[c-nsp] "Compressed" IPv6 ACLs on Cat6500

[Mack McBride]

This is not correct.
The field is actually 288 bits (v4 uses 144 bits).
Some of these bits are used for protocol, flags and such, 2 bits are used for IPv6 address type.
The remaining available for IPv6 addresses + ports is 256.
Source and destination are each allotted 128 bits.
The bits removed are [39:24] ie. 3333:3333:3333:3333:3333:33xx:xx33:AAAA in the OP example.
The part marked x is removed.

See the following for specifics: 
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/acl.html#wp1090842

Embedded IPv4 removes the upper 16 bits as these are all zero.
Link local discard bits 95:80 which are zero.
All other formats remove bits [39:24]

The misunderstanding is anything with a prefix longer than /88 includes discarded bits in the subnet portion 
as opposed to the host portion.

Mack McBride
Network Architect

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Saku Ytti
Sent: Wednesday, December 08, 2010 1:38 PM
To: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] "Compressed" IPv6 ACLs on Cat6500


> Where did you arrive to 112? My understanding of the compressed mode is
> 128-src_port-dst_port-flags = 128-16-16 = 88 usable bits for addresses.

omitted -8 there,

flags = 8bits, so 128-16-16-8 = 88. 

-- 
  ++ytti
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