100-490 General Networking Knowledge
General Networking Knowledge
Detailed list of 100-490 knowledge points
General Networking Knowledge (Additional Content) Detailed Explanation
1. Common Network Commands
Understanding basic network-related commands is critical for troubleshooting and diagnosing connectivity issues. These commands are typically used on Windows or Cisco CLI and are frequently tested in entry-level certifications.
ping
Purpose: Tests basic connectivity between two devices.
Example:
ping 192.168.1.1Explanation: Sends ICMP Echo Requests to the target IP and waits for replies. If successful, it confirms that the remote device is reachable.
ipconfig (Windows)
Purpose: Displays current IP configuration of the host.
Example:
ipconfigExplanation: Shows IP address, subnet mask, and default gateway. Useful for checking if the host has a valid address.
tracert (Windows)
Purpose: Shows the path a packet takes to reach a destination.
Example:
tracert 8.8.8.8Explanation: Helps identify where delays or failures occur across the network path.
netstat
Purpose: Displays current network connections and listening ports.
Example:
netstat -anExplanation: Useful for checking which applications are using which ports, or for detecting suspicious activity.
nslookup
Purpose: Resolves a domain name to an IP address using DNS.
Example:
nslookup www.cisco.comExplanation: Helps verify DNS server functionality and name resolution.
2. Data Encapsulation and De-encapsulation in the OSI Model
When data travels across a network, it undergoes encapsulation (on the sender side) and de-encapsulation (on the receiver side). Each layer of the OSI model adds or removes a header (and sometimes a trailer) to the data.
Encapsulation (Sender Side)
As data flows from the Application Layer (Layer 7) down to the Physical Layer (Layer 1):
| OSI Layer | Unit Name | What’s Added |
|---|---|---|
| Application / Presentation / Session | Data | No header; logical data |
| Transport Layer | Segment | TCP/UDP header |
| Network Layer | Packet | IP header |
| Data Link Layer | Frame | MAC header + trailer |
| Physical Layer | Bits | Converted to electrical/optical signals |
De-encapsulation (Receiver Side)
At the receiving device, the process reverses:
- Each layer removes its corresponding header and processes the data before passing it up to the next layer.
Exam Tip:
Questions often ask:
"At which layer is the IP address added?" → Network Layer
3. VLAN and NAT – Key Concepts
While not the core focus of the 100-490 exam, both concepts appear indirectly in configuration-based and troubleshooting questions.
VLAN (Virtual Local Area Network)
Definition: A VLAN logically segments a physical network into multiple broadcast domains.
Use Case: Helps isolate traffic (e.g., staff vs. guest networks).
Device Involved: Switch
Example: VLAN 10 for HR, VLAN 20 for IT
NAT (Network Address Translation)
Definition: Translates private IP addresses to a public IP address (and vice versa).
Use Case: Enables multiple internal devices to share a single public IP for internet access.
Device Involved: Router or firewall
Types: Static NAT, Dynamic NAT, PAT (Port Address Translation)
4. Terminology Matching Table (Layer Functions, Protocols, Devices)
To help with “match the concept to the layer” questions, here is a quick-reference chart:
| OSI Layer | Function | Example Protocols | Devices |
|---|---|---|---|
| Application (7) | User interface and network apps | HTTP, FTP, SMTP | - |
| Presentation (6) | Data formatting/encryption | SSL, JPEG, ASCII | - |
| Session (5) | Manages sessions/connections | NetBIOS, RPC | - |
| Transport (4) | End-to-end communication | TCP, UDP | - |
| Network (3) | Logical addressing/routing | IP, ICMP | Router |
| Data Link (2) | MAC addressing, framing | Ethernet, ARP | Switch, Bridge |
| Physical (1) | Transmission of bits | Cables, voltages | Hub, Cable, NIC |
Frequently Asked Questions
What is the key difference between the OSI model and the TCP/IP model when explaining network communication?
The OSI model is a conceptual 7-layer framework for understanding networking functions, while the TCP/IP model is a practical 4-layer model used in real-world Internet protocols.
The OSI model separates networking tasks into seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. This separation helps engineers diagnose problems by isolating where communication fails. The TCP/IP model consolidates these functions into four layers: Network Interface, Internet, Transport, and Application. Real networking protocols such as IP, TCP, UDP, and HTTP align directly with the TCP/IP model. Cisco troubleshooting often references OSI layers conceptually but actual device operations and protocol stacks follow the TCP/IP structure. A common mistake is assuming OSI layers directly map one-to-one with protocol implementations.
Demand Score: 72
Exam Relevance Score: 88
Why might a device successfully respond to ping but fail to establish an SSH connection?
Ping may succeed because ICMP traffic is allowed, while SSH fails due to blocked TCP port 22 or disabled SSH services on the device.
Ping uses ICMP Echo Requests at the network layer, which only confirms IP connectivity. SSH, however, operates at the transport and application layers using TCP port 22. If an access control list blocks TCP port 22, or if the SSH service is not enabled on the device, the SSH connection will fail even though ICMP traffic works. Another possibility is that the device has reached its maximum number of VTY sessions. Network engineers frequently misinterpret successful pings as proof that all services are reachable, but ping only verifies basic IP reachability, not application-level access.
Demand Score: 67
Exam Relevance Score: 82
What information does a subnet mask provide in an IPv4 address?
A subnet mask identifies which portion of an IPv4 address represents the network and which portion represents the host.
An IPv4 address consists of 32 bits. The subnet mask separates these bits into network and host segments. For example, the mask 255.255.255.0 corresponds to a /24 prefix where the first 24 bits represent the network and the remaining 8 bits represent host addresses within that network. Devices compare their own network portion with the destination address to determine whether traffic should be sent locally or forwarded to a router. Incorrect subnet masks often cause devices to treat remote hosts as local, leading to connectivity failures. Understanding this division is essential when configuring device interfaces or troubleshooting routing issues.
Demand Score: 61
Exam Relevance Score: 85
What is the functional difference between Telnet and SSH when accessing a network device?
Telnet provides unencrypted remote access, while SSH provides encrypted and secure remote access.
Telnet transmits all session data, including usernames and passwords, in plain text. This makes it vulnerable to packet sniffing and man-in-the-middle attacks. SSH uses encryption to protect both authentication credentials and session data during transmission. SSH also supports stronger authentication methods such as public key authentication. Because of these security advantages, SSH has largely replaced Telnet for remote management of network devices. However, Telnet may still appear in legacy environments or troubleshooting scenarios where encryption overhead is not required. Engineers must understand both protocols because Cisco devices historically supported Telnet before SSH became standard.
Demand Score: 59
Exam Relevance Score: 84
