[comp.protocols.tcp-ip] SMTP host number notation

CDC-DDN@DDN3.ARPA (07/30/87)

  
SMTP RFC-821 defines a 'host number' that supplements the regular domain
name and internet address notation.  For example, PERSON @ #354.  (refer
to page 30 in the syntax for <element>, and to page 31 in the last
paragraph.)
  
Does anyone know what this #host number is and where it comes from?  Is
this a feature that is normally expected to be implemented in SMTP, or is
it a left-over from years past (NCP?).  Any pointers will be appreciated.
  

Mike Sanders

CDC-DDN @ DDN3.ARPA