By default, traffic between subnets within the same Azure Virtual Network (VNet) is permitted by Azure's system routes and default Network Security Group (NSG) rules. The AllowVnetInBound rule specifically allows all protocols, including ICMP, from any source within the VNet. However, the Windows Server operating system has its own host-based firewall (Windows Defender Firewall) which, by default, is configured to block incoming ICMP Echo Requests as a security measure. Therefore, while the Azure network delivers the ping packet to the Windows VM, the VM's operating system firewall drops it, preventing a reply.

A. You have not created a VPN to allow traffic between those subnets: A VPN is not required for communication between subnets within the same VNet. Azure's internal routing handles this connectivity by default. B. By default, Azure does not allow ICMP traffic between subnets: This is incorrect. The default AllowVnetInBound security rule in an NSG explicitly permits all traffic, including ICMP, between resources in the same VNet. D. You have not configured a user-defined route for this traffic: Azure's default system routes are sufficient for direct communication between subnets in the same VNet. A UDR is only needed to override this default behavior.

1. Microsoft Azure Documentation on Default Security Rules: "By default, security groups have a

set of default rules... The AllowVnetInBound rule... allows traffic originating from any IP address in

the address space of the virtual network." This rule permits all protocols, including ICMP.

Source: Microsoft Learn, "Network security groups," Default security rules section.

URL: https://learn.microsoft.com/en-us/azure/virtual-network/network-security-groups-overview#d

efault-security-rules

2. Microsoft Windows Documentation on Firewall Behavior: "By default, Windows Firewall blocks

incoming ICMP echo requests, which is the network traffic sent by the Ping tool." To allow it, a

specific firewall rule must be enabled.

Source: Microsoft Learn, "Allow ICMP traffic."

URL: https://learn.microsoft.com/en-us/windows/security/operating-system-security/network-secur

ity/windows-firewall/allow-icmp-traffic

3. Fortinet Documentation on Azure Troubleshooting: Fortinet's Azure troubleshooting guides

frequently point to checking both Azure NSGs and the guest OS firewall as primary steps for

resolving connectivity issues.

Source: Fortinet Docs, "FortiGate Public Cloud for Azure 7.4.2 Administration Guide,"

Troubleshooting section.

URL: https://docs.fortinet.com/document/fortigate-public-cloud/7.4.2/azure-administration-guide/5

82526/troubleshooting