Networking Master Reference

A simple networking reference for study, interviews, troubleshooting, and daily work.

Overview

This guide is meant to be:

Easy to read quickly
Clear enough for fast review
Clean in GitHub
Useful for students, sysadmins, network engineers, and interview prep

1. Network Models
2. IPv4 Addressing Basics
3. IPv6 Basics
4. DNS Reference
5. Common Ports
6. Core Network Protocols
7. Switching, VLANs, and Layer 2
8. Routing Basics
9. NAT, ACLs, Firewalls, and VPNs
10. DHCP Reference
11. Wireless Networking
12. Ethernet, Cabling, and Media
13. Essential Network Commands
14. Troubleshooting Flow
15. Common Network Numbers and Defaults
16. Network Design Concepts
17. Very Common Exam / Interview Terms
18. Mini Glossary

1. Network Models

OSI Model

Layer	Name	Typical Scope
1	Physical	Cables, signals, optics, radio
2	Data Link	MAC, Ethernet, VLAN, STP, switching
3	Network	IPv4, IPv6, routing, ICMP
4	Transport	TCP, UDP
5	Session	Session control
6	Presentation	Encryption, formatting, encoding
7	Application	HTTP, HTTPS, DNS, DHCP, SMTP, SSH

TCP/IP Model

Layer	Name	Typical Scope
1	Link	Physical + Data Link
2	Internet	IP, ICMP, ARP/ND, routing
3	Transport	TCP, UDP
4	Application	DNS, HTTP, SMTP, DHCP, SSH, and more

Common PDU Names

Layer	PDU
L1	Bits
L2	Frame
L3	Packet
L4	Segment (TCP) / Datagram (UDP)

2. IPv4 Addressing Basics

IPv4 Address Size

32 bits
Example: 192.168.1.10

Address Types

Unicast -> one sender to one receiver
Broadcast -> one sender to all devices in a subnet
Multicast -> one sender to a group
Anycast -> one sender to the nearest available destination

Private IPv4 Ranges

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

Special IPv4 Ranges

Range / Address	Purpose
`0.0.0.0`	Unspecified / default route context
`127.0.0.0/8`	Loopback
`169.254.0.0/16`	APIPA / Link-local
`224.0.0.0/4`	Multicast
`240.0.0.0/4`	Reserved
`255.255.255.255`	Limited broadcast
`100.64.0.0/10`	Carrier-Grade NAT (CGNAT)

Subnetting Formula

Total addresses = 2^(32 - prefix)
Usable hosts = 2^(32 - prefix) - 2

Exceptions

/31 is often used on point-to-point links
/32 is a single host route

CIDR / Netmask / Wildcard / Total / Usable

Prefix	Netmask	Wildcard	Total Addresses	Usable Hosts
`/8`	`255.0.0.0`	`0.255.255.255`	16777216	16777214
`/16`	`255.255.0.0`	`0.0.255.255`	65536	65534
`/24`	`255.255.255.0`	`0.0.0.255`	256	254
`/25`	`255.255.255.128`	`0.0.0.127`	128	126
`/26`	`255.255.255.192`	`0.0.0.63`	64	62
`/27`	`255.255.255.224`	`0.0.0.31`	32	30
`/28`	`255.255.255.240`	`0.0.0.15`	16	14
`/29`	`255.255.255.248`	`0.0.0.7`	8	6
`/30`	`255.255.255.252`	`0.0.0.3`	4	2
`/31`	`255.255.255.254`	`0.0.0.1`	2	P2P use
`/32`	`255.255.255.255`	`0.0.0.0`	1	1 host

Useful Subnetting Notes

Network address = first address in the subnet
Broadcast address = last address in the subnet
First usable = network + 1
Last usable = broadcast - 1

Example

For 192.168.1.0/24:

Network: 192.168.1.0
Usable range: 192.168.1.1 - 192.168.1.254
Broadcast: 192.168.1.255

3. IPv6 Basics

IPv6 Address Size

128 bits
Example: 2001:db8::1

IPv6 Key Points

There is no broadcast
IPv6 uses multicast and anycast
Addresses are usually written in hexadecimal
Leading zeros can be removed
One continuous block of zeros can be shortened with ::

Common IPv6 Ranges

Prefix	Purpose
`::1/128`	Loopback
`::/0`	Default route
`fe80::/10`	Link-local
`fc00::/7`	Unique Local Address (ULA)
`2000::/3`	Global unicast
`ff00::/8`	Multicast

Common IPv6 Prefixes

/64 is the standard subnet size
/128 is a single host

IPv6 Essentials

SLAAC -> Stateless Address Autoconfiguration
DHCPv6 -> IPv6 address and option assignment
NDP -> Neighbor Discovery Protocol
ICMPv6 -> required for normal IPv6 operation

4. DNS Reference

What DNS Does

DNS turns names into IP addresses and can also return other types of records.

Common Public DNS Resolvers

Provider	Primary	Secondary
Google	`8.8.8.8`	`8.8.4.4`
Cloudflare	`1.1.1.1`	`1.0.0.1`
Quad9	`9.9.9.9`	`149.112.112.112`
OpenDNS	`208.67.222.222`	`208.67.220.220`
AdGuard	`94.140.14.14`	`94.140.15.15`

Common DNS Record Types

Record	Purpose
`A`	Hostname to IPv4
`AAAA`	Hostname to IPv6
`CNAME`	Alias to another name
`MX`	Mail server
`NS`	Authoritative name server
`TXT`	Text data, SPF, domain verification
`PTR`	Reverse lookup
`SOA`	Start of authority
`SRV`	Service locator
`CAA`	Certificate authority authorization
`NAPTR`	Rewrite / service discovery
`DS`	DNSSEC delegation signer
`DNSKEY`	DNSSEC public key
`RRSIG`	DNSSEC signature

DNS Ports

Port	Protocol	Use
`53`	UDP	Standard queries
`53`	TCP	Zone transfers / large responses
`853`	TCP	DNS over TLS (DoT)
`443`	TCP	DNS over HTTPS (DoH)

DNS Lookup Order (Simplified)

Client -> Resolver -> Root -> TLD -> Authoritative -> Answer

5. Common Ports

Port	Protocol	Service
`20/21`	TCP	FTP
`22`	TCP	SSH
`23`	TCP	Telnet
`25`	TCP	SMTP
`53`	TCP/UDP	DNS
`67/68`	UDP	DHCP
`69`	UDP	TFTP
`80`	TCP	HTTP
`88`	TCP/UDP	Kerberos
`110`	TCP	POP3
`123`	UDP	NTP
`135`	TCP	RPC Endpoint Mapper
`137-139`	TCP/UDP	NetBIOS
`143`	TCP	IMAP
`161/162`	UDP	SNMP / Traps
`179`	TCP	BGP
`389`	TCP/UDP	LDAP
`443`	TCP	HTTPS
`445`	TCP	SMB/CIFS
`500`	UDP	IKE / IPsec
`514`	UDP	Syslog
`520`	UDP	RIP
`546/547`	UDP	DHCPv6
`636`	TCP	LDAPS
`853`	TCP	DNS over TLS
`993`	TCP	IMAPS
`995`	TCP	POP3S
`1194`	UDP/TCP	OpenVPN
`1433`	TCP	Microsoft SQL Server
`1521`	TCP	Oracle DB
`1701`	UDP	L2TP
`1723`	TCP	PPTP
`1812/1813`	UDP	RADIUS Auth/Acct
`2049`	TCP/UDP	NFS
`3306`	TCP	MySQL
`3389`	TCP	RDP
`4500`	UDP	IPsec NAT-T
`5060`	UDP/TCP	SIP
`5061`	TCP	SIP-TLS
`5432`	TCP	PostgreSQL
`5900`	TCP	VNC
`8080`	TCP	HTTP Alternate
`8443`	TCP	HTTPS Alternate

6. Core Network Protocols

Layer 2 / Neighboring

ARP -> IPv4 address to MAC resolution
NDP -> IPv6 neighbor discovery
STP -> prevents switching loops
LLDP -> device discovery
LACP -> link aggregation

Layer 3 / Routing

IPv4
IPv6
ICMP / ICMPv6
OSPF
RIP
EIGRP
BGP

Layer 4 / Transport

TCP -> reliable and connection-oriented
UDP -> fast and connectionless

Application / Services

DNS
DHCP / DHCPv6
HTTP / HTTPS
SMTP / IMAP / POP3
SNMP
NTP
SSH
LDAP
RADIUS / TACACS+

ICMP Message Examples

Echo request / reply
Destination unreachable
Time exceeded
Redirect

TCP 3-Way Handshake

SYN
SYN-ACK
ACK

TCP Common Flags

SYN
ACK
FIN
RST
PSH
URG
ECE
CWR

7. Switching, VLANs, and Layer 2

Key Terms

MAC Address -> Layer 2 hardware address
CAM Table -> switch MAC address table
Broadcast Domain -> devices that receive broadcasts
Collision Domain -> the area where collisions can happen on shared media

Switch Port Types

Access Port -> carries one VLAN
Trunk Port -> carries multiple VLANs
Native VLAN -> untagged VLAN on an 802.1Q trunk

VLAN Basics

A VLAN is logical separation at Layer 2
The common default VLAN is VLAN 1
Inter-VLAN routing needs a Layer 3 device or an SVI

802.1Q Tagging

Adds a VLAN tag to an Ethernet frame
Native VLAN traffic is usually untagged

STP / RSTP

Purpose: stop loops in switched networks

Common RSTP states:

Discarding
Learning
Forwarding

Link Aggregation

LACP is the IEEE standard for bundling links
It improves bandwidth and redundancy

PoE Standards

Standard	Common Name
`802.3af`	PoE
`802.3at`	PoE+
`802.3bt`	PoE++

8. Routing Basics

Routing Concepts

Connected route
Static route
Default route
Dynamic routing
Longest prefix match always wins

Default Routes

IPv4 -> 0.0.0.0/0
IPv6 -> ::/0

Common Routing Protocols

RIP

Distance vector
Metric: hop count
Best for small and simple networks

OSPF

Link-state
Metric: cost
Supports areas
Common in enterprise internal routing

EIGRP

Advanced distance vector / hybrid
Common in Cisco-focused environments

BGP

Path vector
Used between autonomous systems
Built for Internet-scale routing

First Hop Redundancy

HSRP
VRRP
GLBP

Routing Terms

Next hop
Metric
Administrative distance
Convergence
Redistribution
Summarization

9. NAT, ACLs, Firewalls, and VPNs

NAT Types

Static NAT -> one private IP maps to one public IP
Dynamic NAT -> uses a pool of public IPs
PAT / NAPT -> many private IPs share one public IP by using ports

Translation Directions

SNAT -> Source NAT
DNAT -> Destination NAT

ACLs

Standard ACL -> filters mainly by source
Extended ACL -> filters by source, destination, port, and protocol

Firewall Types

Stateless firewall
Stateful firewall
Next-generation firewall (NGFW)

VPN Types

Site-to-site VPN
Remote access VPN
SSL VPN
IPsec VPN
GRE tunnel

IPsec Terms

Term	Meaning
AH	Authentication Header
ESP	Encapsulating Security Payload
IKE	Key exchange
NAT-T	IPsec through NAT using UDP 4500

10. DHCP Reference

DHCP Purpose

DHCP gives devices IP settings automatically.

DHCP DORA Process

Discover
Offer
Request
Acknowledge

Common DHCP Parameters

IP address
Subnet mask
Default gateway
DNS server
Lease time

DHCP Ports

Client: UDP 68
Server: UDP 67

11. Wireless Networking

Wi-Fi Standards

Standard	Band
`802.11a`	5 GHz
`802.11b`	2.4 GHz
`802.11g`	2.4 GHz
`802.11n`	2.4 / 5 GHz
`802.11ac`	5 GHz
`802.11ax`	2.4 / 5 / 6 GHz (Wi-Fi 6 / 6E)

Common Wi-Fi Terms

SSID -> network name
BSSID -> radio MAC identity
Channel Width -> 20 / 40 / 80 / 160 MHz
MIMO -> multiple antennas / streams
RSSI -> signal strength indicator

2.4 GHz Notes

Longer range
More interference
Common non-overlapping channels: 1, 6, 11

5 GHz Notes

Higher throughput
Shorter range
More channels

6 GHz Notes

Cleaner spectrum
Newer devices
Common in Wi-Fi 6E / Wi-Fi 7 setups

Wireless Security

WEP -> old and insecure
WPA -> legacy
WPA2 -> common standard
WPA3 -> stronger modern standard
PSK -> pre-shared key
Enterprise -> 802.1X / RADIUS

12. Ethernet, Cabling, and Media

Common Ethernet Speeds

10BASE-T
100BASE-TX
1000BASE-T
10GBASE-T
25G / 40G / 100G / 400G Ethernet in higher-end environments

Copper Cable Categories

Cable	Typical Use
Cat5e	Common for 1G
Cat6	Better performance
Cat6a	Better for 10G and lower crosstalk

Typical Twisted Pair Distance

Up to 100 meters for standard Ethernet runs

Fiber Types

Multimode Fiber (MMF)
Single-mode Fiber (SMF)

Common Fiber Connector Terms

LC
SC
ST

Transceiver Terms

SFP
SFP+
SFP28
QSFP+
QSFP28

Duplex Settings

Half-duplex
Full-duplex

Important Layer 1 / Layer 2 Issues

Speed mismatch
Duplex mismatch
Bad cable
CRC errors
Interface flaps

13. Essential Network Commands

Windows

ipconfig /all
ping 8.8.8.8
tracert example.com
nslookup example.com
arp -a
route print
netstat -ano
pathping example.com

Linux / macOS

ip addr
ip route
ping 8.8.8.8
traceroute example.com
dig example.com
nslookup example.com
arp -a
ss -tulpn
netstat -tulpn
tcpdump -i eth0
mtr example.com
curl -I https://example.com
nc -vz host port

Cross-Platform Useful Tools

Wireshark -> packet capture and analysis
Nmap -> port scanning and service discovery
iperf3 -> throughput testing
telnet / nc -> simple TCP connectivity testing

Quick Examples

Ping the gateway

ping 192.168.1.1

Check a DNS A record

nslookup example.com
dig example.com A

Trace the path

tracert 8.8.8.8
traceroute 8.8.8.8

Check listening ports

netstat -ano
ss -lntup

14. Troubleshooting Flow

Work Layer by Layer

1) Physical

Power
Cable
Link light
Speed / duplex
SFP / transceiver

2) Data Link

Correct VLAN
MAC learning
STP blocking
Port security

3) Network

IP address
Subnet mask / prefix
Default gateway
Route table
ARP / NDP
ACL / firewall

4) Transport

TCP / UDP port open
Session setup
RST / timeout

5) Application

DNS records
Service status
Certificates
Authentication
Application misconfiguration

Quick Isolation Questions

Can I ping localhost?
Can I ping my own IP?
Can I ping the gateway?
Can I ping a remote IP?
Can I resolve DNS?
Can I reach the service port?
Is the firewall blocking it?

Common Symptoms

Symptom	Likely Cause
No link	Cable, NIC, or switch problem
APIPA address	DHCP failure
Can ping IP but not name	DNS problem
Same VLAN works only	Routing problem
Intermittent drops	Duplex, RF, bad cable, congestion
Slow network	Errors, congestion, MTU, QoS, server load

15. Common Network Numbers and Defaults

General

Item	Common Value
Ethernet MTU	`1500` bytes
Jumbo frame MTU	`9000` bytes
TCP MSS over 1500 MTU	`1460` bytes
HTTPS default port	`443`
DNS default port	`53`
SSH default port	`22`

TTL / Hop Limit

IPv4 uses TTL
IPv6 uses Hop Limit

Common starting values you often see:

64
128
255

QoS / DSCP Terms

Best Effort
AF classes
EF (often used for voice)

Voice / Real-Time Considerations

Low latency
Low jitter
Low packet loss
QoS matters

16. Network Design Concepts

Segmentation

VLANs separate Layer 2 domains
Subnets separate Layer 3 domains
Firewalls enforce policy between zones

Redundancy

Dual links
Dual switches
Link aggregation
First-hop redundancy
Dynamic routing failover

High Availability

Remove single points of failure
Use redundant power and uplinks
Monitor devices and links

Security Best Practices

Disable unused ports
Use SSH instead of Telnet
Use SNMPv3 instead of SNMPv1/v2c when possible
Change default credentials
Use ACLs and firewalls
Segment users, servers, voice, and management networks
Keep firmware and software updated
Use WPA2/WPA3 for wireless
Log to Syslog / SIEM
Protect management plane access

17. Very Common Exam / Interview Terms

Broadcast domain
Collision domain
CIDR
Subnet mask
Wildcard mask
Default gateway
ARP
NAT / PAT
VLAN
STP / RSTP
Trunk / Access port
OSPF / BGP
TCP vs UDP
DNS A / AAAA / CNAME / MX / TXT
DHCP DORA
MTU / MSS
SLAAC / DHCPv6
HSRP / VRRP
QoS
VPN
Load balancer
Reverse proxy

18. Mini Glossary

Term	Meaning
ACL	Access Control List
AF	Assured Forwarding
AH	Authentication Header
APIPA	Automatic Private IP Addressing
ARP	Address Resolution Protocol
AS	Autonomous System
BGP	Border Gateway Protocol
CAM	Content Addressable Memory
CIDR	Classless Inter-Domain Routing
CRC	Cyclic Redundancy Check
DHCP	Dynamic Host Configuration Protocol
DNS	Domain Name System
ESP	Encapsulating Security Payload
FHRP	First Hop Redundancy Protocol
HSRP	Hot Standby Router Protocol
ICMP	Internet Control Message Protocol
IKE	Internet Key Exchange
IPsec	Internet Protocol Security
LACP	Link Aggregation Control Protocol
LDAP	Lightweight Directory Access Protocol
LLDP	Link Layer Discovery Protocol
MAC	Media Access Control
MSS	Maximum Segment Size
MTU	Maximum Transmission Unit
NAT	Network Address Translation
NDP	Neighbor Discovery Protocol
NTP	Network Time Protocol
OSPF	Open Shortest Path First
PAT	Port Address Translation
PDU	Protocol Data Unit
PoE	Power over Ethernet
QoS	Quality of Service
RADIUS	Remote Authentication Dial-In User Service
RSTP	Rapid Spanning Tree Protocol
SFP	Small Form-factor Pluggable
SIP	Session Initiation Protocol
SLAAC	Stateless Address Autoconfiguration
SMB	Server Message Block
SNMP	Simple Network Management Protocol
SOA	Start of Authority
SSH	Secure Shell
SSID	Service Set Identifier
STP	Spanning Tree Protocol
SVI	Switched Virtual Interface
TCP	Transmission Control Protocol
TLS	Transport Layer Security
UDP	User Datagram Protocol
ULA	Unique Local Address
VLAN	Virtual Local Area Network
VPN	Virtual Private Network
VRRP	Virtual Router Redundancy Protocol
WEP	Wired Equivalent Privacy
WPA	Wi-Fi Protected Access

License

This project is licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0).

You are free to share and adapt the content, including for commercial purposes, as long as proper credit is given.

See the LICENSE file for details.

Name		Name	Last commit message	Last commit date
Latest commit History 44 Commits
01-network-models.md		01-network-models.md
02-ipv4-addressing-basics.md		02-ipv4-addressing-basics.md
03-ipv6-basics.md		03-ipv6-basics.md
04-dns-reference.md		04-dns-reference.md
05-common-ports.md		05-common-ports.md
06-core-network-protocols.md		06-core-network-protocols.md
07-switching-vlans-and-layer-2.md		07-switching-vlans-and-layer-2.md
08-routing-basics.md		08-routing-basics.md
09-nat-acls-firewalls-and-vpns.md		09-nat-acls-firewalls-and-vpns.md
10-dhcp-reference.md		10-dhcp-reference.md
11-wireless-networking.md		11-wireless-networking.md
12-ethernet-cabling-and-media.md		12-ethernet-cabling-and-media.md
13-essential-network-commands.md		13-essential-network-commands.md
14-troubleshooting-flow.md		14-troubleshooting-flow.md
15-common-network-numbers-and-defaults.md		15-common-network-numbers-and-defaults.md
16-network-design-concepts.md		16-network-design-concepts.md
17-very-common-exam-interview-terms.md		17-very-common-exam-interview-terms.md
18-mini-glossary.md		18-mini-glossary.md
LICENSE		LICENSE
README.md		README.md

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