CIDR FAQ

Fri Aug 18 18:00:21 UTC 1995

At 8:47 PM 8/17/95, Gary Wright wrote:
>Renumbering isn't necessarily required if the blocks are given out
>such that there is room for growth should the smaller IP come back
>for more addresses.  For example when I allocate individual class
>C addresses, I don't give them out consecutively. Initially I leave
>at least three unused addresses after the one I allocate.  If
>someone comes back and needs more, their allocation grows to fill
>up the holes.  I try to judge who will grow (e.g., a town network)
>and who won't (a BBS) and leave my options open as long as possible.

One can also accomplish this with a lot less thought by simply making each
allocation be as far away from all other allocations as possible within a
given CIDR block.  For example, allocate xx.xx.0/24 first, xx.xx.128/24
second, xx.xx.64/24 third, xx.xx.192/24 fourth, xx.xx.32/24 fifth, etc.

If one only allocates 50% of the space using that kind of an algorithm,
then every single prefix can be doubled to become a /23.  If one only
allocates 25% or the space, then every single prefix can be quadrupled to
become a /22.

Of course, we'd have to convince the authorities that 25% to 50%
utilization of a CIDR block in this manner is a good thing so we could get
new /16 or bigger blocks...