VPN customers often manage their own networks and use the RFC-1918 private address space. If globally unique IPv4 addresses are not used, the same 32-bit IPv4 address can be used to identify different systems in different VPNs. This causes routing problems because BGP assumes that each IPv4 address that it carries is globally unique. To solve this problem, BGP/MPLS VPN converts non-unique IPv4 addresses into unique VPN-IPv4 address families. Each IPv4 address is prepended with an 8-bytes long Route Distinguisher (RD).
A VPN-IPv4 address is a 12-byte quantity composed of an 8-byte RD and 4 byte IPv4 address. The 8-byte RD is composed of a 2-bytes type field and a 6-bytes value field. The value of the Type field determines the lengths of the Value field‘s two subfields (Administrator and Assigned Number), as well as the semantics of the Administrator field.
Most commonly, the Admin field holds a 2-byte non-private AS number and the Assigned number field holds a 4-byte number assigned by the VPN service provider. Another common format for the RD is the Admin field holds a global 4-byte IPv4 address, and the Assigned number field holds 2 byte number assigned by the VPN service provider.