Networking Concepts

What this domain actually tests

The largest domain, and the foundation the other four stand on.

The OSI model as a working tool, not a mnemonic. You need to know which devices, protocols, and — crucially — problems live at each layer, because troubleshooting questions are frequently framed as "at which layer is this issue?"

Layer	Lives here
7 Application	HTTP, DNS, SMTP — the app itself
6 Presentation	Encryption, encoding, compression
5 Session	Session setup and teardown
4 Transport	TCP/UDP, ports, segmentation
3 Network	IP, routers, packets — routing and addressing
2 Data Link	MAC, switches, frames, VLANs, ARP
1 Physical	Cables, connectors, signal — is it plugged in

Learn to place the symptom: a cable fault is Layer 1. A VLAN misconfiguration is Layer 2. A wrong default gateway is Layer 3. A blocked port is Layer 4.

The traps

Subnetting, and being slow at it

Slow subnetting is the top exam-killer on Network+ — not wrong subnetting, slow subnetting. It turns your most reliable marks into a time sink, and then the clock beats you on the questions you actually found hard.

Use the block-size method. For a /26, the mask ends in .192, so the block is 256 − 192 = 64. Subnets fall on .0, .64, .128, .192, and each broadcast is one below the next network.

Worked example: which subnet does 172.16.45.130/26 belong to? Block size 64 → blocks are 0–63, 64–127, 128–191, 192–255. So: network .128, broadcast .191, usable .129–.190, 62 hosts.

Ten problems a day, every day, from week one. Not a subnetting week — a subnetting habit.

Ports, drilled in both directions

Around two dozen matter: 20/21 FTP, 22 SSH, 23 Telnet, 25 SMTP, 53 DNS, 67/68 DHCP, 69 TFTP, 80 HTTP, 110 POP3, 123 NTP, 143 IMAP, 161/162 SNMP, 389 LDAP, 443 HTTPS, 445 SMB, 514 Syslog, 636 LDAPS, 3389 RDP.

Drill them both ways — "what port is LDAP?" and "what runs on 389?" — because PBQs make you drag protocols onto port numbers and partial credit is not guaranteed. Practising in only one direction leaves you half-prepared, and it is a very common mistake.

TCP versus UDP matters and is checked. DNS uses UDP (falling back to TCP for large transfers). DHCP, SNMP, and Syslog are UDP. Anything needing reliable ordered delivery is TCP.

How to study it

Treat subnetting and ports as habits, not topics. Ten subnetting problems and ten minutes of port flashcards, daily, from day one. By week four both are automatic, and you have banked the marks and the clock time.

Then drill the OSI model by symptom placement rather than by reciting the layers. Given "the user's cable is unplugged," "the VLAN is wrong," "the gateway is misconfigured" — name the layer. That is how the exam asks.

At 23%, this domain is the biggest on the paper, and it is almost entirely made of skills that reward drilling over understanding.