# Understanding VPN-IPv4 Addresses

Because each VPN has its own private address space, the same IP address might be used in several VPNs. To provide for more than one route to a given IPv4 address (each route unique to a single VPN), BGP/MPLS VPNs use route distinguishers (RDs) followed by an IPv4 address to create unique VPN-IPv4 addresses. A route can have only one RD.

The RD contains no routing information; it simply enables you to create unique VPN-IPv4 address prefixes. You can specify the RD in either of the following ways:

- An autonomous system (AS) number followed by a 32-bit assigned number. If the AS number is from the public address space, it must have been assigned to the service provider by the Internet Assigned Numbers Authority (IANA). The service provider can choose the assigned number. We recommend you do not use numbers from the private AS number space.
- An IP address followed by a 16-bit assigned number. If the IP address is from the public IP address space, it must have been assigned to the service provider by IANA. The assigned number may be chosen by the service provider. Use of numbers from the private IP address space is strongly discouraged.

You can create unique VPN-IPv4 addresses by assigning a unique RD to each VRF in your network. However, the optimal strategy depends on the configuration of your network. For example, if each VRF always belongs to only one VPN, you might use a single RD for all VRFs that belong to a particular VPN.