clear in_list flag
FOR each line in the data file
  IF line matches first line of a list THEN
    extract IP
    save IP as source
    set in_list flag
  ELSE
    IF in_list flag set THEN
      SWITCH line
        CASE line matches second list line
          ignore it
        CASE line matches list-address
          save addresses, indexed to current source
        CASE default (i.e. no longer in list)
          clear in_list flag
      ENDSWITCH
    ENDIF
  ENDELSE
ENDFOR