CIDR FAQ - V2
HANK at taunivm.tau.ac.il
Mon Aug 21 15:25:04 UTC 1995
All followups to cidrd only plz...
The CIDR FAQ
| Version 2
| 21 August 1995
The following document is a collection of Frequently Asked Questions
about CIDR. This document is not meant to be a networking/routing
guide and tutorial. Where appropriate pointers to other documents
of a more general nature have been mentioned.
Updates from a previous version are marked with a '|' in column 1.
If you have any questions you would like added, please send them to
the editor mentioned below:
Hank Nussbacher (Tel Aviv University and IBM Israel)
hank at vm.tau.ac.il or hank at ibm.net.il
If you would like to "discuss" items from this FAQ please send
your mail to cidrd at iepg.org
This FAQ is being distributed to the following groups and lists:
nanog at merit.edu
inet-access at earth.com
iap at vma.cc.nd.edu
local-ir at ripe.net
cidrd at iepg.org
To retrieve the most up-to-date version of this document:
- anonymous FTP: ftp.ibm.net.il/pub/docs/cidr.faq
1. What does CIDR stand for?
CIDR stands for Classless Inter-Domain Routing and is documented
in RFC1517/1518/1519/1520. CIDR is an effective method to
stem the tide of IP address allocation as well as routing table
overflow. Without CIDR having been implemented in 1994 & 1995,
the Internet would not be functioning today.
Basically, CIDR eliminates the concept of class A, B, and C
networks and replaces this with a generalized "IP prefix". CIDR
can be used to perform route aggregation in which a single route
can cover the address space of several "old-style" network
numbers and thus replace a lot of old routes. This lessens the
local administrative burden of updating external routing, saves
routing table space in all backbone routers and reduces route
| flapping (rapid changes in routes), and thus CPU load, in all
| backbone routers. CIDR will also allow delegation of pieces of
what use d to be called "network numbers" to customers, and
therefore make it possible to utilize the available address space
See question #6 below for details on "IP prefix"s.
2. What is an ASN?
ASN stands for Autonomous System Number and acts to merge many
networks into a logical domain.
3. What is BGP?
BGP stands for Border Gateway Protocol and is the de-facto
standard for routing between Autonomous Systems in the Internet.
All communications between Internet Service Providers (ISP) is
handled via BGP4 which supports CIDR.
4. Why should I make the effort and convert my routing to be
The routing tables in the Internet have been growing as fast
as the Internet and the router technology specifically and
computer technology in general has not been able to
keep pace. In December 1990 there were 2190 routes and 2 years
later there were over 8500 routes. In July 1995 there are now over
| 29,000 routes, which require approximately 10 MB in a router with a
| single peer. Routers at interconnection points (or multi-homed
| hosts doing full routing with many peers) receive these routes from
| several peers, and need several dozen megabytes of RAM (and the
| appropriate CPU horsepower) to handle this. A list of those routers
| that can handle this appears at the end of this question. Routers
with 64MB of memory have the capacity for approximately 60,000
routes after which some routes will just have to be left out of
the global routing tables, and the more likely ones to be left out
are routes covering small pieces of address space.
Without the CIDRization work that has gone on for the past 2 years
the routing tables would be in excess of 65,000 routes. By
CIDRizing you help the Internet reduce the routing overload
as well as increasing the liklihood that in the future your
routes will be carried by all ISPs.
The major benefit of CIDR is to allow for continuous, uninterrupted
growth of the Internet. For a significant percentage of sites
connected to the Internet the value of the Internet increases with
the total number of sites connected to the Internet. Therefore,
taking steps needed to allow for continuous uninterrupted growth
(like CIDRizing, or renumbering) is beneficial to such sites.
The routers today that are available to handle the full routing
| cisco 7000 w/ 64Mb
cisco 4500 w/ 32Mb
| IBM ENSS w/ 64Mb
| BayNetworks models AN/ASN/BLN/BCN w/ 32Mb
5. Can you give an example of a simple CIDR configuration for a
The following example creates 2 aggregates and suppresses
any more specific addresses that may be contained within
those aggregates. The access-list causes only those nets
to be distributed as listed, and not any others that may
exist in the BGP routing tables.
router bgp 64100
aggregate-address 172.16.0.0 255.248.0.0 summary-only
aggregate-address 192.168.50.0 255.255.255.0 summary-only
neighbor 192.168.54.2 remote-as 65000
neighbor 192.168.54.2 distribute-list 12 out
access-list 12 permit 192.168.50.0 0.0.0.255
access-list 12 permit 172.16.0.0 0.7.255.255
An alternate method is via network and route statements:
router bgp 64100
network 172.16.0.0 mask 255.248.0.0
network 192.168.50.0 mask 255.255.255.0
neighbor 192.168.54.2 remote-as 65000
ip route 172.16.0.0 255.248.0.0 Null0 254
ip route 192.168.50.0 255.255.255.0 Null0 254
In this case, only those routes explicitly mentioned in "network"
statements will be announced with BGP. For these routes to be
announced, there has to be a corresponding route in the IP
forwarding table, thus the need to create the static routes. The
static routes will also serve as "pull-ups" for the route
advertisements and thus prevent route flapping: these routes will
always be announced with BGP by this router. Note that as long
as more specific routes exist internally in your network, these
will be used in preference to the static "less specific" route
entries (longest prefix matching is being used).
A good rule to follow is to never redistribute IGP learnt routes
directly into BGP, but to rather use network or aggregate-address
statements. And if you must redistribute dynamically learnt IGP
routes into BGP, you MUST use filtering.
The reasons for this advice are several, some of which are:
1) if your IGP is classful (e.g. RIP or IGRP) you will by
default not do any route aggregation
2) if you have an internal stability problem (accidents do
happen), this will be reflected as a "route flap" in the whole
routing system, globally burning CPU cycles better spent on
3) if the IGP -> BGP transition is unrestricted, this can lead to
false routing information escaping from your network
(especially if you do not fully have administrative control
over your IGP)
6. What do all these /16s and /24s mean in my BGP tables?
This refers to the number of bits of the network part of the
IP address. A former class B may appear as 188.8.131.52/16,
which is the same as 256 class C's which can appear as
184.108.40.206/16. A single class C appears as 220.127.116.11/24.
These "things" are often called an "IP prefix", which
consists of an IP address and a mask length. The mask length
specifies the number of leftmost contiguous significant bits in
the corresponding IP address. Thus, an IP prefix with a prefix
length of 15 (denoted /15) covers the address space of 128k IP
addresses, and a /17 covers the address space of 32k IP
Here is a table of the more popular CIDR blocks:
CIDR class C
/16 256 = 1 former class B
In general, advertising a prefix covering less address space than
a /24 prefix will probably not get into the global routing
tables, and global Internet connectivity is less likely to
happen. Note that for you as an administrator of an AS, it is a
good idea to announce as few prefixes as possible and to utilize
the address space as much as possible.
7. Do I need to carry the full Internet routing table? When would it
be necessary? What routers on the Internet carry full routing
tables and how much memory is needed?
No you do not need to carry the full Internet routing table. If
you are single-homed, meaning you have a single connection to an
ISP, then all you need to do is point a default route to the
ISP and tell your ISP not to send you the full routing table. If
you are multi-homed, you will want to know which nets to route
via connection A and which nets to route via connection B. The
easiest way to do this is to request a partial routing table
from one ISP - with those nets that are closest to them, and default
everything else to the other ISP. This way your routing tables
need not contain the entire Internet universe but only a small
The closer you get to the hub or nexus of the Internet, the larger
your routing tables need to be. For example, those connected to
public exchange points (like the NAPs, CIX, STIX, LINX, dGIX) in
general, carry full routing tables and run without a default
8. What is there in the Internet to stop me from making a mistake and
announcing via BGP an aggregate that is larger than the nets I am
in charge of?
In principle there is nothing to stop you. The responsibility falls
on both ends of the BGP link - you are responsible to filter what
you announce and the receiving end - if it has its act together -
filters also what it *thinks* it should be hearing from you so as
prevent mistakes on your part. Those sites that do not work with
access lists and filters and just readily accept what is sent to
them are just waiting for a problem to happen.
Filtering can either be done at the IP network level or at the
BGP path (BGP orgin AS) level. See question 20 below for details
on a tool to assist in filtering.
9. Who has to renumber with CIDR ?
Sites that move from one ISP to another, and who had been allocated
addresses from their original ISP's CIDR block, in all likelihood
will have to return those addresses as part of the move. The reason
is to keep the number of prefixes in the global routing system
within the limits of current technology.
10. I have a /16 but have registered parts of it as /24s in the RADB.
I now want to CIDRize. The problem is parts of the /16s are
missing and are routed via a different ASN. Can you explain how
more specific routes override more general ones and will I hurt my
routing if I just advertise the /16 and not a bunch of /20s and
There are two aspects to the answer:
1) Real (BGP) world: Given there are several AS's sharing addresses
out of a /16 prefix, every AS should advertise exactly those
prefixes which it is really originating. However, if there is one
AS "originating" a significant majority of this address space, the
concerned AS's might agree that this one and only advertises the /16
and all others their more specifics. The more specifics always take
precedence over the less specific.
2) Routing registry: The registry DB, of course, should always
reflect reality. If in the above example the AS's agree on the "big
AS" announcing the /16, the "big AS" should document with the
route-object that it's not really originating the whole aggregate
by using "hole" attributes (see ripe.181, 5. The Route Object).
11. How can I redistribute our IGP routes (IGRP) so that they become
aggregated when sent out via BGP?
It is strongly discouraged to redistribute IGPs into BGP, because
local IGP configuration errors might easily corrupt routing
information of the whole Internet. If, however, you have to do
it anyway, you MUST use strict distribute-lists with explicit
permits (or route-maps) for redistribution. Here is an example
for a Cisco configuration:
router bgp 64100
aggregate-address 192.168.0.0 255.255.192.0 summary-only
aggregate-address 172.16.0.0 255.254.0.0 summary-only
redistribute igrp 64100 route-map origin-AS64100
! redistribute igrp 64100
! distribute-list 10 out igrp 64100
route-map origin-AS64100 permit 10
match ip address 10
access-list 10 permit 192.168.0.0 0.0.63.0
access-list 10 permit 172.16.0.0
access-list 10 permit 172.17.0.0
This example would generate one route 192.168/18 and one route
172.16/15 if any of the contained networks is in the IGP.
12. I am multihomed to three ISPs and can only CIDRize to two of them
but to the third I need to still announce specific nets. What
damage will this do to my AS?
No damage can be done if the non-CIDR peer does not further
announce your specifics to the global Internet. If your non-CIDR
ISP DOES announce your specifics to the global Internet those
specifics will have preference over the less specifics and
therefore all traffic to you will get routed through the non-CIDR
13. I don't want to CIDRize. Can someone do proxy aggregation for me?
Proxy aggregation should only be done with great care and should
be avoided if you are not single-homed ! If you are single-homed
ask your ISP.
| Others may proxy aggregate over your address range without your
| consent, and send your traffic over paths/links not of your
| choosing. Use of Routing Registries may help to identify and
| correct these problem areas.
|14. What routers on the market today do support CIDR (classless
| Routers that are capable of handling CIDR are:
| - all Cisco routers running 10.0 or higher
| - all Bay Networks routers running 8.01 or higher
| - 3Com Netbuilder II and Netbuilder Remote Office
| - Telebit EMPB
| - Unix w/ BSD/OS 2.0 w/ gated 3.5alpha_11 + radix-fixes
| - IBM 2210 routers
15. How do I reach other parts of a subnetted old-style network when
I have only partial routing information for that same old-style
There are actually three ways to solve this particular problem
with Cisco's software. Which of them applies will depend on
what software version is involved:
o Preferred solution: turn on "ip classless" in your routers and
use a default route inside your AS. The "ip classless"
command prevents the existence of a single "subnet" route from
blocking access via the default route to other subnets of the
same old-style network.
o Workaround for 9.1 or later software where the "ip classless"
command is not available: install a "default network route"
like this: "ip route 18.104.22.168 255.0.0.0 next-hop" along the
axis your default route would normally take.
o Workaround for 9.0 or older software: create a "default
subnet route": "ip route 39.x.y.0 next-hop" combined with "ip
default-network 39.x.y.0", otherwise as the 9.1 fix.
Both of the latter solutions rely on static routes, and in the
long run these will be impossible to maintain. In some
topologies the use of static routes can be a problem (e.g. if you
have more than one possible exit point from your AS to choose
The following information is presented as supplemental information
that is related to the CIDRization process.
16. What is the Internet Routing Registry?
The IRR is a way for ASN's to publicize their own intended
routing policies without having to request a change from a
| The RAdb which stands for the Routing Arbiter Data Base, which
is part of the IRR, is part of a joint project between Merit and
ISI. For full details contact:
The Routing Arbiter is a project of the US National Science
Foundation. As part of that project, it runs a routing
That database (the RAdb) forms part of the IRR collection
of databases. The RIPE database is not part of the RAdb
but does participate in the IRR. At present, there are
five entities that contribute to the IRR effort and more
are expected. Today, all the contributing registries use the
RIPE-181 database format.
| All IRR participants can be contacted via automail handlers
that accept batch updates via email. An example of a routing
update appears below:
*ch: hank at aristo.tau.ac.il 950724
The *rt: tag identifies the net and the routing policy is based on
*or: tag. An example of a routing policy is presented below:
descr: Israeli Academic and Research Network
as-in: from AS1755 100 accept ANY
as-in: from AS174 100 accept ANY
as-in: from AS3339 100 accept AS3339
as-out: to AS1755 announce AS378 AS3339
as-out: to AS174 announce AS378 AS3339
as-out: to AS3339 announce ANY
default: AS174 10
default: AS1755 20
default: AS3339 30
guardian: HANK at vm.biu.ac.il
admin-c: Hank Nussbacher
tech-c: Hank Nussbacher
changed: hank at vm.tau.ac.il 950627
For further details read over ripe-120.ps, ripe-121.ps and
ripe-181.ps (via anonymous ftp from info.ripe.net/ripe/docs).
17. How do I update the registered routing information for my ASN?
You need to submit a "route" object update and perhaps an
"aut-num" object update (see examples above). Route objects
add new nets to your autonomous system (or you can remove nets
from your autonomous system) and the Autonomous-system object
describes the type of routing you wish to have.
18. Which Routing database takes precedence? RIPE? RADB? MCI? Do I
have to update all of them?
| Each provider is allowed to select the preference order for
| authentic data. For example, ANS uses the following precedence:
| ANS, CANET, MCI, RIPE, RADB
If there are two routes (with different origins) within one
database, the changed date is used as a tiebreaker. Else, only
database precedence is used. Thus, if the RADB entry has a more
recent changed date than the RIPE, ANS will use the RIPE entry.
| You should only have to register in one of the IRR component
|19. How do I check what is registered in the IRR?
The tool to use is whois. A few examples make the command
whois -h whois.ra.net 22.214.171.124
whois -h whois.ripe.net as378
whois -h whois.canet.ca 126.96.36.199
20. Is there a tool to automatically create route filters based
on IRR information?
rlc is a route list compiler which is a subset of nlc/alc that
allows the generation of route based filters (cisco access-
lists) by extracting the nets belonging to an AS or AS MACRO
from a routing database (i.e. Ripe Routing Database). In
addition, it supports a limited set of functions to generate
AS based filter lists.
rlc is fully classless, and hence supports CIDR routes and
subnets, as well as host routes.
Author: Jean-Michel Jouanigot, CERN <jimi at dxcoms.cern.ch>
Christian Panigl - Vienna University, Austria
Bill Manning - ISI
Tony Li - Cisco Systems
Havard Eidnes - SINTEF, Norway
Yakov Rekhter - Cisco Systems
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