Networking Fundamentals​, Quick Notes – OSI Model based Troubleshooting (Basic)

Some common issues:

  • Unable to connect to the Internet
  • Unable to use a particular network application
  • Unable to access another computer on the same network
  • Unable to access shared folders or printers

General questions:

  • Was it working before?
  • Any recent changes made?
  • Does it happen to a single computer or all computers?
  • Is it application specific?
  • If it’s slow, how many active wired and/or wireless connections? Is the guest access enabled?

 Note: Some software installations might include a 3rd party software to be installed without user’s knowledge which may cause specific issues such as automatic redirection in a browser, blocking services or applications, or even technical issues in operating systems; in such cases, use add/remove programs to filter and remove such programs if necessary.

For effective troubleshooting, it is recommended to follow a standard approach such as:

  • Physical Layer
  • Wired
    • Check if the SOHO Router / Network switch is turned on
    • Check if the SOHO Router has Internet connectivity
      • Solutions
        • Check Internet Status by logging on to the Router or status indicator
        • Check for any loose connections or DSL filter issues (Replace filter)
        • Restart the Router
      • Check if network cable is secure (no loose pins or damaged RJ-45 Jacks) at both ends
        • Solutions
          • Use alternate patch cable
        • Wireless
          • Check if signal strength is good or below average
          • Check placement of Wi-Fi Routers or Access Points
          • Check if there are other devices causing interference to the signal
          • Solutions (Analyze using Wi-Fi Analyzers)
            • Turn Off interfering devices
            • Adjust SOHO Router or Access Point location (Placement of antennas)
            • Use a less crowded channel
            • Use 5 GHz (if available) if there are many 2.4 GHz based devices
          • Data Link Layer
            • Check if the NIC is installed & Working properly
              • Solution
                • Use Device Manager to check NIC status
                • Update Device Drivers (or rollback if an update has caused the problem)
                • Re-install Device Drivers
                • Use a different NIC (or a different slot if it’s PCI/PCIe or different port if it’s USB)
              • Check for incorrect settings or changes made to the NIC
            • Network Layer
              • Check DHCP service is enabled on the SOHO Router
              • Check if the Network Adapters have valid IP addresses
              • Check if the Network Adapter has a valid IP address & Gateway
              • Check if the Network Adapter has valid DNS servers
              • Check if there are any issues communicating to Default Gateway
              • Check if the website is actually working
              • Check if DNS server is reachable & resolving domain names
                • Solutions
                  • Use ipconfig /release & ipconfig /renew
                  • Use Static IP Address & Gateway (If DHCP is NOT working) – Assign IP in the same same range of SOHO Router
                  • Use public DNS Servers for faster name resolution, such as:
                    • Google Public DNS – 8.8.8.8 (Primary) & 8.8.4.4 (Secondary)
                    • OpenDNS Home – 208.67.222.222 (Primary) & 208.67.220.220 (Secondary)
                  • Transport, Session Layer, Presentation & Application Layer
                    • Check Network Application Specific settings
                      • Web Browser
                        • Determine if it is very slow or no connectivity at all
                        • Close unwanted tabs & Windows
                        • Use an alternate browser to check, if available
                        • Check another website, not all websites serve content at super speeds
                        • Clear Browser Cache
                        • Disable Browser Plugins
                      • Email Application
                        • Match Email Settings as per ISP
                        • Check Webmail access, if email application is not working
                        • Check POP3 / IMAP / SMTP, Security & Authentication Settings
                      • 3rdParty Applications
                        • Determine application settings as per vendor including port numbers
                      • Check if Anti-Virus or Firewall is active & blocking any connections
                        • Solutions
                          • Disable Anti-Virus & Firewall Temporarily and Check
                          • Add Executable/Service to exception list of Anti-Virus & Firewall as required
                          • Enable Anti-Virus & Firewall

Unable to access another computer (not Internet), File Sharing Service (Microsoft Windows)

  • Check if the Network is set to “Private”
  • Check if the Network Adapters have valid IP addresses
  • Check Computer Name (Spelling)
  • Check Network Connectivity between computers by Computer Name
  • Check if it’s accessible by IPv4
  • Check if NETBIOS over TCP/IP is enabled
  • Check for Authentication issues (User accounts & Passwords)
  • Check if the remote folder is shared & appropriate permissions are set
  • Check Share Name (May not be same as a the folder name)
  • Solutions
    • Create a new user account for file & print sharing purposes
    • Use HOSTS file to map IPv4 to computer names
    • Add File & Print Sharing Services to Firewall Exceptions List

Unable to connect to a printer on another computer, Print Sharing Service (Microsoft Windows)

  • Check Share Name (May not be same as a the printer name)
  • Check Printer Permissions (Sharing)

Note: Above is NOT applicable for network printers connected to a network switch; applicable only for printers that are connected directly to a computer with Microsoft Windows.

Using Device Manager

  • Management console to manage hardware devices
  • Has pre-defined error codes, ease of troubleshooting

“Device Drivers” or “Drivers” refers to software that controls the hardware.

  • START > RUN > DEVMGMT.MSC

Device manager listing devices

  • Select an NIC, Right-Click & Select “Properties”
  • Check message under “Device Status”

Reference(s):

Steps to install an NIC

  1. Insert the NIC (PCI/PCIe/USB)
  2. Install Device Drivers &
  3. Modify Settings (Only if required)
  4. Insert cable (Wired NIC)
  5. Select SSID (Wireless NIC)

Most recent versions of Microsoft Windows, Linux are designed to detect, install device drivers and configure device automatically (device drivers for many models are included with the operating system package), but this may not work for recently released devices (as device drivers may not be available within the OS package).  In such cases, device drivers may be installed from the CD/DVD included with the device package or, device drivers can be downloaded through Windows Update or, device drivers can be downloaded from respective vendor website to complete the installation.

Package types from vendors:

  • Ready-to-install packages are usually available from most vendors, that should be downloaded and installed. Recommended for most users, as it requires minimal technical intervention.
  • Driver Only Package (usually smaller in size) requires administrator efforts for installations; use Update Driver option in Device Manager (Microsoft Windows) to install device driver.

To view device driver details:

  • START  > RUN > DEVMGMT.MSC
  • Select Driver tab

Driver tab

Common Issues:

A.Unknown device (Device listed like “PCI Device” with no further details)
a)Check the Model/Manufacturer of the NIC
b)Download appropriate drivers (Select exact Operating System / 32 or 64 bit version)
c)Select Update Driver
d)Point to the folder where drivers are extracted
B.Yellow Question Mark (devices drivers not installed)

To install and/or update device drivers
Select “Update Driver Software…”

Options

Select “Search automatically for updated driver software…” – Latest devices drivers will be automatically installed if available.
Select “Browse my computer for driver software” – use this if the device driver has been downloaded from vendor’s website (use this method if the device drivers are downloaded and/or available through CD/DVD).

To Re-install NIC
Right-Click on a device
Select “Uninstall”
Select “Scan for hardware changes” (or restart computer)

Sometimes an updated device driver may cause specific problems; in such scenarios, previous version of a device driver (restore is possible only if the previous device driver was updated, not on a fresh installation) can be restored through the Rollback driver option.

  • Select NIC, Right-Click & Select “Properties”
  • Select “Driver” Tab
  • Select “Roll Back Driver”

Tip: Always check a) Windows Update b) Vendor website for device drivers. Never download drivers from unknown sources as it may cause other problems. Download specific device driver version if required.

  • Check NIC Settings
    • START > RUN > DEVMGMT.MSC
    • Select NIC, Right-Click & Select “Properties”
    • Select “Advanced” Tab

Settings of a wired NIC (Top) and Wireless NIC (Bottom)

Note: Settings may differ depending on the model, requires technical expertise to modify. It is highly recommended to leave the settings to “AUTO”; this means the settings will be managed by the operating system. If unsure, reinstall the NIC. MODIFY THESE SETTINGS, only if you have a clear understanding.

  • Assign Static IP Address & Gateway (If DHCP server has not assigned any IP address):
  • CMD > netsh interface ipv4 set address ”CONNECTIONNAME” static ipv4 subnet mask gateway

C:\netsh interface ipv4 set address “Local Area Connection” static 192.168.1.51 255.255.255.0 192.168.1.1

Sample input to setup an IPv4 Address

Sample Output displaying DHCP Status as disabled and, assigned static Pv4 address

  • Assign static DNS IPv4 address via command line:
  • CMD > netsh interface ipv4 set dns name=”CONNECTIONNAME”static DNSIP1

Sample input to use 8.8.8.8 as the primary DNS server

Output listing DNS Server IPv4 Address

  • CMD > netsh interface ip add dns name=”CONNECTIONNAME” DNSIP2 index=x

Sample Input to add secondary DNS

  • Use DNS Servers from DHCP settings
    • CMD > netsh interface ip set dnsservers name=”CONNECTIONNAME” source=dhcp

Input to use DNS servers from DHCP

  • Add second IPv4 address
    • CMD > netsh interface ipv4 add address “CONNECTIONNAME” IPv4 Subnet Mask Gateway

 Input to add second IPv4 address

  • CMD > ipconfig

Output listing two IPv4 addresses assigned to a single connection

  • Enable DHCP Client:
  • CMD > netsh interface ipv4 set address name=”CONNECTIONNAME” dhcp

Sample input to enable DHCP client

  • Force IPv4 address to be released (before requesting for a new IP address)
    • CMD >ipconfig /release

Input to release existing dynamic IP address (only on DHCP Clients)

  • Request for IPv4 address
    • CMD > ipconfig /renew

Input to request for a IP address (only on DHCP Clients)

Note: DHCP client may be offered the same IP address as before, if that IP is not leased to any other client. DHCP Client can receive the same IP address, if it’s reserved on a DHCP Server.

Tip: Any number of IP addresses can be added to a single network card, though there may be technical limitations imposed through operating system.

  • Advanced Troubleshooting
  • Reset Windows Socket
    • CMD > netsh winsock reset

 Input to reset Windows Socket

  • Reset TCP/IP
    • CMD > netsh int ip reset

Input to reset TCP/IP

  • Restart computer
  • To create a log file for reference:
    • CMD > netsh int ip reset > C:\Log.txt

Addressing SOHO Router Problems

Similar to a computer, a SOHO Router has a CPU+ROM+RAM and a micro operating system to manage all the internal operations of a Router. Most routers are never turned off, which may cause unknown and/or unexplainable issues due to prolonged use; in such scenarios, perform a cold reboot and confirm if the issue is resolved.

Some manufacturers fix specific issues and release updated micro operating systems referred to as “Firmware” time-to-time, which should be updated if available. SOHO Routers can also be reset to factory settings, but details related to network & ISP settings must be available to reconfigure (if required). Updating Firmware requires technical expertise, it is recommended to take extra caution.

Tip: Download Firmware only from Manufacturer’s website.

Server Message Block

  • SMB protocol is used for file, print & resource sharing on Microsoft Windows Networks
  • Used with NetBIOS over TCP/IP
  • TCP ports: 137 & 139
  • UDP ports: 137 & 138

NBTSTAT is a utility for troubleshooting NetBIOS over TCP/IP (Microsoft Windows based networks).

  • To check settings for NETBIOS over TCP/IP:
    • START > RUN > NCPA.CPL, Right-click “Local Area Connection” & Select “Properties”
    • Select “Internet Protocol Version 4 (TCP/IP)” & Select “Properties”
    • Select “Advanced” & Select “WINS” Tab

TCP/IP settings, Enable NETBIOS over TCP/IP option

  • Select “Enable NetBIOS over TCP/IP”, Select “OK” Twice
  • Select “Close”
  • View Statistics
    • CMD > nbtstat -r

Input to view statistics

  • View Cache
    • CMD > nbtstat -c

Input to view cache

  • View Sessions
    • CMD > nbtstat -s

Input to view sessions

  • Add File & Print Sharing Services to Windows Firewall Exceptions List
    • START > RUN > FiREWALL.CPL
    • Select “Allow an app or feature through Windows Defender Firewall”
    • Select “Change Settings”

  • Scroll and locate “File and Printer Sharing”

  • Check under Private, Select OK
  • Change Public to Private Network via Powershell
    • CMD > Powershell
    • Powershell >  Get-NetConnectionProfile

  • Powershell > Set-NetConnectionProfile -Name “CONNECTION” -NetworkCategory Private

 
   

Using Troubleshooter

Troubleshooter included within the Operating System (Microsoft Windows) may be used for identifying and fixing certain issues.

  • To launch troubleshooter:
    • Control Panel> Troubleshooting

Troubleshooting Applet

  • For example, select “Connect to the Internet” under Network and Internet to initiate troubleshooting Internet connectivity related issues.

Summary

Hope the articles were useful enough to know the basics of networking; though not covered in-depth, we request our readers to learn more through authentic websites/tutorials for further knowledge.

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