The identifier is a domain name which can be matched against
the user domain as received in the PPP user authentication
information. For example if a local PPP layer was connected to
a downlink of this Layer 2 Dial layer, and it's remote PPP peer
requested that an address be assigned to it, the local PPP could
request this address from the Layer 2 Dial layer, which would
allocate it from a subnet defined in this table.