Internet & Networking Cheat Sheet

Internet & Networking Cheat Sheet: IP Addresses, DNS, Protocols, Ports & Network Basics

Computer networking connects devices so they can exchange information and share resources. The internet is a global network of interconnected networks that communicate using standardized protocols.

Understanding networking fundamentals is useful for IT support, software development, cloud computing, cybersecurity, system administration, and technical interviews.

This cheat sheet covers network types, IP addresses, DNS, routers, switches, protocols, ports, subnetting, troubleshooting, network security, and essential terminology.

1. What Is Computer Networking?

Computer networking allows devices such as computers, smartphones, servers, printers, and smart devices to communicate.

Common Networking Objectives

• Share files and resources.

• Access websites and online applications.

• Communicate through email and messaging.

• Connect distributed offices.

• Access cloud services.

• Support remote work.

• Exchange data between applications and servers.

Common Network Types

A VPN is a networking technology, not a geographic network-size category in the same sense as LAN or WAN.

2. How the Internet Works

When you open a website, several networking processes may occur.

1. You enter a domain name, such as example.com.

2. Your device obtains the information needed to locate the destination, often through DNS.

3. The device establishes a connection using the applicable network and transport protocols.

4. For HTTPS, the browser establishes a TLS-protected connection.

5. The browser sends an HTTP request.

6. The server processes the request and returns a response.

7. The browser renders the returned content.

Caching, proxies, content delivery networks, connection reuse, and other mechanisms can change the exact sequence.

Important Internet Components

A home gateway may combine several functions, including routing, switching, firewalling, and Wi-Fi access.

3. IP Addresses

An IP address identifies a network interface or endpoint for communication using the Internet Protocol.

IPv4

IPv4 addresses are 32 bits long and are commonly written as four decimal numbers separated by periods.

Example:

192.168.1.10

Each decimal number ranges from 0 to 255.

IPv4 has a limited address space, so networks use techniques such as private addressing and Network Address Translation.

IPv6

IPv6 addresses are 128 bits long and are written in hexadecimal groups separated by colons.

Example:

2001:db8::10

The 2001:db8::/32 block is reserved for documentation and examples, so this example is not intended to identify a real public destination.

IPv6 provides a vastly larger address space than IPv4.

Public vs. Private IP Addresses

Private IPv4 Ranges

The three standard private IPv4 ranges are:

A private IP address can be reused in many different private networks.

Static vs. Dynamic IP

Static IP: An address configured to remain fixed or assigned with a stable allocation.

Dynamic IP: An address assigned or managed dynamically, often through DHCP.

Dynamic addressing reduces manual configuration, while static or reserved addresses can be useful for infrastructure and services.

4. MAC Addresses

A Media Access Control (MAC) address is a link-layer address used in local network communication.

Example:

00:1A:2B:3C:4D:5E

IP Address vs. MAC Address

A router generally uses IP addressing to forward traffic between networks. On an Ethernet network, devices use link-layer addressing to deliver frames to the next hop.

5. DNS — Domain Name System

DNS translates domain names into records used by internet services.

For example, a browser can use DNS to discover an IP address associated with a domain name.

Common DNS Record Types

How DNS Resolution Works

A simplified resolution process is:

1. An application requests the address for a hostname.

2. The device or local resolver checks available cached information.

3. A recursive resolver obtains the answer if it is not already cached.

4. DNS infrastructure may direct the resolver through root, top-level-domain, and authoritative servers.

5. The resolver returns the answer to the client.

6. The answer may be cached for its configured time to live (TTL).

Actual resolution can vary because of caching, forwarding, encrypted DNS, and network configuration.

Useful DNS Commands

Windows:

nslookup example.com
ipconfig /displaydns
ipconfig /flushdns

Linux or macOS, where available:

nslookup example.com
dig example.com

Clearing a local DNS cache does not clear every intermediate or authoritative DNS cache.

6. TCP vs. UDP

TCP and UDP are transport-layer protocols used by applications communicating over IP networks.

UDP applications can implement their own reliability mechanisms. Some modern web traffic uses QUIC, which operates over UDP and provides features such as reliable streams.

When to Use Each

TCP is useful when the application needs reliable, ordered delivery.

UDP is useful when the application benefits from a simpler transport or needs to manage latency, retransmission, and delivery behavior itself.

The best choice depends on the protocol and application requirements rather than speed alone.

7. Common Network Protocols

ARP is associated with IPv4 on Ethernet-style networks. IPv6 uses Neighbor Discovery instead of ARP.

8. Common Network Ports

A port identifies an application endpoint within a transport protocol such as TCP or UDP.

These are common defaults, not mandatory assignments. Applications can use other ports, and some protocols use additional transport options.

Port Security

• Allow only necessary services through firewalls.

• Avoid exposing database services directly to the internet without a secure, justified design.

• Restrict remote administration to authorized users and networks.

• Disable unused services.

• Keep internet-facing services updated.

• Monitor unexpected connections.

9. Routers, Switches, and Access Points

Router

A router forwards packets between different IP networks.

For example, a home router forwards traffic between the home network and the ISP network.

Switch

A network switch connects devices within a local network and forwards Ethernet frames based on learned MAC addresses.

Managed switches may also support VLANs, monitoring, and other network-management features.

Wireless Access Point

An access point allows wireless devices to connect to a network through Wi-Fi.

Modem and ONT

A modem or optical network terminal (ONT) connects customer equipment to the ISP's access technology.

These devices have different roles, although consumer products often combine several of them.

10. OSI Model

The Open Systems Interconnection (OSI) model describes networking using seven conceptual layers.

The OSI model is a learning and troubleshooting framework. Real-world protocols do not always map neatly to a single layer.

Easy Way to Remember the Layers

From Layer 1 to Layer 7:

Physical → Data Link → Network → Transport → Session → Presentation → Application

A commonly used mnemonic is: “Please Do Not Throw Sausage Pizza Away.”

11. TCP/IP Model

The TCP/IP model describes networking in a more practical internet-protocol framework.

Some references split the link layer into separate data-link and physical layers. Naming conventions vary across textbooks.

OSI vs. TCP/IP

OSI is a seven-layer conceptual model. TCP/IP is commonly represented as a four-layer model aligned with the protocols used for internet communication.

Both models help explain how data moves from an application to the network and back.

12. Subnetting Fundamentals

Subnetting divides an IP network into smaller logical networks.

It helps organize addressing, control broadcast domains in IPv4 networks, and support network planning.

CIDR Notation

CIDR stands for Classless Inter-Domain Routing.

Example:

192.168.1.0/24

The /24 indicates that 24 of the 32 IPv4 address bits are part of the network prefix.

That leaves 8 bits for the remaining address portion.

A conventional IPv4 /24 subnet contains 256 total addresses. In an ordinary subnet, 254 are typically usable host addresses because the network and broadcast addresses are reserved.

Special-purpose networks and point-to-point configurations can have different allocation rules.

Common IPv4 Prefixes

For a conventional IPv4 subnet with network and broadcast addresses, the number of typical usable host addresses is total addresses minus two.

Subnetting Example

For 192.168.1.0/24:

• Network address: 192.168.1.0

• Typical usable host range: 192.168.1.1 to 192.168.1.254

• Broadcast address: 192.168.1.255

• Total addresses: 256

• Typical usable host addresses: 254

Subnetting is an important topic for networking, cloud, and cybersecurity interviews.

13. DHCP — Dynamic Host Configuration Protocol

DHCP automatically provides network configuration to devices.

A DHCP server may provide:

• IP address.

• Subnet mask or prefix information.

• Default gateway.

• DNS resolver addresses.

• Lease duration and other configuration options.

DORA Process

The classic DHCPv4 address-assignment process is commonly remembered as DORA:

1. Discover: Client searches for DHCP servers.

2. Offer: A server offers configuration.

3. Request: Client requests an offered configuration.

4. Acknowledge: Server confirms the assignment.

The process can differ in renewal and other situations.

DHCP Troubleshooting

If a device cannot obtain an address:

• Check the network connection.

• Confirm that the correct Wi-Fi or Ethernet network is selected.

• Check whether the DHCP service is working.

• Check the available address pool.

• Review VLAN and relay configuration where relevant.

• Look for address conflicts.

• Confirm the default gateway and DNS settings.

14. NAT — Network Address Translation

NAT modifies address information as traffic passes through a network device.

A common home-network configuration allows multiple private IPv4 devices to share one public IPv4 address through a form of NAT called Port Address Translation (PAT).

Why NAT Is Used

• Conserves public IPv4 addresses.

• Allows private address spaces to be reused.

• Supports certain network boundary designs.

NAT is not a replacement for a firewall. It does not, by itself, provide complete security or guarantee that internal systems cannot be accessed.

15. Wi-Fi and Wireless Networking

Wi-Fi enables wireless network connectivity using IEEE 802.11 standards.

Important Wireless Concepts

Actual speed and range depend on equipment, interference, walls, distance, channel width, and network configuration.

Wi-Fi Security Best Practices

• Use WPA3 when supported, or a properly configured WPA2 mode.

• Set a strong Wi-Fi password.

• Change default administrator credentials.

• Keep router firmware updated.

• Disable insecure legacy features when practical.

• Create a guest network for visitors when useful.

• Avoid exposing router administration interfaces unnecessarily.

16. Network Security Fundamentals

Network security protects data, devices, and services from unauthorized access and disruption.

Common Controls

Firewall: Filters traffic according to configured rules.

VPN: Protects traffic between VPN endpoints, depending on its configuration.

IDS: Detects suspicious activity and produces alerts.

IPS: Detects and may block suspicious traffic.

Network segmentation: Separates systems into different network zones.

Zero-trust approach: Requires appropriate verification and authorization rather than automatically trusting a user or device because of its network location.

Common Network Threats

• Denial-of-service attacks.

• Unauthorized network access.

• Credential theft.

• Traffic interception.

• DNS abuse.

• Malware communication.

• Misconfigured network services.

• Rogue wireless access points.

Security Best Practices

• Apply least-privilege network access.

• Keep firmware and software updated.

• Use secure protocols.

• Restrict exposed ports.

• Monitor network logs.

• Segment critical systems.

• Protect administrative interfaces.

• Use strong authentication.

• Maintain incident response and recovery plans.

17. Common Network Troubleshooting Commands

Commands vary by operating system, and some require elevated privileges or installed utilities.

Windows

ipconfig
ipconfig /all
ping example.com
tracert example.com
nslookup example.com
netstat -ano

Linux

ip addr
ip route
ping example.com
traceroute example.com
dig example.com
ss -tuln

macOS

ifconfig
netstat -rn
ping example.com
traceroute example.com
nslookup example.com

On Linux and macOS, traceroute, dig, and other utilities may need to be installed separately. Some systems provide alternatives such as tracepath.

What These Commands Do

A failed ping does not necessarily mean a service is unavailable. Firewalls may block ICMP while allowing application traffic.

18. Step-by-Step Network Troubleshooting

When a website or application cannot be reached, investigate systematically.

Step 1: Check Physical or Wireless Connectivity

• Confirm Wi-Fi is enabled or the cable is connected.

• Check whether other devices can connect.

• Review access-point or router status.

Step 2: Check IP Configuration

• Confirm the device has a valid IP address.

• Verify the subnet and default gateway.

• Check for address conflicts or unexpected link-local addresses.

Step 3: Test Local Gateway Reachability

Try reaching the default gateway where permitted.

If this fails, investigate the local network before assuming the problem is with the internet.

Step 4: Test External Connectivity

Test a known external IP address or another suitable destination.

A successful test may indicate that basic IP connectivity works, but it does not prove that all internet services are working.

Step 5: Check DNS

Query the intended hostname and compare the result with the expected configuration.

If IP connectivity works but name resolution fails, investigate DNS settings, resolver availability, and cached records.

Step 6: Check the Application

• Confirm the hostname and URL are correct.

• Verify that the service is running.

• Check the expected port.

• Review firewall and proxy rules.

• Inspect application logs and TLS certificate errors.

Step 7: Document and Verify

Record the symptoms, tests, findings, changes, and final result. After making a change, verify the service and monitor for recurrence.

19. Internet and Networking Interview Questions

What is an IP address?

An IP address identifies an interface or endpoint for communication using the Internet Protocol.

What is the difference between IPv4 and IPv6?

IPv4 uses 32-bit addresses, while IPv6 uses 128-bit addresses.

What is DNS?

DNS is a naming system that provides records used to locate services and resolve domain names.

What is the difference between TCP and UDP?

TCP provides reliable, ordered byte-stream delivery. UDP provides datagram delivery without built-in guarantees of delivery or ordering.

What is a router?

A router forwards packets between IP networks.

What is a switch?

A switch connects devices within a network and commonly forwards Ethernet frames using MAC addresses.

What is DHCP?

DHCP automatically provides network configuration to clients.

What is NAT?

NAT modifies network address information as traffic passes through a device.

What is a port?

A port identifies an application endpoint within a transport protocol.

What is the difference between HTTP and HTTPS?

HTTPS protects HTTP communication using TLS.

What is subnetting?

Subnetting divides an IP network into smaller logical networks.

What is the OSI model?

It is a seven-layer conceptual model used to understand network communication and support troubleshooting.

What does a firewall do?

A firewall applies rules to control network traffic.

What is a VPN?

A VPN creates a protected logical connection between endpoints or networks.

What is the purpose of ping?

Ping helps test reachability and, when responses are received, provides information about round-trip time and packet loss.

20. Internet and Networking Quick Reference

21. Networking Learning Checklist

Fundamentals

• Explain what an IP address does.

• Distinguish IPv4 and IPv6.

• Understand private and public IPv4 addresses.

• Explain DNS and DHCP.

• Distinguish routers, switches, and access points.

Protocols

• Understand TCP and UDP.

• Recognize common network protocols.

• Learn frequently used ports.

• Explain HTTP and HTTPS.

• Understand MAC addressing and ARP.

Practical Skills

• Use IP configuration commands.

• Test connectivity with ping.

• Investigate DNS issues.

• Read routing information.

• Follow a structured troubleshooting process.

Security

• Understand firewalls and VPNs.

• Apply basic Wi-Fi security practices.

• Understand network segmentation.

• Recognize unnecessary exposed services.

• Learn the basics of network monitoring.

Conclusion

Internet and networking fundamentals explain how devices communicate, how domain names lead to network destinations, how data travels between systems, and how network services are protected.

Start with IP addressing, DNS, DHCP, TCP and UDP, common ports, and troubleshooting commands. Then explore subnetting, routing, network security, and cloud networking through practical exercises in authorized environments.