[c-nsp] subnet mask confusion?

Gert Doering gert at greenie.muc.de
Fri Apr 19 03:44:04 EDT 2013


On Thu, Apr 18, 2013 at 10:21:17PM -0700, sky vader wrote:
> when using the following mask errors out as bad mask when used on an 
> interface.
> labasa(config-if)# ip address
> ERROR: Bad mask for address

This is no longer meaningful, and thus not allowed.

> works on an access-list,
> labasa(config-if)#access-list 101 extended permit ip any 

This is not a netmask, but a "ignore these bits" wildcard mask (and
particularily for normal networks, it's the *inverse* of the netmask,
so to match everything inside a /24 you'd use in the ACL).

> Just wondering what am I missing?

Interface config needs to build a strictly hierarchical "longest match
first" routing structure, so the netmask needs to be left-contiguous
(nowadays, IOS 9 or 10 still permitted discontiguous netmasks).

ACLs match by clearing ignore bits and then comparing with the given
address, which can operate on any bits in the ACL mask.


USENET is *not* the non-clickable part of WWW!
Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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