Several options, such as Record Route and Timestamp, contain slots into which a router inserts its address when forwarding the packet. However, each such option has a finite number of slots, and therefore a router may find that there is not free slot into which it can insert its address. No requirement listed below should be construed as requiring a router to insert its address into an option that has no remaining slot to insert it into. Section [5.2.5] discusses how a router must choose which of its addresses to insert into an option.