Given a site that has a need for a certain number of addresses, but probably will never need additional networks to fill out a power of two allocation. Should we allocate the larger block to the site anyway to reduce the size of the routing table, or should we allocate two CIDR blocks?
For example, if a site will only realisticly need 6 class C's, should we allocate a block of 8, or a block of four and a block of two.
Given that the immediate threat is the growth of routing tables, not IP address space exhaustion it's better to allocate powers of 2-sized blocks and announce only one route. The IP space overhead is about 30% but routing table savings are at least 50%. Actually, CIDR allocation helps to slow down IP address allocation by providing most of organizations an alternative to class B. We have sliced an eqivalent of 4 calss B to about 100 customers! So the 2008 doomsday estimate may be kind of pessimistic :-) --vadim
Folks, here is something I wrote some time ago on a very related subject. The last paragraph actually answers the question: To my mind it is quite OK to "reserve" a small amount of address space by rounding up. The emphasis here is on *small* in absolute terms. Enjoy Daniel CIDR Assignment Strategy Previously the RIPE NCC has recommended IRs to reserve some address space contiguous to assigned address space for future expansion. The reasoning was that this would further aggregation and keep the routing table sizes down in the long term, while being slightly inefficient on address space usage in the short term. However experience has shown that the address space usage problems created by those reservations outweigh the possible aggregation benefits. If a block of equal size to the one actually assigned is reserved, address space usage is halved at the expense of doubled routing table size. Relatively speaking this looks like a good tradeoff. In absolute terms however substantial amounts of address space are traded in for 1 (in words: one) additional CIDR route. If no reser- vation were made at all and more address space is needed then another non-contiguous block of appropriate size can always be assigned. Since there are now two blocks these can be aggregated into two routes instead of the one which would have resulted if a suitable reservation had been made. Note that this small absolute gain can only be realised if the reserved space indeed fits the need. The reserved space not only decreases address space usage but also creates fragmentation of the address space which makes it difficult to find block of appropriate size for new requests. Considering this it does not make sense to reserve anything but very small amounts of address space or unused parts of CIDR blocks. Thus the current recommendation is to reserve only address space that is needed to "round" the requested space to a suitable block boundary.
participants (2)
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Daniel Karrenberg -
Vadim Antonov