A /27 range of IP Addresses inside my /24 Subnet in Azure vNet isn't working

[#1]

If I assign 10.1.2.95 to a VM... It works without issue, but if I try using .96 thru .127 (aka 10.1.2.96/27 CIDR) then I can no longer access the VM directly (only via a jump box on the same subnet as the VM) and the VM cannot reach anything outside the subnet (including DNS servers, or other VMs in peered vNets).

Has anyone seen this happen in Azure before? I checked all the NSG/UDR settings I could find, and already filled a support ticket, but this is driving me crazy!

[#2]

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@Haguromo wrote:

If I assign 10.1.2.95 to a VM... It works without issue, but if I try using .96 thru .127 (aka 10.1.2.96/27 CIDR) then I can no longer access the VM directly (only via a jump box on the same subnet as the VM) and the VM cannot reach anything outside the subnet (including DNS servers, or other VMs in peered vNets).

Has anyone seen this happen in Azure before? I checked all the NSG/UDR settings I could find, and already filled a support ticket, but this is driving me crazy!

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Hello,

If you are experiencing issues with connectivity when using the IP range 10.1.2.96/27 in Azure, it's possible that there is a configuration or routing issue causing the problem. While it's difficult to provide a specific solution without detailed information about your Azure environment, I can suggest some general troubleshooting steps:

1. Double-check Network Security Group (NSG) rules: Ensure that the NSG associated with your subnet or network interface allows the necessary inbound and outbound traffic. Verify that the rules are correctly configured to allow the desired communication.

2. Check User-Defined Routes (UDR): Review any UDRs that might be in place for the subnet or network interface and confirm that they are not causing unintended routing restrictions. Incorrectly configured UDRs can disrupt connectivity.

3. Verify peering settings: If you have virtual network peering configured, confirm that the peering connections are properly established and that the necessary routes are configured between the peered virtual networks.

4. DNS settings: Ensure that the VMs have the correct DNS server settings configured, either through the Azure DNS resolver or custom DNS settings. Incorrect DNS settings can prevent the VM from resolving external addresses.
5. Review subnet delegation and service endpoints: If you have configured subnet delegation or service endpoints, make sure that they are correctly set up and not causing any conflicts or restrictions.

6. Validate IP address availability: Check if there are any conflicting IP assignments or address overlaps in your Azure environment that could be causing conflicts with the IP range you are trying to use.

7. Engage Azure support: It's great that you have already opened a support ticket. Microsoft Azure support can assist you in troubleshooting the issue further and provide guidance specific to your environment.

Remember to provide detailed information about your Azure configuration, network setup, and any error messages or symptoms you are experiencing when working with Azure support. They will be able to assist you in resolving the connectivity issues you are facing.

Best regard,
N1Poole paybyplatema

[#3]

Hello, Double-check Network Security Group (NSG) rules: Ensure that the NSG associated with the subnet or VM allows the necessary inbound and outbound traffic. Check if any rules are blocking access to the VM or resources outside the subnet. Verify User-Defined Routes (UDR): Review any UDRs applied to the subnet or VM and confirm that they are configured correctly. Incorrect UDR settings could cause routing issues and block connectivity. Check Virtual Network Peering: If the VM needs to communicate with resources in peered virtual networks, verify the peering configuration. Ensure that the peering connections are properly established and allow the necessary traffic between the networks. Validate DNS Configuration: Ensure that the DNS server settings on the VM are correct. Check if the VM is configured to use the appropriate DNS servers, which should be able to resolve both internal and external DNS queries. Review Azure Network Virtual Appliance (NVA) configurations: If you are using NVAs in your network setup, confirm that their configurations are correct and do not interfere with the desired connectivity.