Resource Cheatsheet
A+ Core 1: Networking Professor Messer A+ 220-1201 section 2 — IP, TCP/UDP and ports, wireless, network services, DNS/DHCP/VLAN/VPN, devices, IP addressing, connection types and tools.
Updated 22 Sept 2026
Reading time 11 min read
Detail A+ Core 1
Detail Networking
In one line: IP gets packets to the right machine, ports get them to the right program, and a handful of services (DNS, DHCP) make it all automatic.
Companion to: Professor Messer, CompTIA A+ 220-1201 Core 1 course, section 2 (objectives 2.1–2.8).
2.1 IP, TCP, UDP and ports
IP moves packets between addresses.TCP and UDP ride on IP, and ports pick the application.IP address + port = a socket , e.g. 192.168.1.10:443.
TCP
UDP
Connection-oriented — three-way handshake: SYN → SYN-ACK → ACK
Connectionless — just sends
Reliable: acknowledgements, retransmits, ordering, flow control
No delivery guarantee; low overhead
Web, email, SSH, file transfer, RDP
DNS queries, DHCP, streaming, VoIP, gaming
Port range
Use
0–1023
Well-known (non-ephemeral) — servers listen here
Higher ports
Ephemeral — temporary client ports (Windows uses 49152–65535)
Common ports (220-1201 list)
Port
Protocol
Transport
Use
20 / 21
FTP
TCP
File transfer (data / control) — cleartext
22
SSH
TCP
Encrypted remote shell; SFTP and SCP too
23
Telnet
TCP
Cleartext remote shell — legacy
25
SMTP
TCP
Mail between servers
53
DNS
UDP / TCP
Name resolution (TCP for zone transfers and large replies)
67 / 68
DHCP
UDP
Automatic IP configuration (server / client)
80
HTTP
TCP
Web — cleartext
110
POP3
TCP
Download mail
143
IMAP
TCP
Mail kept in sync on the server
389
LDAP
TCP / UDP
Directory queries (Active Directory)
443
HTTPS
TCP
Web over TLS
445
SMB
TCP
Windows file and printer sharing
636
LDAPS
TCP
LDAP over TLS
3389
RDP
TCP
Remote Desktop
Real-world extras not on this exam’s list: 587 SMTP submission · 993 IMAPS · 995 POP3S · 137–139 NetBIOS · 161/162 SNMP · 514 syslog.
Check a port
Command
Can I reach it?
Test-NetConnection server01 -Port 443 · Linux: nc -zv server01 443
What’s listening here?
Get-NetTCPConnection -State Listen · netstat -ano · Linux: ss -tulpn
2.2 Wireless
Band
Traits
2.4 GHz
Longer range, more interference, few channels — non-overlapping 1, 6, 11
5 GHz
Faster, many more channels, shorter range; some channels need radar detection (DFS)
6 GHz
Wi-Fi 6E / 7 — lots of clean spectrum, shortest range
Channel width: 20 / 40 / 80 / 160 MHz (320 MHz on Wi-Fi 7). Wider is faster but more prone to interference.Regulations: allowed channels and power vary by country.
Standard
Name
Bands
802.11n
Wi-Fi 4
2.4 / 5 GHz
802.11ac
Wi-Fi 5
5 GHz
802.11ax
Wi-Fi 6 / 6E
2.4 / 5 GHz (6E adds 6 GHz)
802.11be
Wi-Fi 7
2.4 / 5 / 6 GHz
Other wireless
Notes
Bluetooth
2.4 GHz personal area network; around 10 m
RFID
Tags read by radio — badges, inventory, tolls. Passive tags are powered by the reader
NFC
13.56 MHz, a few cm — payments, pairing, access control
Copy netsh wlan show interfaces # connected SSID, signal, band, channel netsh wlan show networks mode = bssid # nearby networks with channels — a quick Wi-Fi survey Copy nmcli dev wifi list # Linux equivalent 2.3 Network services
Service
Job
DNS server
Names → IP addresses
DHCP server
Hands out IP configuration automatically
File share
Central storage (SMB on Windows, NFS on Linux)
Print server
Central print queues and drivers
Mail server
Sends (SMTP) and stores (IMAP/POP3) email
Syslog
Collects logs from many devices in one place
Web server
Serves websites and web apps
AAA
Authentication, authorisation, accounting — e.g. RADIUS, TACACS+
Spam gateway
Filters email before it reaches users
All-in-one security appliance (UTM)
Firewall, IPS, URL filter, anti-malware and VPN in one box
Load balancer
Spreads traffic across servers; removes failed ones
Proxy server
Makes requests on users’ behalf — caching, filtering, logging
SCADA / ICS
Controls industrial equipment — keep it isolated
Legacy and embedded systems
Old or single-purpose systems that can’t be easily patched
IoT
Smart devices: cameras, sensors, thermostats
2.4 DNS, DHCP, VLANs and VPNs
DNS records
Record
Maps
A
Name → IPv4
AAAA
Name → IPv6
CNAME
Name → another name (alias)
MX
Domain → its mail servers
TXT
Free text — used for email authentication, below
Email authentication (TXT)
Does
Example
SPF
Lists servers allowed to send for the domain
v=spf1 include:spf.protection.outlook.com -all
DKIM
Publishes the public key that verifies mail signatures
At <selector>._domainkey.example.com
DMARC
Tells receivers what to do when SPF/DKIM fail, and where to send reports
v=DMARC1; p=reject; rua=mailto:dmarc@example.com
SPF endings: -all = hard fail, ~all = soft fail.DMARC policies: p=none, quarantine or reject.
Copy nslookup example.com nslookup - type = mx example.com nslookup - type = txt _dmarc.example.com Resolve-DnsName example.com - Type TXT ipconfig / displaydns # local DNS cache ipconfig / flushdns # clear it Copy dig example.com MX +short dig TXT _dmarc.example.com +short DHCP
Term
Meaning
DORA
D iscover → O ffer → R equest → A cknowledge (broadcast; UDP 67 / 68)
Scope / pool
The range of addresses handed out, plus options (gateway, DNS, domain)
Exclusion
Addresses inside the scope that are never handed out
Reservation
A fixed address always given to one MAC address
Lease
How long the client keeps the address; renewal is attempted at 50 %
Copy ipconfig / all # DHCP server, lease obtained and expires ipconfig / release ipconfig / renew VLANs and VPNs
VLAN: splits one physical switch into separate broadcast domains — e.g. staff, guests, IoT. Traffic between VLANs needs a router or layer-3 switch.VPN: an encrypted tunnel over an untrusted network.
Client-to-site: a remote worker’s VPN client connects to a concentrator or firewall.Site-to-site: a permanent tunnel between two offices’ firewalls.
2.5 Network devices
Device
Role
Router
Forwards traffic between networks by IP address; home routers also do NAT, DHCP and Wi-Fi
Unmanaged switch
Plug-and-play; forwards by MAC address; no configuration
Managed switch
Configurable: VLANs, QoS, port mirroring, monitoring
Access point
Bridges wireless clients onto the wired network
Patch panel
Cable runs punched down at the back, patch leads at the front
Firewall
Allows or blocks traffic by rule; next-gen firewalls understand applications
PoE
Power over the Ethernet cable — APs, phones, cameras
Cable modem
ISP handoff over coax (DOCSIS)
DSL modem
ISP handoff over the phone line
ONT
Optical network terminal — fibre in, Ethernet out
NIC
The host’s network interface; holds the MAC address
PoE standard
At the switch
At the device
802.3af (PoE)
15.4 W
12.95 W
802.3at (PoE+)
30 W
25.5 W
802.3bt (PoE++)
60 W / 90 W
51 W / 71.3 W
2.6 IPv4, IPv6 and assigning addresses
IPv4
Detail
Structure
32 bits as four octets (0–255), e.g. 192.168.1.20
Subnet mask
Splits network from host: 255.255.255.0 = /24
Private ranges
10.0.0.0/8 · 172.16.0.0/12 (172.16–172.31) · 192.168.0.0/16 — NAT translates them to a public IP
Loopback
127.0.0.1 — “this machine”
APIPA
169.254.x.x — self-assigned when DHCP failed ; only reaches the local segment
IPv6
Detail
Structure
128 bits, eight groups of four hex digits
Shortening
Drop leading zeros; replace one run of all-zero groups with ::. 2001:0db8:0000:0000:0000:0000:0000:0001 → 2001:db8::1
Link-local
Starts fe80:: — every interface has one
Loopback
::1
Assignment
When
Static
Servers, printers, network gear — set IP, mask, gateway and DNS by hand
Dynamic (DHCP)
Clients — automatic
DHCP reservation
Static-like, but managed centrally
APIPA
Automatic fallback when DHCP doesn’t answer — a symptom, not a fix
Copy Get-NetIPConfiguration New-NetIPAddress - InterfaceAlias "Ethernet" - IPAddress 192.168 . 1.50 - PrefixLength 24 - DefaultGateway 192.168 . 1.1 Set-DnsClientServerAddress - InterfaceAlias "Ethernet" - ServerAddresses 192.168 . 1.1 ncpa.cpl # the GUI route Copy ip a # addresses ip route # default gateway resolvectl status # DNS servers (systemd systems) 2.7 Internet connection and network types
Connection
Notes
Fibre
Fastest; ONT at the premises
Cable
Coax (DOCSIS); bandwidth shared with the neighbourhood
DSL
Phone line; download faster than upload; slows with distance from the exchange
Satellite
Geostationary: high latency. Low-earth-orbit constellations: much lower
Cellular
4G / 5G routers and hotspots
WISP
Wireless ISP — fixed antenna, line of sight to a tower
Network type
Scale
PAN
Personal — Bluetooth around one person
LAN
One building or site
WLAN
Wireless LAN
WAN
Connects distant sites — the internet is the largest
SAN
Storage area network — block storage for servers
Tool
Use
Crimper
Attaches RJ45 connectors to cable
Cable tester
Checks continuity and pinout
Punch-down tool
Seats wires into patch panels and keystone jacks
Tone generator and probe
Traces a cable to its other end through walls or a patch panel
Loopback plug
Tests a port or NIC by looping its signal back
Wi-Fi analyzer
Shows networks, channels, signal strength and interference
Network tap
Copies traffic for monitoring or analysis
🔐 Security notes
Cleartext protocols expose credentials:
Insecure
Use instead
Telnet 23
SSH 22
FTP 20/21
SFTP 22
HTTP 80
HTTPS 443
LDAP 389
LDAPS 636
Never expose RDP 3389 or SMB 445 to the internet. Both are leading ransomware entry points. Put RDP behind a VPN or gateway.
Rogue DHCP servers can hand out an attacker’s gateway or DNS server. Managed switches block them with DHCP snooping. Check ipconfig /all shows the expected DHCP and DNS servers.
SPF, DKIM and DMARC are the first things to check on a spoofed-email ticket. A domain with no DMARC record is easy to impersonate.
Segment with VLANs. IoT devices, cameras, guests and SCADA shouldn’t share a network with staff PCs.
Change default credentials on routers, APs, cameras and IoT devices, and keep their firmware updated.
Know what’s listening: Get-NetTCPConnection -State Listen or ss -tulpn. Unexpected listeners are worth investigating.
Practice drills
1. A PC shows 169.254.33.7. What happened and what's your first step?
APIPA — DHCP didn’t answer. Check the cable or Wi-Fi link, then ipconfig /release and ipconfig /renew. Then check the DHCP server or scope.
2. Which ports would you allow for secure web, remote shell and Remote Desktop?
443, 22 and 3389 — with RDP only over a VPN.
3. Mail from your domain is landing in spam at other companies. Which records do you check?
SPF, DKIM and DMARC — e.g. nslookup -type=txt example.com and nslookup -type=txt _dmarc.example.com.
4. Shorten fe80:0000:0000:0000:0a1b:0000:0000:0042.
fe80::a1b:0:0:42 — only one :: is allowed, so the second zero run is written out.
5. A printer's address keeps changing. Two fixes?
A DHCP reservation for its MAC address, or a static IP outside the DHCP scope.
6. You need to find which wall port a cable in the comms room goes to. Which tool?
Tone generator and probe.
Key takeaways
TCP is reliable and connection-based; UDP is fast and fire-and-forget. Ports pick the application.
Memorise the 220-1201 port list — and know the secure alternatives.
2.4 GHz = range and 1/6/11; 5 and 6 GHz = speed.
DNS records (A, AAAA, CNAME, MX, TXT plus SPF/DKIM/DMARC), DHCP (DORA, scope, reservation, lease), VLANs split networks, VPNs tunnel them.
169.254.x.x means DHCP failed. Private ranges are 10/8, 172.16/12 and 192.168/16.