Questions on RFC 2317

[Joseph S D Yao]

> " Let us assume we have assigned the address spaces to three different
> parties as follows:
> 
>            192.0.2.0/25   to organization A
>            192.0.2.128/26 to organization B
>            192.0.2.192/26 to organization C"
> 
> Sounds silly, but what do those slashes mean? They seem crucial to the
> whole RFC, but they have me lost.

Since 1993, there has been a move afoot to completely get rid of the
notions of "Class A", "Class B", and "Class C" networks.  To the extent
that widely deployed software still implicitly believes in those
notions, this move is still struggling.  But most current systems now
accept the concpet of Classless Inter-Domain Routing (CIDR).

The basic premise is that, instead of defined breaks in the 32-bit host
address at 8, 16, or 24 bits, one is allowed to specify any number of
bits for the network.  The remaining bits would go to host addresses -
one "host" address would be the network address (all zeroes); one
"host" address would be the broadcast address (all ones); and the rest
would be "real" [including virtual ;-)] hosts.

The /25 and /26 above designate the number of bits that designate a
network address.  E.g., in 192.0.2.128/26 and 192.0.2.192/26, the first
26 bits:
		11000000 00000000 00000010 10......
		11000000 00000000 00000010 11......
will be the same for all hosts on that network.  The remaining 6 bits
in the 32-bit address, 0-63, are the "host" addresses.  The 0 is the
network address, of course - 192.0.2.128 or 192.0.2.192.  The 63 is the
broadcast address - 192.0.2.191 or 192.0.2.255.  

If you haven't quite got it yet, see the "Sidebar on CIDR" in "DNS and
BIND," or ask.

Here are the RFCs on CIDR, from a quick scan.  Some are OBE already.

1467 Status of CIDR Deployment in the Internet. C. Topolcic. August
     1993. (Format: TXT=20720 bytes) (Obsoletes RFC1367) (Status:
     INFORMATIONAL)

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)

1817 CIDR and Classful Routing. Y. Rekhter. August 1995. (Format:
     TXT=3416 bytes) (Status: INFORMATIONAL)

2317 Classless IN-ADDR.ARPA delegation. H. Eidnes, G. de Groot, P.
     Vixie. March 1998. (Format: TXT=17744 bytes) (Also BCP0020) (Status:
     BEST CURRENT PRACTICE)

--
Joe Yao				jsdy at cospo.osis.gov - Joseph S. D. Yao
COSPO/OSIS Computer Support					EMT-B
-----------------------------------------------------------------------
      This message is not an official statement of COSPO policies.