alt.certification.network-plus

Author

CIDR

BCSynchroNet

2003-05-28, 3:24 pm

Can someone, put into easy terms, Classless Inter-Domain Routing for me?

Dennis

2003-05-28, 10:24 pm

On 28 May 2003 13:14:01 -0700, bcarder@synchronet.net (BCSynchroNet)
wrote:

>Can someone, put into easy terms, Classless Inter-Domain Routing for me?

I don't claim to be a addressing guru but, basically, CIDR provides a
way to sell network addresses in smaller chunks. The number after the
slash is how many bits are allocated to the network address. Since you
have 32 bits in all you can tell how many bits are available for
machine addresses.

I'm pasting in a chart I used to help me remember - hopefully it won't
become unrecognizable due to line wrap.

CIDR Class Mask # of Addresses
Slash 8 A 255.0.0.0 16,000,000
Slash 12 255.240.0.0 1,000,000
Slash 16 B 255.255.0.0 65,536
Slash 20 255.255.240.0 4,096
Slash 21 255.255.248.0 2,048
Slash 22 255.255.252.0 1,024
Slash 23 255.255.254.0 512
Slash 24 C 255.255.255.0 256
Slash 25 255.255.255.128 128
Slash 26 255.255.255.192 64
Slash 27 255.255.255.224 32
Slash 28 255.255.255.240 16
Slash 29 255.255.255.248 8
Slash 30 255.255.255.252 4

If this is still confusing I'll try again or someone else may have a
clearer explanation.

aznluvsmc

2003-05-29, 8:17 am

CIDR basically describes the concept of using non default subnet masks. This allows us to create subnets by borrowing 1 or more bits from the host address.

Any IP address that does not use it's own default subnet mask is considered to be a Classless IP address.

The following are the default subnet masks for the respective IP classes:

Class A - 255.0.0.0
Class B - 255.255.0.0
Class C - 255.255.255.0

Crazymiclo30

2003-05-29, 10:24 am

This really isn't a subject easily explained. You should read a couple of
books and visit some websites on the subject.

--
crazymiclo
A+, Network+, MCP

"BCSynchroNet" <bcarder@synchronet.net> wrote in message
news:10623e23.0305281214.23c7bd64@posting.google.com...
> Can someone, put into easy terms, Classless Inter-Domain Routing for me?

Charles M. Kozierok

2003-05-29, 10:24 am

In article <10623e23.0305281214.23c7bd64@posting.google.com>,
BCSynchroNet <bcarder@synchronet.net> wrote:
} Can someone, put into easy terms, Classless Inter-Domain Routing for me?

As others have said, this is not a subject easily summarized.

Here's an introduction to the subject of the different types of
addressing that may help.. This is actually a small portion of a much
larger explanation of CIDR, and as you can see, it is still not very
short, yet it is the bare minimum I consider sufficient to explain CIDR.

Note that the term "CIDR" is itself somewhat confusing. It really refers
to a new method of both addressing and routing.

The following is © Copyright 2003 Charles M. Kozierok. All rights reserved.

The fundamental division of the 32 bits in an IP address is into the
network identifier (network ID) and host identifier (host ID). The
network ID is used for routing purposes while the host ID uniquely
identifies each network interface on the network. In order for devices
to know how to use IP addresses on the network they must be able to tell
which bits are used for each ID. However, the "dividing line" is not
predefined. It depends on the type of addressing used in the network.

Understanding how these IDs are determined leads to a larger discussion
of the three main categories of IP addressing schemes. Each of these
uses a slightly different system of indicating where in the IP address
the host ID is found. The three are:

* Conventional ("Classful") Addressing: The original IP addressing scheme
is set up so that the dividing line occurs only in one of a few
locations: on octet boundaries. Three main classes of addresses, A, B
and C are differentiated based on how many octets are used for the
network ID and how many for the host ID. For example, class C addresses
devote 24 bits to the network ID and 8 to the host ID. This type of
addressing is now often referred to by the made-up word "classful" to
differentiate it from newer "classless" scheme.

This simplest addressing type uses the simplest method to divide the
network and host identifiers: the class type, and therefore the dividing
point, are encoded into the first few bits of each address. Routers can
tell from these bits which octets belong to which identifier.

* Subnetted "Classful" Addressing: In this system the two-tier
network/host division of the IP address is made into a three-tier system
by taking some number of bits from a class A, B or C host ID and using
them for a subnet identifier. The network ID is unchanged. The subnet ID
is used for routing within the different subnetworks that constitute a
complete network, providing extra flexibility for administrators. For
example, consider a class C address that normally uses the first 24 bits
for the network ID and remaining 8 bits for the host ID. The host ID can
be split into, say, 3 bits for a subnet ID and 5 for the host ID.

This system is based on the original classless scheme, so the dividing
line between the network ID and "full" host ID is based on the first few
bits of the address as before. The dividing line between the subnet ID
and the "sub-host" ID is indicated by a 32-bit number called a subnet
mask. In the example above, the subnet mask would be 27 ones followed by
5 zeroes--the zeroes indicate what part of the address is the host. In
dotted decimal notation, this would be 255.255.255.224.

* Classless Addressing (CIDR): In this system the classes of the original IP
addressing scheme are tossed out the window. The division between the
network ID and host ID can occur at an arbitrary point, not just on
octet boundaries like in the "classful" scheme.

The dividing point is indicated by putting the number of bits used for
the network ID, called the prefix length, after the address (recall that
the network ID bits are also sometimes called the network prefix). For
example, if 227.82.157.177 is part of a network where the first 27 bits
are used for the network ID, that network would be specified as
227.82.157.160/27. The "/27" is conceptually the same as the
255.255.255.224 subnet mask, since it has 27 one bits followed by 5
zeroes.

....

CIDR provides numerous advantages over the "classful" addressing scheme,
whether or not subnetting is used:

* Efficient Address Space Allocation: Instead of allocating addresses in
fixed-size blocks of low "granularity", under CIDR addresses are
allocated in sizes of any binary multiple. So, a company that needs
2,000 addresses can be assigned a block of 2,046 instead of 65,534. Or,
to think of it another way, the equivalent of a single Class B network
can be shared amongst 30 or more companies that each need 2,046 or fewer
IP addresses.

* Elimination of Classes: There are no more class A, B and C networks, so
there is no problem with some portions of the address space being widely
used while others are neglected.

* Efficient Routing Entries: CIDR's multiple-level hierarchical structure
allows a small number of routing entries to represent a large number of
networks. Network descriptions can be "aggregated" and represented by a
single entry. Since CIDR is hierarchical, the detail of lower-level,
smaller networks can be hidden from routers that move traffic between
large groups of networks.

* No Separate Subnetting Method: CIDR implements the concepts of
subnetting within the internet itself. An organization can use the same
method used on the Internet to subdivide its internal network into
subnets of arbitrary complexity without needing a separate subnetting
mechanism.

Since the main benefit of "classful" addressing was its simplicity, it's
no surprise that the main drawback of CIDR is its greater complexity.
One issue is that it is no longer possible to determine by looking at
the first octet to determine how many bits of an IP address represent
the network ID and how many the host ID. A bit more care needs to be
used in setting up routers as well, to make sure that routing is
accomplished correctly.

© Copyright 2003 Charles M. Kozierok. All rights reserved.

peace,

-*-
charles

Charles M. Kozierok

2003-05-29, 10:24 am

User1

2003-05-30, 10:25 pm

"Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
news:bb575a$mfa$1@reader1.panix.com...
> As others have said, this is not a subject easily summarized.
>
> Here's an introduction to the subject of the different types of
> addressing that may help.. This is actually a small portion of a much
> larger explanation of CIDR, and as you can see, it is still not very
> short, yet it is the bare minimum I consider sufficient to explain CIDR.
>
> Note that the term "CIDR" is itself somewhat confusing. It really refers
> to a new method of both addressing and routing.
>
> The following is © Copyright 2003 Charles M. Kozierok. All rights
reserved.

Thanks for the write up. I can use this in my new book.
You can't copyright something posted to UseNet!

Charles M. Kozierok

2003-05-31, 8:24 am

In article <a1VBa.89538$eJ2.76692@fed1read07>,
User1 <user1@microsoft.com> wrote:
} "Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
} news:bb575a$mfa$1@reader1.panix.com...
} > As others have said, this is not a subject easily summarized.
} >
} > Here's an introduction to the subject of the different types of
} > addressing that may help.. This is actually a small portion of a much
} > larger explanation of CIDR, and as you can see, it is still not very
} > short, yet it is the bare minimum I consider sufficient to explain CIDR.
} >
} > Note that the term "CIDR" is itself somewhat confusing. It really refers
} > to a new method of both addressing and routing.
} >
} > The following is © Copyright 2003 Charles M. Kozierok. All rights
} reserved.
}
} Thanks for the write up. I can use this in my new book.
} You can't copyright something posted to UseNet!

I assume you are joking and did not mean to admit to everyone here that
you are a thief?

That material was not written just for that posting. It was taken from
another document.

That I am the author of the material is now on record since it
was posted, and you have now admitted that you are going to take it for
your document. While you may be anonymous here, if you republish my
stolen writing, your identity will be known, and it will still be clear
that you took it from me.

So if you want to try, all I can say is that I could use the money.
It would not be the first time I have successfully taken action against a
publishing company. You may find yourself more than a bit embarrassed at
the questions your publisher will ask you when they find out your "book"
is filled with material stolen from others on Usenet.

As for your claim about copyright, I suggest that you educate yourself.
Material from various source documents is posted to Usenet all the time,
and that does not invalidate their copyright.

http://www.templetons.com/brad/copymyths.html

10 Big Myths about copyright explained
==============================
========

3) "If it's posted to Usenet it's in the public domain."

False. Nothing modern is in the public domain anymore unless the owner
explicitly puts it in the public domain. Explicitly, as in you have a
note from the author/owner saying, "I grant this to the public domain."
Those exact words or words very much like them.

Some argue that posting to Usenet implicitly grants permission to
everybody to copy the posting within fairly wide bounds, and others feel
that Usenet is an automatic store and forward network where all the
thousands of copies made are done at the command (rather than the
consent) of the poster. This is a matter of some debate, but even if the
former is true (and in this writer's opinion we should all pray it isn't
true) it simply would suggest posters are implicitly granting
permissions "for the sort of copying one might expect when one posts to
Usenet" and in no case is this a placement of material into the public
domain. It is important to remember that when it comes to the law,
computers never make copies, only human beings make copies. Computers
are given commands, not permission. Only people can be given permission.
Furthermore it is very difficult for an implicit licence to supersede an
explicitly stated licence that the copier was aware of.

peace,

-*-
charles

JK_Deth

2003-05-31, 9:24 am

Why exactly do you feel usenet has anything to do with it. You might want to
take a look at

http://www.copyright.gov/help/faq/faq-general.html

Exerpt:

1.. When is my work protected?
Your work is under copyright protection the moment it is created and fixed
in a tangible form that it is perceptible either directly or with the aid of
a machine or device.

2.. Do I have to register with your office to be protected?
No. In general, registration is voluntary. Copyright exists from the
moment the work is created. You will have to register, however, if you wish
to bring a lawsuit for infringement of a U.S. work. See Circular 1,
Copyright Basics, section "Copyright Registration."

Of course I'm sure of a couple things, you won't check out the site, you
won't be writing a book, you are simply a lUser trying to assinine.

"User1" <user1@microsoft.com> wrote in message
news:a1VBa.89538$eJ2.76692@fed1read07...
> "Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
> news:bb575a$mfa$1@reader1.panix.com...
> > As others have said, this is not a subject easily summarized.
> >
> > Here's an introduction to the subject of the different types of
> > addressing that may help.. This is actually a small portion of a much
> > larger explanation of CIDR, and as you can see, it is still not very
> > short, yet it is the bare minimum I consider sufficient to explain CIDR.
> >
> > Note that the term "CIDR" is itself somewhat confusing. It really refers
> > to a new method of both addressing and routing.
> >
> > The following is © Copyright 2003 Charles M. Kozierok. All rights
> reserved.
>
> Thanks for the write up. I can use this in my new book.
> You can't copyright something posted to UseNet!
>
>

Charles M. Kozierok

2003-06-01, 11:24 am

In article <vdhdb51ii71838@corp.supernews.com>,
JK_Deth <john1@telpage.net> wrote:
} Why exactly do you feel usenet has anything to do with it. You might want to
} take a look at
}
} http://www.copyright.gov/help/faq/faq-general.html

Thanks.

I try to respond to questions here to help people out, and I
occasionally use materials from documents I have written or am writing.
That doesn't mean I am granting permission for them to be used in any
way other than reading and learning from them.

Frankly, I often wonder why I bother. If people are going to steal my
writing then I will certainly think twice before posting content again
in the future.

peace,

-*-
charles

Bruce

2003-06-02, 10:24 am

Thanks for posting your guide to CIDR, I found it most informative. I
hope you don't mind but I noticed an inconsistency in your article.
You refered to the original scheme as being "Classful" but later on
said the original scheme was "Classless". See below:

ixlu@PCGuide.com (Charles M. Kozierok) wrote in message news:<bb578k$mfa$2@reader1.panix.com>...
> In article <10623e23.0305281214.23c7bd64@posting.google.com>,
> BCSynchroNet <bcarder@synchronet.net> wrote:
> } Can someone, put into easy terms, Classless Inter-Domain Routing for me?
>
> As others have said, this is not a subject easily summarized.
>

<snip>

>
> * Conventional ("Classful") Addressing: The original IP addressing scheme

<snip>

>
> This system is based on the original classless scheme, so the dividing
^^^^^^^^^^^^^^^^^^
Do you mean "Classful"?

<snip>

>
> © Copyright 2003 Charles M. Kozierok. All rights reserved.
>
> peace,
>
> -*-
> charles

Bruce.

Charles M. Kozierok

2003-06-02, 1:24 pm

In article <6a8eb75a.0306020648.c348575@posting.google.com>,
Bruce <codwars@hotmail.com> wrote:
} Thanks for posting your guide to CIDR, I found it most informative. I
} hope you don't mind but I noticed an inconsistency in your article.
} You refered to the original scheme as being "Classful" but later on
} said the original scheme was "Classless".

Yep! My mistake. Thanks for mentioning it and glad you found the writeup
useful.

peace,

-*-
charles

chromeDome

2003-06-22, 1:25 am

Classless Inter-Domain Routing is the process of throwing away the old Class A-E network heirarchy. When it became clear that IP addresses weren't going to last being dished out in the classful form, the entire heirarchy had to be re-thought. An innovati
ve idea was to reduce the wasted addresses that remained unused by discarding the heirarcy. In this form a subnet mask was included with all propagated routes to identify the network more clearly and allow for more suitable network boundaries. Allong wit
h this, route summarization was implemented, which said that if I protect a block of networks, instead of propagating each network separately, a single route would be provided for the entire block (where possible).

Example:

I own 192.168.0.0/24-192.168.1.0/24

Instead of representing this group of networks with a /24, I could summarize by using /23 and only 1 route is pushed to my directly connected neighbors.

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