Yes, but a subnet mask and a wildcard mask are two entirely different
things, with two entirely different functions.
A subnet mask determines which portion of an IP address is the network
portion, and which portion is the host portion.
A wildcard mask merely indicates which IP addresses match a particular
addressing requirement.
Just to rant a teeny bit, it is unfortunate that in a number of the study
guides, wildcard masks are presented and treated as if they of necessity are
related to subnet masks. I.e. as an "inverse" of the subnet mask. So if I
want to filter a network with an access list I use the "inverse" mask, or if
I want to place a network into the OSPF process, I use the "inverse" mask.
While I appreciate the difficulty of properly explaining the concepts, and I
particularly appreciate the difficulty most of us have in understanding and
internalizing binary math, the fact remains that the things implied in the
books and presentations I have read are wrong. For example, in the
statement
Access-list 1 permit 209.100.100.64 0.0.0.63
All you are doing is permitting hosts with addresses in the range of
209.100.100.64 through 209.100.100.127 You may make assumptions about the
subnet mask associated with 209.100.100.64, but you might be wrong. That
access-list is valid, and matches that range of hosts, whether the subnet
mask is 255.255.255.0, 255.255.255.192, 255.255.0.0, 255.0.0.0, or
255.248.0.0 for that matter. The mask is merely the means the router CPU
uses to easily match bits, using Boolean logic.
Well, enough ranting. Got things to do and wives to please. Enjoy the rest
of this deliciously long weekend.
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
Sent: Monday, September 04, 2000 1:53 AM
To: [EMAIL PROTECTED]; [EMAIL PROTECTED]; [EMAIL PROTECTED];
[EMAIL PROTECTED]
Subject: RE: Subnet Question
It is true in regard of wild-cards.
-----Original Message-----
From: Albert Ip [mailto:[EMAIL PROTECTED]]
Sent: ??? ????? 03 ?????? 2000 19:14
To: 'Chuck Larrieu'; Aaron Moreau-Cook; [EMAIL PROTECTED]
Subject: RE: Subnet Question
Chuck,
Just tried it on a 3662 with IOS 12.1T and it didn't work.
Rotuer(config-if)#ip address 10.1.1.1 0.255.255.0
Bad mask 0xFFFF00 for address 10.1.1.1
Too bad, it would had made a interesting trouble-shooting lab.
Albert
-----Original Message-----
From: Chuck Larrieu [mailto:[EMAIL PROTECTED]]
Sent: Sunday, September 03, 2000 3:06 AM
To: Aaron Moreau-Cook; [EMAIL PROTECTED]
Subject: RE: Subnet Question
I hate questions like this. Can, May, Always, Never.
RFC 1812 strongly discourages this practice.
On the other hand, the world won't end if you do. You may even create a
permanent income for yourself by setting up your network like this. ;-> All
the TCP stacks I have worked with allow this on the host side. It occurs to
me I've never tried this on a Cisco router, even after the long discussion
on the topic a few months ago. Next time I'm in the routers, I'll see what
happens and report.
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Aaron Moreau-Cook
Sent: Saturday, September 02, 2000 5:48 PM
To: [EMAIL PROTECTED]
Subject: Subnet Question
Question taken from the CCNA Exam Cram book by Walters, Rees, and Coe.
A subnet mask can have a value of 0.255.255.0
A) True
B) False
The Cisco answer would dictate that it is false, and in all functionality it
is true. Hypothetically though it could be true, I rememeber this discusion
a while ago, but I'm looking to see if I am smoking the proverbial crack.
Thanks all!
Aaron Moreau-Cook
Finally taking his CCNA test this coming Friday....
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