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CompTIA A+: Networking Concepts

Importance ★★★★☆ · 23% (estimate)

Overview

This chapter covers how devices talk to each other on a network. You need to know common port numbers, the job of each network device, Wi-Fi basics, IP addressing, and the services that make a network work, such as DHCP and DNS. It is a big part of Core 1, and it is also the base for the troubleshooting chapter.

Each network service listens on a port number. For example, SSH uses 22, DNS uses 53, HTTP uses 80, HTTPS uses 443, and Remote Desktop uses 3389. TCP checks that data arrives and resends lost pieces, while UDP is faster but does not check, which suits things like streaming and DNS lookups.

DHCP hands out IP addresses automatically, and DNS turns names like www.example.com into IP addresses. If a Windows PC cannot reach a DHCP server, it gives itself an APIPA address that starts with 169.254 and can only talk to the local network. Private address ranges such as 192.168.x.x, 10.x.x.x, and 172.16.x.x to 172.31.x.x are used inside homes and offices.

Devices each have a role: a switch connects devices inside one network, a router connects different networks, an access point adds Wi-Fi, and a firewall filters traffic. Technicians use tools such as a cable tester, a crimper, a punchdown tool, a toner probe, and a Wi-Fi analyzer to build and check networks.

Key concepts

  • TCP vs. UDP

    TCP is connection-based and resends lost data. UDP sends without checking delivery, so it is faster with less overhead.

    Memory tipMust arrive complete = TCP. Speed over perfection = UDP.

  • DHCP

    A service that gives devices an IP address, subnet mask, default gateway, and DNS server automatically. Uses UDP ports 67 and 68.

    Memory tipNo DHCP reply on Windows = 169.254.x.x address.

  • DNS

    A service that translates host names into IP addresses. Uses port 53.

    Memory tipCan ping an IP but not a name = check DNS.

  • APIPA

    Automatic Private IP Addressing. A Windows PC that cannot get a DHCP address gives itself an address in 169.254.0.0/16.

    Memory tip169.254 = DHCP problem.

  • Switch vs. router

    A switch forwards traffic between devices on the same network using MAC addresses. A router forwards traffic between different networks using IP addresses.

    Memory tipSame network = switch. Between networks = router.

  • 2.4 GHz vs. 5 GHz

    2.4 GHz reaches farther and passes walls better but has fewer channels and more interference. 5 GHz is faster with more channels but shorter range.

    Memory tipIn 2.4 GHz, use channels 1, 6, or 11 to avoid overlap.

  • Toner probe

    A two-part tool: the tone generator puts a signal on a cable, and the probe finds which cable or wall jack carries it.

    Memory tipWhich cable goes to which jack? = toner probe.

Often tested

  • Port pairs to know: FTP 20/21, SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67/68, HTTP 80, POP3 110, IMAP 143, HTTPS 443, SMB 445, RDP 3389.
  • Telnet sends everything, including passwords, in plain text; SSH encrypts the session.
  • 127.0.0.1 is the loopback address; pinging it tests the computer's own network stack.
  • A punchdown tool seats wires into a patch panel or keystone jack; a crimper attaches RJ45 plugs.
  • SNMP (161/162) is used to monitor network devices; LDAP (389) is used to query directory services.

Easy to confuse: Secure and insecure protocols

TaskInsecureSecure choice
Remote command lineTelnet (23)SSH (22)
Web pagesHTTP (80)HTTPS (443)
Remote desktop over the internetOpen RDP (3389) to the internetRDP through a VPN

3 sample questions

Verified practice questions from this unit.

Practice questionReviewedCore 1 (220-1201) › Networking★★★★☆

Question 1. Which port does HTTPS use by default?

  1. ① 80
  2. ② 3389
  3. ③ 22
  4. ④ 443
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ 443

Key point: HTTPS = TCP port 443

HTTPS is web traffic protected by encryption, and it uses port 443 by default. Plain HTTP uses port 80. Remember the pair: 80 for HTTP, 443 for HTTPS.

Wrong choices

  • 80: Port 80 is used by HTTP, the unencrypted version of web traffic.
  • 3389: Port 3389 is used by Remote Desktop Protocol (RDP).
  • 22: Port 22 is used by SSH for secure remote command-line access.
Practice questionReviewedCore 1 (220-1201) › Networking★★★★☆

Question 2. A technician needs a secure, encrypted command-line connection to manage a Linux server remotely. Which protocol should the technician use?

  1. ① Telnet
  2. ② FTP
  3. ③ SSH
  4. ④ RDP
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ③ SSH

Key point: SSH (port 22) = encrypted remote command line

SSH (Secure Shell) uses port 22 and encrypts the whole session, including the login. It is the standard way to manage Linux servers and network devices from the command line.

Wrong choices

  • Telnet: Telnet also gives remote command-line access, but it sends everything, including passwords, in plain text.
  • FTP: FTP transfers files. It is not used for a remote command line, and basic FTP is not encrypted.
  • RDP: RDP gives a graphical remote desktop, mainly for Windows, not a command-line session to a Linux server.
Practice questionReviewedCore 1 (220-1201) › Networking★★★★☆

Question 3. A user's Windows PC cannot reach the internet. The technician sees that its IP address is 169.254.10.5. What does this address mean?

  1. ① The DNS server is not answering
  2. ② The PC has a correctly set static IP address
  3. ③ The PC has a public internet address from the ISP
  4. ④ The PC could not get an address from a DHCP server and gave itself one
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ The PC could not get an address from a DHCP server and gave itself one

Key point: 169.254.x.x (APIPA) = DHCP failed

When a Windows PC is set to get an address automatically but no DHCP server answers, it assigns itself an APIPA address in the 169.254.x.x range. With this address it can only talk to nearby devices on the same network, so it cannot reach the internet. Check the cable or Wi-Fi link and the DHCP server.

Wrong choices

  • The DNS server is not answering: A DNS problem stops names from resolving, but it does not cause the PC to give itself a 169.254 address.
  • The PC has a correctly set static IP address: A 169.254 address is assigned automatically when DHCP fails. It is not a normal static address.
  • The PC has a public internet address from the ISP: 169.254.x.x is a link-local (APIPA) range. It is never a public internet address.
Practice 5 questions from this unit →

AI-written · Verified · Written by AI. Only units that passed a separate AI check (re-reading facts and numbers) are published. Tell us if you find a mistake.