# Notes and Constraints

When using a NAT gateway:

- Multiple rules for one NAT gateway can use the same EIP, but the rules for different NAT gateways must use different EIPs.
- Each VPC can only have one NAT gateway.
- Manually adding the default route for a VPC is not allowed.
- Each VPC subnet can only be used in one SNAT rule.
- SNAT and DNAT rules cannot share the same EIP.
- DNAT rules do not support the mapping between an EIP and a virtual IP address.
- If both an EIP and a NAT gateway are configured for a server, data will be forwarded through the EIP.
- When you add an SNAT rule, if the rule is used in the VPC scenario, the custom CIDR block must be a subset of the NAT gateway's VPC subnets. If the rule is used in the Direct Connect scenario, the custom CIDR block must be a CIDR block of a Direct Connect connection and cannot overlap with the NAT gateway's VPC subnets.
- You can configure only one DNAT rule for each port of a server. One port can be mapped to only one EIP.

Parent topic: Overview