Route Distinguishers are used to create VPN-IPv4 addresses, as specified in
[RFC4364]. The RDs are structured so that every
service provider can administer its own "numbering space" (i.e., can make
its own assignments of RDs), without conflicting with the RD assignments
made by any other service provider. An RD consists of three fields: a
type field, an administrator field, and an assigned number field.
The value of the type field determines the lengths of the other two fields,
as well as the semantics of the administrator field. The administrator
field identifies an assigned number authority, and the assigned number
field contains a number which has been assigned, by the identified
authority, for a particular purpose.