>At 01:53 PM 2/1/01, Fred Danson wrote:
>>I dont understand why the answer "C) Routing protocols must carry the
>>prefix length with the 32bit address" doesn't apply to the question of
>>"When you develop a subent routing scheme, to which guideline must
>>you adhere?"
>
>"Subnet routing scheme" isn't a technical term. You'll notice that the most
>experienced people on the group just scratched their heads when they heard
>it. Maybe Cisco has started using the term, but it would be unfortunate if
>they were. The technical term for the number of bits that mean network
>instead of host is "prefix length." Routing based on prefix length rather
>than classes is called classless interdomain routing (CIDR).
I'd quibble with you a little bit here. The basic mechanism is simply
classless routing, which, IIRC, was introduced under the term
supernetting:
RFC 1338 Supernetting: an Address Assignment and Aggregation Strategy. V.
Fuller, T. Li, J. Yu, K. Varadhan. June 1992.
The broader CIDR effort was a little later, and the main documents are:
1517 Applicability Statement for the Implementation of Classless
Inter-Domain Routing (CIDR). Internet Engineering Steering Group, R.
Hinden. September 1993. (Format: TXT=7357 bytes) (Status: PROPOSED
STANDARD)
1518 An Architecture for IP Address Allocation with CIDR. Y. Rekhter,
T. Li. September 1993. (Format: TXT=72609 bytes) (Status: PROPOSED
STANDARD)
1519 Classless Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy. V. Fuller, T. Li, J. Yu, K. Varadhan. September
1993. (Format: TXT=59998 bytes) (Obsoletes RFC1338) (Status: PROPOSED
STANDARD)
1520 Exchanging Routing Information Across Provider Boundaries in the
CIDR Environment. Y. Rekhter, C. Topolcic. September 1993. (Format:
TXT=20389 bytes) (Status: INFORMATIONAL)
The distinction I'd make between CIDR and generic prefix routing:
1. CIDR was specifically intended to solve some address exhaustion
and routing table size problems in the public Internet.
2. CIDR does not consider the "subnetting a subnet" or VLSM approach.
VLSM certainly is an arbitrary prefix length technique, but its
applicability is inside enterprises rather than among providers.
VLSM is truly a terrible name -- subnet masks are always 32 bits long.
They can contain different numbers of one bits.
3. CIDR includes administrative procedures for global address assignment.
4. There are an assortment of problems caused by workarounds to
classful addressing, such as discontiguous networks, that are
primarily enterprise problems and outside the CIDR scope.
>
>CIDR allows routers to group routes together in order to cut down on the
>quantity of routing information carried by core routers. With CIDR, several
>IP networks appear to networks outside the group as a single, larger
>entity. The grouping of routes is also known as summarization, aggregation,
>or supernetting.
>
>Somebody at Cisco (and we need to find them and take them out) learned
>subnetting and thinks that supernetting is the same thing.
It's probably the same copywriter that, when Sprint announced they
were installing GSR's, boasted how the new hardware was going to
provide customers with greater bandwidth and latency.
>I ran into this
>before. The e-learning version of Designing Cisco Networks had this problem
>too. (I think they pulled that product, I complained so bitterly about it,
>though it might have resurfaced. &;-)
>
>Priscilla
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