Private NAT for Network Connectivity Center spokes

Private NAT lets you create a Private NAT gateway that works in conjunction with Network Connectivity Center spokes to perform network address translation (NAT) between the following networks:

  • Virtual Private Cloud (VPC) networks: In this scenario, the VPC networks that you want to connect are attached to a Network Connectivity Center hub as VPC spokes.
  • VPC networks and networks outside of Google Cloud (Preview): In this scenario, one or more VPC networks are attached to a Network Connectivity Center hub as VPC spokes and connected to your on-premises or other cloud provider networks through hybrid spokes.

Specifications

In addition to the general Private NAT specifications, Private NAT for Network Connectivity Center spokes has the following specifications:

  • Private NAT uses a NAT configuration of type=PRIVATE to let networks with overlapping subnet IP address ranges communicate. However, only non-overlapping subnets can connect to each other.
  • You need to create a custom NAT rule by referencing a Network Connectivity Center hub. The NAT rule specifies a NAT IP address range from a subnet of purpose PRIVATE_NAT that Private NAT uses to perform NAT on traffic between your connected networks.
  • A Private NAT gateway is associated with subnet IP address ranges in a single region in a single VPC network. This means a Private NAT gateway created in one VPC network cannot provide NAT to VMs in other spokes of the Network Connectivity Center hub, even if the VMs are in the same region as the gateway.

Traffic between VPC networks

The following additional specifications apply for traffic between VPC networks (Inter-VPC NAT):

  • To enable Inter-VPC NAT between two VPC networks, each VPC network must be configured as a VPC spoke of a Network Connectivity Center hub. When you create the spoke, you must ensure that there are no overlapping IP address ranges across your VPC spokes. For more information, see Create a VPC spoke.
  • Inter-VPC NAT supports NAT between Network Connectivity Center VPC spokes only, and not between VPC networks connected using VPC Network Peering.
  • Inter-VPC NAT supports address translation for VPC subnets within a region as well as across regions.

Traffic between VPC networks and other networks

The following additional specifications apply for traffic between VPC spokes and networks outside of Google Cloud (Preview):

  • To enable Private NAT between a VPC network and an on-premises or other cloud provider network:
    1. The VPC network must be configured as a VPC spoke of a Network Connectivity Center hub. If a Network Connectivity Center hub has more than one VPC spoke, you must ensure that there are no subnet overlaps across the VPC spokes. For more information, see Create a VPC spoke.
    2. A hybrid spoke must be attached to the same Network Connectivity Center hub to establish connectivity between the VPC spoke and the network outside of Google Cloud. Hybrid spokes support VLAN attachments for Cloud Interconnect, Cloud VPN tunnels, and Router appliance VMs. For more information, see About connectivity between VPC spokes and hybrid spokes.
  • The Private NAT gateway must be configured in the workload VPC network, not in the routing VPC network that is associated with the hybrid spoke. For more information about workload and routing VPC networks, see Route exchange with VPC spokes.

Basic configuration and workflow

The following diagram shows a basic Private NAT configuration for traffic between two VPC spokes:

Inter-VPC NAT translation example.
Inter-VPC NAT translation example (click to enlarge).

In this example, Private NAT is set up as follows:

  • The pvt-nat-gw gateway is configured in vpc-a to apply to all the IP address ranges of subnet-a in the us-east1 region. Using the NAT IP ranges of pvt-nat-gw, a virtual machine (VM) instance in subnet-a of vpc-a can send traffic to a VM in subnet-b of vpc-b, even though subnet-a of vpc-a overlaps with subnet-c of vpc-b.
  • Both vpc-a and vpc-b are configured as spokes of a Network Connectivity Center hub.
  • The pvt-nat-gw gateway is configured to provide NAT between VPC networks that are configured as VPC spokes in the same Network Connectivity Center hub.

Example workflow

In the preceding diagram, vm-a with the internal IP address 192.168.1.2 in subnet-a of vpc-a needs to download an update from vm-b with the internal IP address 192.168.2.2 in subnet-b of vpc-b. Both the VPC networks are connected to the same Network Connectivity Center hub as VPC spokes. Assume that vpc-b contains another subnet 192.168.1.0/24 that overlaps with the subnet in vpc-a. For subnet-a of vpc-a to communicate with subnet-b of vpc-b, you need to configure a Private NAT gateway, pvt-nat-gw, in vpc-a as follows:

  • Private NAT subnet: Before configuring the Private NAT gateway, create a Private NAT subnet of purpose PRIVATE_NAT, for example, 10.1.2.0/29. Ensure that this subnet doesn't overlap with an existing subnet in any of the VPC spokes attached to the same Network Connectivity Center hub.

  • A NAT rule whose nexthop.hub matches the Network Connectivity Center hub URL.

  • NAT for all address ranges of subnet-a.

The following table summarizes the network configuration specified in the preceding example:

Network name Network component IP address/range Region
vpc-a

 subnet-a 192.168.1.0/24 us-east1
vm-a 192.168.1.2
pvt-nat-gw 10.1.2.0/29
vpc-b

 subnet-b 192.168.2.0/24 us-west1
vm-b 192.168.2.2
subnet-c 192.168.1.0/24
vm-c 192.168.1.3

Private NAT for Network Connectivity Center spokes follows the port reservation procedure to reserve the following NAT source IP address and source port tuples for each of the VMs in the network. For example, the Private NAT gateway reserves 64 source ports for vm-a: 10.1.2.2:34000 through 10.1.2.2:34063.

When the VM uses the TCP protocol to send a packet to the update server 192.168.2.2 on destination port 80, the following occurs:

  1. The VM sends a request packet with these attributes:

    • Source IP address: 192.168.1.2, the internal IP address of the VM
    • Source port: 24000, the ephemeral source port chosen by the VM's operating system
    • Destination address: 192.168.2.2, the update server's IP address
    • Destination port: 80, the destination port for HTTP traffic to the update server
    • Protocol: TCP

  2. The pvt-nat-gw gateway performs source network address translation (SNAT or source NAT) on egress, rewriting the request packet's NAT source IP address and source port:

    • NAT source IP address: 10.1.2.2, from one of the VM's reserved NAT source IP address and source port tuples
    • Source port: 34022, an unused source port from one of the VM's reserved source port tuples
    • Destination address: 192.168.2.2, unchanged
    • Destination port: 80, unchanged
    • Protocol: TCP, unchanged

  3. The update server sends a response packet that arrives on the pvt-nat-gw gateway with these attributes:

    • Source IP address: 192.168.2.2, the update server's internal IP address
    • Source port: 80, the HTTP response from the update server
    • Destination address: 10.1.2.2, which matches the original NAT source IP address of the request packet
    • Destination port: 34022, which matches the source port of the request packet
    • Protocol: TCP, unchanged

  4. The pvt-nat-gw gateway performs destination network address translation (DNAT) on the response packet, rewriting the response packet's destination address and destination port so that the packet is delivered to the VM that requested the update with the following attributes:

    • Source IP address: 192.168.2.2, unchanged
    • Source port: 80, unchanged
    • Destination address: 192.168.1.2, the internal IP address of the VM
    • Destination port: 24000, matching the original ephemeral source port of the request packet
    • Protocol: TCP, unchanged

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