IPv4 Addressing and Subnetting -...

IPv4 Addressing

and Subnetting

G. Gianini

Summary

• Addressing basics in IPv4

• Limits and problems

• Fixed Mask Subnetting

• Variable Lenght Subnet Masking

• A look at CIDR and IPv6

IPv4 Addressing basics

IPv4 Header

The IPv4 address space consists of a 32 bit field,

or the equivalent of some 4.5 billion values

Dotted-decimal notation

IP address classes

The three classes we focus on

Some special addresses

Zero and All One Host Numbers

• The values 0 and -1 (all ones) have always special

meanings when used in Host Numbers :

– The value zero means “this” host

– The value -1 is used as a broadcast address to mean all

hosts of the indicated network

• As a consequence if n bits are reserved for the host

addressing only 2^n -2 different hosts can be

given an address.

Zero Network Numbers

• In Network numbers of class A,B or C, the zero network number has a special meaning in the three following cases:

– 0.x.x.x means this network within a class A network

– 0.0.x.x means this network within a class B network

– 0.0.0.x means this network within a class C network

The all zero address

• As a consequence of the zero rules

for nets and host

– 0.0.0.0 means this host

The all Ones Network Number

• By convention

– 255.255.255.255 indicates

a broadcast on the

local network

Class A Networks (/8 Prefixes)

Class B Networks (/16 Prefixes)

Class D Networks (/24 Prefixes)

How many nets? How many hosts?

A

B

C

_

2^31 IP addresses (=2,147,483,648) distributed over

2^7 possible network addresses

each with

2^24 hosts (16,777,216)

From those figures one must subtract special addresses as mentioned above

2^30 IP addresses (=1,073,741,824) distributed over

2^14 possible network addresses

each with

2^16 hosts (=65,536)


2^29 IP addresses (=536,870,912) distributed over

2^21 network addresses (=2,097,152)

each with

2^8 hosts (=256)


0

5

10

15

20

25

30

0 5 10 15 20 25

Network size (log2 n.hosts)

Number of networks (log2 n)

Network

sizeNumber of

Networks

7 14 21

A

C

B

Problems

What if reality does not fit the theory?

• Think of systems of objects of different sizes

(such as vehicles) and of their distribution (if a

parking lot doesn’t fit the actual

vehicles’distribution we are unhappy)

• Think of different case studies of object naming

grouped objects:

– people addresses in cities (mail addressing)

– telephone numbers

– car plates in provinces and states

– computer addresses and organizations

Unforeseen Limitations to

Classful Addressing

The original designers never envisioned that the Internet

would grow into what it has become today.

(Unforseen developements which clash against

the insufficient allocation of a resource

are quite common in many areas: think of Y2K)

Many of the problems that the Internet is facing today

can be traced back to the early decisions that were made

during its formative years.

Depletion of address spaceDuring the early days of the Internet, the seemingly unlimited address space

allowed IP addresses to be allocated to an organization

based on its request rather than its actual need.

As a result, addresses were freely assigned to those who asked for them

without concerns about the eventual depletion of the IP address space.

The decision to standardize on a 32-bit address space

meant that there were only 2^32 = (4,294,967,296) IPv4 addresses available.

A decision to support a slightly larger address space

would have exponentially increased the number of addresses,

and eliminated (or postponed) the current address shortage problem.

No support for medium-sized

organizations

The classful A, B, and C octet boundaries were easy to understand and implement,

but they did not foster the efficient allocation of a finite address space.

Problems resulted from the lack of a network class that was designed

to support medium-sized organizations.

A /24, which supports 254 hosts, is too small

while a /16, which supports 65,534 hosts, is too large.

In the past, the Internet has assigned sites with several hundred hosts a single /16 address

instead of a couple of /24s addresses.

Unfortunately, this has resulted in a premature depletion of the /16 network address space.

The only readily available addresses for medium-size organizations are /24s which have

the potentially negative impact of increasing the size of the global Internet's routing table.

IETF

Short for Internet Engineering Task Force,

the main standards organization for the Internet.

The IETF is a large open international community

of network designers, operators, vendors, and researchers

concerned with the evolution of the Internet architecture

and the smooth operation of the Internet.

It is open to any interested individual.

From Webopedia

IANA

Short for Internet Assigned Numbers Authority,

an organization working under the auspices

of the Internet Architecture Board (IAB) that is

responsible for assigning new Internet-wide IP addresses.

From Webopedia

INTERNET PROTOCOL V4 ADDRESS SPACE (last updated 03 August 2004)

Originally, all the IPv4 address spaces was managed directly by the IANA. Later parts of

the address space were allocated to various other registries to manage for particular purposes

or regional areas of the world. RFC 1466 [RFC1466] documents most of these allocations.

Block Date Registry - Purpose Notes or Reference

----- ------ --------------------------- ------------------

000/8 Sep 81 IANA - Reserved



003/8 May 94 General Electric Company

004/8 Dec 92 Bolt Beranek and Newman Inc.

005/8 Jul 95 IANA - Reserved

006/8 Feb 94 Army Information Systems Center

007/8 Apr 95 IANA - Reserved


009/8 Aug 92 IBM

010/8 Jun 95 IANA - Private Use See [RFC1918]

011/8 May 93 DoD Intel Information Systems

012/8 Jun 95 AT&T Bell Laboratories

013/8 Sep 91 Xerox Corporation

014/8 Jun 91 IANA - Public Data Network

015/8 Jul 94 Hewlett-Packard Company

016/8 Nov 94 Digital Equipment Corporation

017/8 Jul 92 Apple Computer Inc.

018/8 Jan 94 MIT

019/8 May 95 Ford Motor Company

020/8 Oct 94 Computer Sciences Corporation

021/8 Jul 91 DDN-RVN

022/8 May 93 Defense Information Systems Agency


024/8 May 01 ARIN - Cable Block (Formerly IANA - Jul 95)

025/8 Jan 95 Royal Signals and Radar Establishment

026/8 May 95 Defense Information Systems Agency


028/8 Jul 92 DSI-North

029/8 Jul 91 Defense Information Systems Agency

030/8 Jul 91 Defense Information Systems Agency


032/8 Jun 94 Norsk Informasjonsteknology

033/8 Jan 91 DLA Systems Automation Center

034/8 Mar 93 Halliburton Company

035/8 Apr 94 MERIT Computer Network

036/8 Jul 00 IANA - Reserved (Formerly Stanford University - Apr 93)


038/8 Sep 94 Performance Systems International


040/8 Jun 94 Eli Lily and Company

041/8 May 95 IANA - Reserved


043/8 Jan 91 Japan Inet

044/8 Jul 92 Amateur Radio Digital Communications

045/8 Jan 95 Interop Show Network


047/8 Jan 91 Bell-Northern Research

048/8 May 95 Prudential Securities Inc.

049/8 May 94 Joint Technical Command (Returned to IANA Mar 98)

050/8 May 94 Joint Technical Command (Returned to IANA Mar 98)

051/8 Aug 94 Deparment of Social Security of UK

052/8 Dec 91 E.I. duPont de Nemours and Co., Inc.

053/8 Oct 93 Cap Debis CCS

054/8 Mar 92 Merck and Co., Inc.

055/8 Apr 95 Boeing Computer Services

056/8 Jun 94 U.S. Postal Service

057/8 May 95 SITA

058/8 Apr 04 APNIC (whois.apnic.net)




062/8 Apr 97 RIPE NCC (whois.ripe.net)

063/8 Apr 97 ARIN (whois.arin.net)

064/8 Jul 99 ARIN (whois.arin.net)

065/8 Jul 00 ARIN

066/8 Jul 00 ARIN

067/8 May 01 ARIN

068/8 Jun 01 ARIN

069/8 Aug 02 ARIN

070/8 Jan 04 ARIN

071/8 Aug 04 ARIN

072/8 Aug 04 ARIN








080/8 Apr 01 RIPE NCC


082/8 Nov 02 RIPE NCC














































128/8 May 93 Various Registries

































































193/8 May 93 RIPE NCC (whois.ripe




197/8 May 93 IANA - Reserved


199/8 May 93 ARIN (whois.arin

200/8 Nov 02 LACNIC (whois.lacn

201/8 Apr 03 LACNIC (whois.lacn

202/8 May 93 APNIC (whois.apni

203/8 May 93 APNIC (whois.apni

204/8 Mar 94 ARIN (whois.arin

205/8 Mar 94 ARIN (whois.arin

206/8 Apr 95 ARIN (whois.arin

207/8 Nov 95 ARIN (whois.arin

208/8 Apr 96 ARIN (whois.arin

209/8 Jun 96 ARIN (whois.arin

210/8 Jun 96 APNIC (whois.apni

211/8 Jun 96 APNIC

212/8 Oct 97 RIPE NCC

213/8 Mar 99 RIPE NCC

214/8 Mar 98 US-DOD

215/8 Mar 98 US-DOD

216/8 Apr 98 ARIN

217/8 Jun 00 RIPE NCC

218/8 Dec 00 APNIC

219/8 Sep 01 APNIC

220/8 Dec 01 APNIC

221/8 Jul 02 APNIC

222/8 Feb 03 APNIC


224/8 Sep 81 IANA - Multicast
































Summary Table for Specialized Address Blocks

Address Block Present Use Reference

---------------------------------------------------------------------

000.000.000.000/8 "This" Network [RFC1700, page 4]

010.000.000.000/8 Private-Use Networks [RFC1918]

014.000.000.000/8 Public-Data Networks [RFC1700, page 181]

024.000.000.000/8 Cable Television Networks --

039.000.000.000/8 Reserved but subject to allocation [RFC1797]

127.000.000.000/8 Loopback [RFC1700, page 5]

128.000.000.000/16 Reserved but subject to allocation --

169.254.0.0/16 Link Local --


191.255.0.0/16 Reserved but subject

to allocation --


192.0.2.0/24 Test-Net

192.88.99.0/24 6to4 Relay Anycast [RFC3068]


198.18.0.0/15 Network Interconnect

Device Benchmark Testing [RFC2544]


224.0.0.0/4 Multicast [RFC3171]

240.0.0.0/4 Reserved for Future Use [RFC1700, page 4]

The Internet Assigned Numbers Authority (IANA) has reserved the

following three blocks of the IP address space for private

internets:

10.0.0.0 - 10.255.255.255 (10/8 prefix)

172.16.0.0 - 172.31.255.255 (172.16/12 prefix)

192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

We will refer to

the first block as "24-bit block",

the second as "20-bit block", and to

the third as "16-bit" block.

Note that (in pre-CIDR notation)

the first block is nothing but a single class A network number,

the second is a set of 16 contiguous class B network numbers,

and the third is a set of 256 contiguous class C network numbers.

IPv4 Address Space

Unicast 219.92 /8s 85.91%

Multicast 16.00 /8s 6.25%

IETF Res. 20.08 /8s 7.84%

Total address space:

- 256 chunks or "/8's",

each of which spans

16,777,216

address values.

The blocks of addresses

- from 224.000.000.000

to 239.255.255.255

reserved for Multicast use.

- from 240.000.000.000

to 255.255.255.255

reserved for future definition.

The address blocks

0.0.0.0/8, 14.0.0.0/8, 127.0.0.0/8

are reserved, as are the address

ranges used for private networks

and other reserved uses.

See RFC 3330.

The remaining addresses, the

equivalent of 219.92 /8 address

blocks form the pool of unicast

addresses which are used for

the Internet.

IANA allocations nowdays

Allocated 142.92 /8s 55.83%

IANA Pool 77.00 /8s 30.08%

Multicast 16.00 /8s 6.25%

IETF Res. 20.08 /8s 7.84%

IPv4 IANA ProjectionsThe post-1995 data has been fitted to an exponential growth model

(a model that assumes growth is proportional to the total size of the network)

The extrapolation of this model to the point of address pool exhaustion is shown here.

The END of IPv4IPv4 Address Space Exhaustion Predictors:

Application of best fit models to historical data relating to the growth in the address space

advertised in the BGP routing table. The underlying assumptions made in this predictive

model is that the previous drivers in address consumption will continue to determine future

consumption rates, and that growth in consumption rates will continue to operate

in a fashion where the growth rate is constant rather than increasing or decreasing.

Source: http://bgp.potaroo.net/ipv4/

Prediction updated: 23 October 2005 (now)

Exhaustion of the IPv4 Unallocated Address Pool

March 2013

Complete Exhaustion of all available IPv4 Address Space:

August 2022 !!!

Summary of problems

- Address space depletion

- Bloating of Internet routing tables.

- Bourocratic loads: local administrators had to request

another network number from the Internet

before a new network could be installed at their site.

The subsequent history of Internet addressing is focused

on a series of steps that overcome these addressing issues

and have supported the growth of the global Internet.

Subnetting

Subnetting

• One solution is to allow a network to be

split in several parts for internal use, but

still act as a single network to the outside

world.

Example A campus network

Here each of the ethernets has his own router

connected to the main router

How does it work

• When a packet comes into the main router, how

does this know which subnet (Ethernet) to give it

to?

• Having a host table with 65K entries each with the

responsable router is impractical

• A better way is that of devoting a part of the host

address to the specification of the router address

Fixed Length Mask Subnetting

In practice some bits are taken away from the host number to

create a subnet number

This adds another level of hierarchy to the IP addressing structure.

Instead of the classful two-level hierarchy, subnetting supports a three-

level hierarchy.

Subnet Mask

To implement subnetting the main router needs a subnet mask that indicates

the split between the network+subnetwork number and host: the subnet mask

tells the net router where the host addresses starts. The bits of the subnet

mask are set to 1 if the system examining the address should treat the

corresponding bit in the IP address as part of the extended-network- prefix.

The bits in the mask are set to 0 if the system should treat the bit as part of

the host-number.

Extended-Network-Prefix LengthThe standards describing modern routing protocols often refer to

the extended-network-prefix- length rather than the subnet mask.

The prefix length is equal to the number of contiguous

one-bits in the traditional subnet mask.

However, it is important to note that modern routing protocols

still carry the subnet mask. There are no Internet standard routing

protocols that have a one-byte field in their header that contains

the number of bits in the extended-network prefix. Rather,

each routing protocol is still required to carry the complete four-octet subnet mask.

How does it work?

In order to route an incoming packet

the main router uses the mask by performing

a logical AND operation, so as to extract the

network address from the overall address, and hands

the packet to the corresponding router.

Address: 11000000 10101000 00010010 10110111

Subnet Mask: 11111111 11111111 11111111 11000000

AND -------- -------- -------- --------

Network ID: 11000000 10101000 00010010 10000000

In the last column of the above example

we have a class C address with a mask of length 26

which tells us that the host portion of the address

10110111 must be split into

the subnet prefix 10

and the host address 110111

How it works without subnetting

• Each router has a table listing

some number of (network, 0) IP addresses and

some number of (this-network, host) IP addresses:

associated with each table is the network interface

to use to reach the destination.The first table is for distant

networks, the second for local hosts.

• When an IP packet arrives its destination address is looked up

in the routing table: if it is for a distant network it is

forwarded to the router indicated in the table; if it is for a

local host (e.g. on the touter LAN) it is sent directly to dht

destination.

How it works with subnetting• When subnetting is introduced the routing tables are

changed, adding entries of the form

(this-network, subnet, 0) and

(this-network, this-subnet, host)

• The first is used to reach other subnets,

the second to reach the hosts of the local subnet.

• Notice that in this way the router does not have to know

the details about the hosts on other subnets: the router will

- take the IP address

- perform an AND with the subnet mask

getting rid of the host number

- look up the resulting subnet number in the routing table.

BenefitsThe size of the global Internet routing table does not grow

because the site administrator does not need to obtain additional

address space and the routing advertisements for

all of the subnets are combined into a single routing table entry.

The local administrator has the flexibility to deploy

additional subnets without obtaining a new network

number from the Internet.

Route flapping (i.e., the rapid changing of routes)

within the private network does not affect the

Internet routing table since Internet routers

do not know about the reachability of the individual

subnets - they just know about the reachability

of the parent network number.

Subnet Design Considerations

The deployment of an addressing plan requires careful thought on the part of the network

administrator. There are four key questions that must be answered before any design

should be undertaken:

1) How many total subnets does the organization need today?

2) How many total subnets will the organization need in the future?

3) How many hosts are there on the organization's largest subnet today?

4) How many hosts will there be on the organization's largest subnet in the future?

All Zero and all one hosts

Recall that according to Internet practices,

the host-number field of an IP address

cannot contain all 0-bits or all 1-bits:

- the all-0s host-number identifies the base network

(or subnetwork) number,

-the all-1s host-number represents the broadcast address

for the network (or subnetwork).

In practice with n bits one will be able to address 2^n-2 hosts

To subnet a network, extend the natural mask using some of the bits

from the host ID portion of the address to create a subnetwork ID.

For example, given a Class C network of 204.15.5.0 which has a

natural mask of 255.255.255.0, you can create subnets in this manner:

204.15.5.0 - 11001100.00001111.00000101.00000000

255.255.255.224 - 11111111.11111111.11111111.11100000

--------------------------|sub|----

By extending the mask to be 255.255.255.224, you have taken

three bits (indicated by "sub") from the original host portion

of the address and used them to make subnets. With these three bits,

it is possible to create eight subnets.

With the remaining five host ID bits, each subnet can have

up to 32 host addresses, 30 of which can actually

be assigned to a device since host ids of all zeros or all ones

are not allowed. So, with this in mind, these subnets have been created.

204.15.5.0 255.255.255.224 host address range 1 to 30








How to subnet a network

Three bits are reserved for the subnet addresses

Five bits are reserved for the host addresses

This means that there is going to be room

for 2^3 = 8 subnets each with at most

2^5-2 = 30 hosts

Example

Subnetting a class C network

More subnets => less hosts

This brings up an interesting point.

The more host bits you use for a subnet mask,

the more subnets you have available.

However, the more subnets available,

the less host addresses available per subnet.

For example, a Class C network of 204.17.5.0

and a mask of 255.255.255.224 (/27) allows you

to have eight subnets, each with 32 host addresses

(30 of which could be assigned to devices).

If you use a mask of 255.255.255.240 (/28),

the break down is:

204.15.5.0 - 11001100.00001111.00000101.00000000

255.255.255.240 - 11111111.11111111.11111111.11110000

--------------------------|sub |---

Since you now have four bits to make subnets with,

you only have four bits left for host addresses.

So in this case you can have up to 16 subnets,

each of which can have up to 16 host addresses

(14 of which can be assigned to devices).

Class C Host/Subnet Table

Class C Subnet Effective Effective Number of Subnet

Bits Mask Subnets Hosts Mask Bits

------- --------------- --------- --------- --------------

1 255.255.255.128 2 126 /25

2 255.255.255.192 4 62 /26

3 255.255.255.224 8 30 /27

4 255.255.255.240 16 14 /28

5 255.255.255.248 32 6 /29

6 255.255.255.252 64 2 /30

7 255.255.255.254 128 2* /31

Notice that an exception to the 2^n-2 rule is 31-bit prefixes,

marked with an asterisk ( * ).

Subnetting a Class B network

Take a look at how a Class B network might be subnetted.

If you have network 172.16.0.0 ,then you know that its natural

mask is 255.255.0.0 or 172.16.0.0/16. Extending the mask

to anything beyond 255.255.0.0 means you are subnetting.

You can quickly see that you have the ability to create

a lot more subnets than with the Class C network.

If you use a mask of 255.255.248.0 (/21), how many subnets

and hosts per subnet does this allow for?

172.16.0.0 - 10101100.00010000.00000000.00000000

255.255.248.0 - 11111111.11111111.11111000.00000000

-----------------| sub |-----------

You are using five bits from the original host bits for subnets.

This will allow you to have 32 subnets (25). After using

the five bits for subnetting, you are left with 11 bits

for host addresses. This will allow each subnet

so have 2048 host addresses (211), 2046 of which

could be assigned to devices.

Example

Subnetting a class B network

Nine bits are reserved for the subnet addresses

Seven bits are reserved for the host addresses

This means that there is going to be room

for 2^9 = 512 subnets each with at most

2^7-2 = 126 hosts

Class B Host/Subnet Table

Class B Subnet Effective Effective Number of Subnet

Bits Mask Subnets Hosts Mask Bits

------- --------------- --------- --------- -------------

1 255.255.128.0 2 32766 /17

2 255.255.192.0 4 16382 /18

3 255.255.224.0 8 8190 /19

4 255.255.240.0 16 4094 /20

5 255.255.248.0 32 2046 /21

6 255.255.252.0 64 1022 /22

7 255.255.254.0 128 510 /23

8 255.255.255.0 256 254 /24

9 255.255.255.128 512 126 /25

10 255.255.255.192 1024 62 /26

11 255.255.255.224 2048 30 /27

12 255.255.255.240 4096 14 /28

13 255.255.255.248 8192 6 /29

14 255.255.255.252 16384 2 /30

15 255.255.255.254 32768 2* /31

Class A Host/Subnet TableClass A

Number of

Bits Borrowed Subnet Effective Number of Number of Subnet

from Host Portion Mask Subnets Hosts/Subnet Mask Bits

------- --------------- --------- ------------- -------------

1 255.128.0.0 2 8388606 /9

2 255.192.0.0 4 4194302 /10

3 255.224.0.0 8 2097150 /11

4 255.240.0.0 16 1048574 /12

5 255.248.0.0 32 524286 /13

6 255.252.0.0 64 262142 /14

7 255.254.0.0 128 131070 /15

8 255.255.0.0 256 65534 /16

9 255.255.128.0 512 32766 /17

10 255.255.192.0 1024 16382 /18

11 255.255.224.0 2048 8190 /19

12 255.255.240.0 4096 4094 /20

13 255.255.248.0 8192 2046 /21

14 255.255.252.0 16384 1022 /22

15 255.255.254.0 32768 510 /23

16 255.255.255.0 65536 254 /24

17 255.255.255.128 131072 126 /25

18 255.255.255.192 262144 62 /26

19 255.255.255.224 524288 30 /27

20 255.255.255.240 1048576 14 /28

21 255.255.255.248 2097152 6 /29

22 255.255.255.252 4194304 2 /30

23 255.255.255.254 8388608 2* /31

Subnetting Example

The first entry in the Class A table (/10 subnet mask) borrows two bits (the leftmost bits)

from the host portion of the network for subnetting, then with two bits you have

four (22) combinations, 00, 01, 10, and 11. Each of these will represent a subnet.

Binary Notation Decimal Notation

-------------------------------------------------- -----------------

xxxx xxxx. 0000 0000.0000 0000.0000 0000/10 ------> X.0.0.0/10

xxxx xxxx. 0100 0000.0000 0000.0000 0000/10 ------> X.64.0.0/10

xxxx xxxx. 1000 0000.0000 0000.0000 0000/10 ------> X.128.0.0/10

xxxx xxxx. 1100 0000.0000 0000.0000 0000/10 ------> X.192.0.0/10

Note: The subnet zero and all-ones subnet are included in the effective number of subnets

as shown in the third column.

Time to work up

• Refer to the file Sample exercises.pdf to see

a few worked out examples of fixed mask

subnetting.

Variable Length Subnet Masks

(VLSM)

• In 1987, RFC 1009 specified that a subnetted network could use more than one subnet mask.

• When an IP network is assigned more than one subnet mask, it is considered a network with variable length subnet masks..

VLSM

• Benefits

– Efficient use of the organization’ s assigned

IP address space.

– Route aggregation.

Efficient Use of the Organization's

Assigned IP Address Space

VLSM supports more efficient use of an organization's assigned IP address space.

One of the major problems with the earlier limitation of supporting only a single

subnet mask across a given network-prefix was that once the mask was selected,

it locked the organization into a fixed-number of fixed-sized subnets.

For example, assume that

a network administrator decided

to configure the 130.5.0.0/16 network

with a /22 extended-network-prefix.

...the waste...A /22 extended-network prefix

permits 64 subnets (2^6 ),

each of which supports

a maximum of 1,022 hosts (2^10 -2).

This is fine if the organization wants to deploy a number of large subnets,

but what about the occasional small subnet containing only 20 or 30 hosts?

Since a subnetted network could have only a single mask, the network administrator

was still required to assign the 20 or 30 hosts to a subnet with a 22-bit prefix.

This assignment would waste approximately 1,000 IP host addresses

for each small subnet deployed!

…avoided.One solution to this problem was to allow

a subnetted network to be assigned more

than one subnet mask.

Assume that the network

administrator is also allowed to configure the 130.5.0.0/16 network

with a /26 extended-network-prefix.

A /16 network address with a /26 extended-network prefix permits 1024 subnets (2^10 ),

each of which supports a maximum of 62 hosts (2^6 -2).

The /26 prefix would be ideal for small subnets with less than 60 hosts, while

the /22 prefix is well suited for larger subnets containing up to 1000 hosts.

VLSM. Route aggregation

• VLSM allows the recursive division of

an organization´s address space.

• It can be aggregated to reduce the

amount of routing information at the top

level.

Route AggregationVLSM also allows the recursive division of an organization's address space so that

it can be reassembled and aggregated to reduce the amount of routing information

at the top level. This allows the detailed structure of routing information

for one subnet group to be hidden from routers in another subnet group.

The 11.0.0.0/8 network is first configured with a /16 extended-network-prefix.

The 11.1.0.0/16 subnet is then configured with a /24 extended-network-prefix and

the 11.253.0.0/16 subnet is configured with a /19 extended-network-prefix.

Conceptually, a network

is first divided into subnets,

some of the subnets

are further divided

into sub-subnets,

and so on.

Reducing Routing Table Size

Notice how Router D

is able to summarize the six subnets behind it into a single advertisement (11.1.253.0/24)

and how Router B

is able to aggregate all of subnets behind it into a single advertisement.

Likewise, Router C

is able to summarize the six subnets behind it into a single advertisement (11.253.0.0/16).

Finally, since the subnet structure is not visible outside of the organization,

Router A injects a single route into the global Internet's routing table -11.0.0.0/ 8 (or 11/8).

A planned and thoughtful

allocation of VLSM

can reduce the size

of an organization's

routing tables.

VLSM operation

• Conceptually, a network is divided into

subnets, some of the subnets are further

divided into sub-subnets, and some of the

sub-subnets are divided into sub2-subnets.

11.0.0.0/8

11.1.0.0/16

11.252.0.0/16

11.3.0.0/16

11.2.0.0/16

11.253.0.0/16

11.254.0.0/16

11.253.32.0/19

11.253.64.0/19

11.253.160.0/19

11.253.192.0/19

11.1.1.0/24

11.1.2.0/24

11.1.253.0/24

11.1.254.0/24

11.1.253.32/27

11.1.253.64/27

11.1.253.160/27

11.1.253.192/27

VLSM permits the

recursive division of a

netrwork prefix

VLSM operation

• The recursive process does not require

the same extended-network-prefix be

assigned at each level of recursion.

• The recursive subdivision can be carried

out as far as the network administrator

needs to take it.

VLSM Design Considerations

At each level of the hierarchy:

• 1) How many total subnets does this level need today?

• 2) How many total subnets does this level need in the future?

• 3) How many hosts are there on this level´s largest subnet today?

• 4) How many hosts will there be on this level´s largest subnet in the future?

VLSM Design Considerations (example)

• Assume a network is spread out over a number

of sites.

• An organization has 3 campuses today.

• It will need 3 bits of subnetting to allow

growth (8 subnets).

• Within each campus a second level of

subnetting will identify a building.

• Within each building a third level of

subnetting will identify an individual

workgroup.


• From this hierarchical model, the top level

is determined by the number of campuses.

• The mid-level by the number of buildings

at each site.

• The lowest level by the number of

workgroups.


• The deployment of a hierarchical subnetting

scheme requires careful planning.

• At the bottom level, the designer must be sure

that the leaf subnets are large enough to support

the required number of hosts.

• The addresses from each site will be aggregable

into a single address block that keeps the

backbone routing tables from becoming too

large.

Requierments for VLSM Deployment

• Three prerequisites:

– The routing protocols must carry extended-network-prefix information with each routing update.

– All routers must implement a consistent forwarding algorithm based on the longest match.

– For route aggregation to occur, addresses must be assigned so that they have topological significance.


Routing protocols

• OSPF, IS-IS, RIP-2, EIGRP allow the

deployment of VLSM by providing the

extended-network-prefix length or mask

value along with each route

advertisement.

• This permits each subnetwork to be

advertised with its corresponding prefix

length or mask.

Requirements for VLSM Deployment

Forwarding algorithm based on longest match

• A route with a longer e-n-p describes a smaller set

of destinations than the same route with a shorter

e-n-p.

• Then, a route with a longer e-n-p is said to be

“more specific”.

• A route with a shorter e-n-p is said to be “less

specific”.

• Routers must use the route with the longest

matching e-n-p (most specific matching route)

when forwarding traffic.


Example

• If a packet destination IP address is 11.1.2.5

and there are 3 network prefixes in the

routing table (11.1.2.0/24, 11.1.0.0/16, and

11.0.0.0/8), the router would select the route

to 11.1.2.0/24 because it has the longest

match with the destination IP address.

Requirements for the Deployment of VLSM

The successful deployment of VLSM has three prerequisites:

•The routing protocols must carry extended-network-prefix

information with each route advertisement.

•All routers must implement a consistent forwarding algorithm

based on the "longest match."

•For route aggregation to occur, addresses must be assigned

so that they have topological significance.


Topological significant address assignment

• Hierarchical routing requires that addresses be assigned to reflect the actual network topology.

• Routing information is reduced by taking the set of addresses assigned to a particular region of the topology, and aggregating them into a single routing update for the entire set.

• This can be done recursively at various points within the hierarchy of the routing topology.


Topological significant address assignment

• If addresses do not have a topological

significance, aggregation cannot be

performed and the size of routing tables

would not be reduced.

Time to work up

• Refer to the files

Sample exercises.pdf

VLSM_01.pdf

VLSM_02.pdf

• to se a few worked out examples of

Variable Lenght Subnet Masking.

Supernetting: Classless

Inter-Domain Routing (CIDR)CIDR was officially documented in September 1993

in RFC 1517, 1518, 1519, and 1520.

CIDR supports two important features that benefit the global Internet routing system:

- CIDR eliminates the traditional concept of Class A, Class B, and Class C

network addresses. This enables the efficient allocation of the IPv4 address space

which will allow the continued growth of the Internet until IPv6 is deployed.

- CIDR supports route aggregation where a single routing table entry

can represent the address space of perhaps thousands of traditional classful routes.

This allows a single routing table entry to specify how to route traffic to many

individual network addresses. Route aggregation helps control the amount

of routing information in the Internet's backbone routers, reduces route flapping

(rapid changes in route availability),

and eases the local administrative burden of updating external routing information.

CIDR and VLSM

CIDR and VLSM are essentially the same thing since they both allow a portion

of the IP address space to be recursively divided into subsequently smaller pieces.

The difference is that with VLSM, the recursion is performed on the address space

previously assigned to an organization and is invisible to the global Internet.

CIDR, on the other hand, permits the recursive allocation of an address block

by an Internet Registry to a high-level ISP, to a mid-level ISP, to a low-level ISP,

and finally to a private organization's network.

/20 Bitwise Contiguous Address Blocks

In a classless environment, prefixes are viewed as bitwise contiguous blocks of the

IP address space. For example, all prefixes with a /20 prefix represent the same

amount of address space (2 12 or 4,096 host addresses): a /20 prefix can be

assigned to a traditional Class A, Class B, or Class C network number.

See the following /20 blocks represent 4,096 host

addresses - 10.23.64.0/20, 130.5.0.0/20, and 200.7.128.0/20.

CIDR Address Blocks

This Table provides

information about

the most commonly

deployed CIDR

address blocks.

CIDR Reduces the Size of Internet

Routing Tables

Organization A Changes Network Providers

IP Version 6 (IPv6):

Expanded Addressing

IPv4 uses 32-bit addresses, which potentially can address up to 232 nodes.

However, the combination of network and local address hierarchy and

reserved address space for special handling such as loopback and broadcast

reduces the number of addressable nodes. At the same time, the exponential

growth of computer networks in recent years indicates the outgrowth of

addressable node using 32-bit addresses.

The IPv6 address size has been increased to 128 bits.

In addition to increased address size, IPv6 eliminated broadcast address

and added the notion of anycast address, which can be used to send

a packet to any one of a group of nodes.

References

• A good reference is the following (it

contains pointers to subnet calculators):– www.njedge.net/activities/nes/kvandev-ipnat-111401.ppt

•

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