This article offers a solution for the issue where the external network becomes unreachable in a guest virtual machine (VM). This issue specifically occurs when the virtual network adapter is attached in the Shared network mode and a Cisco AnyConnect VPN tunnel is established. The root cause of this issue lies in the fact that the Shared Network IPv4 subnet is made routable via the VPN tunnel, leading to connectivity issues with the external network.

Workaround

1. Contact your VPN administrator to suggest a /24 subnet in the subnets 10.0.0.0/8 or 192.168.0.0/16 that is not routed via the VPN tunnel.
   
   Note: This information is crucial and cannot be determined without knowledge of the VPN gateway configuration.

2. Shut down all VMs. Make sure that the VMs are shut down. If any of them are in a suspended state, please run it and then shut down (Actions > Shut Down).

3. Quit Parallels Desktop (click the Parallels icon on the top Mac menu > Quit Parallels Desktop).

     4. Modify the Virtual Network Configuration:

For macOS VMs on Apple Silicon (except the cases when macOS Sonoma is running as a host and VM operating system):

• In your host machine, Open Terminal (Finder > Applications > Utilities > Terminal) and execute the command below:

sudo nano /Library/Preferences/SystemConfiguration/com.apple.vmnet.plist

Note: you will need to enter your Mac password to run the command. No symbols will be shown due to security measures, so just hit the keys and use Return/Enter.

• Locate the following text and replace the text between <string></string> with the subnet address obtained in step (1):

  • <key>Shared_Net_Address</key>

    <string>192.168.64.1</string>

• Save the file by holding down the Control key on your keyboard (labeled as "Ctrl") and pressing the letter "X". Nano will prompt if you want to save any unsaved changes,  press "Y" for "Yes" and then the Enter key.

For Parallels Desktop for AppStore:

• In your host machine, open Terminal (Finder > Applications > Utilities > Terminal) and execute the command below:

open -a TextEdit ~/Library/Group\ Containers/4C6364ACXT.com.parallels.desktop.appstore/Parallels/network.desktop.xml

Note: you will need to enter your Mac password to run the command. No symbols will be shown due to security measures, so just hit the keys and use Return/Enter. 

• In the newly opened TextEdit window, adjust the values of HostIPAddress, DhcpIPAddress, IPScopeStart, and IPScopeEnd for NetworkID = Shared present in the network.desktop.xml file to match the subnet obtained in step (1).

• Go to the "File" menu at the top left of your Mac menu bar and click on "Save" from the dropdown menu.

For Parallels Desktop for Mac with a Standard License:

• In your host machine, open Terminal (Finder > Applications > Utilities > Terminal) and execute the command below:

open -a TextEdit /Library/Preferences/Parallels/network.desktop.xml

Note: you will need to enter your Mac password to run the command. No symbols will be shown due to security measures, so just hit the keys and use Return/Enter.

• In the newly opened TextEdit window, adjust the values of HostIPAddress, DhcpIPAddress, IPScopeStart, and IPScopeEnd for NetworkID = Shared present in the network.desktop.xml file to match the subnet obtained in step (1).

• Go to the "File" menu at the top left of your Mac menubar and click on "Save" from the dropdown menu.

For Other Cases:

• Start Parallels Desktop.
• Click on the Parallels Desktop icon in the Mac menu bar and select Preferences.
• Configure the Shared subnet using the dialog Parallels Desktop > Preferences... > Network > Shared > IPv4.
• Modify the fields to match the subnet address obtained in step (1).