- A. 14
- B. 16
- C. 30
- D. 62
Show answer
Answer: A
A /28 leaves four host bits, giving 16 addresses, of which the network and broadcast addresses are unusable, leaving 14 usable hosts. 16 ignores those two reserved addresses, and 30 and 62 correspond to /27 and /26 respectively.
What to practise
Subnet ten addresses a day, out loud, without a calculator. Six weeks of that makes every subnetting question on the exam free.
Then build a flashcard deck of ports and protocols and run it daily. Between those two habits you cover most of what this domain can ask, and a good share of three other domains as well.