FdSc Computing Technologies
Wide Area Networks and Security
IPv4, CIDR, VLSM
Last update : 15/04/2013
16/03/2016
FdSc Appplied Computing with MMT - Wide
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Objectives
This lecture aims to cover:
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IPv4 re-cap
Limitations of IPv4 and address space depletion
Extending the design of IPv4 - CIDR
Optimising IPv4 - VLSM
Introduction to IPv6
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Introduction…
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Logical addressing is an OSI LAYER 3 technology: IPv4 uses a 32 bit addressing scheme
 e.g. 10.15.1.120
 It incorporates a 32 bit subnet mask to determine
the network and host portion of the address
 e.g. 255.0.0.0 or /8 (8 consecutive 1’s)
 It was ratified in the late 1970’s and therefore could
not foresee the addressing needs of the global
public Internet. It was “deployed on 1st Jan 1983”
(source: iana.org)
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Limitations of IPv4
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Classful addressing:
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Theoretical address space is 2^32 = 4,294,967,296
individual addresses
The original “Classful” addressing schemes
created much wasted address apace
e.g. One “class A” network allows for 16.7 million
hosts 2^24 = 16,777,216 hosts per network
Also, to connect to the Internet all hosts on a
LAN need a unique public Internet address
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LANS, NAT and PAT
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Obviously giving every host on a LAN a public
IP address to connect to the Internet is very
wasteful in IPv4
A method of sharing a single public IP address
or a POOL of public IP addresses was
developed
This was called Network Address Translation
It involved dynamically replacing the IP
packet’s private address with a public address
when access to the Internet was required
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Address Translation, NAT and PAT
Source: Cisco.com
NAT is essentially a 1 to 1 mapping between private and public addresses
PAT extends this to a “1 to many” mapping using dynamic port allocation
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NAT
Adapted from: Cisco.com
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NAT provides one to one mapping
i.e. 192.168.1.20 to 171.69.68.10
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Port Address Translation (PAT)
Source: Cisco.com
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PAT provides “many to 1” or “many to few”
Internet access for larger organisations
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Address space depletion and CIDR
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As the need for addresses grew in the 1990’s,
IPv4 address space was becoming depleted
Innovative ways of extending the protocol
started some 15 years ago
In addition to Address Translation techniques,
one method was introduced called ‘classless
inter-domain routing’ or CIDR
This involved using subnet bits independent of
the address class e.g.172.16.8.1/22
This therefore allowed the creation of smaller
networks or “subnets” from a single network
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Variable Length Subnet Masks
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CIDR however is sometimes inefficient and
wastes valuable IP address space
e.g. 172.16.8.0/22 allows for 2^10-2 host
addresses (1022) per subnet
However some networks in a WAN
environment may need far fewer than that
They may only need TWO host addresses
That is where VLSM can be used to produce
an optimised addressing scheme
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Variable Length Subnet Masking example
adapted from Cisco, (2008-09)
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A class C (/24) address
space has been sub-netted
as a /27 network.
This gives 2^3 networks i.e.
A ,B, C, D, E and 3 unused
We have used 5 but do not
require the full address
space in each (shown by the
shading in each block)
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Variable Length Subnet Masking example
PROBLEM
netA: must support 14 hosts
netB: must support 28 hosts
netC: must support 2 hosts
netD: must support 7 hosts
netE: must support 28 hosts
VLSM METHOD (provides an individual scheme for each network)
 Determine what mask allows the required number of hosts.
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netA: requires a /28 (255.255.255.240) mask to support 14 hosts
netB: requires a /27 (255.255.255.224) mask to support 28 hosts
netC: requires a /30 (255.255.255.252) mask to support 2 hosts
netD*: requires a /28 (255.255.255.240) mask to support 7 hosts
netE: requires a /27 (255.255.255.224) mask to support 28 hosts
(NOTE if you thought netD needs a /29 (255.255.255.248) mask.
Remember this would only allow 6 usable host addresses therefore
netD requires a /28 mask)
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Variable Length Subnet Masking example
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The logical way to assign the subnets
is to assign the largest first. For
example, you could assign in this
manner:
ASSIGN NEW VLSM MASKS
 netB: 204.15.5.0/27 host address
range 1 to 30
 netE: 204.15.5.32/27 host address
range 33 to 62
 netA: 204.15.5.64/28 host address
range 65 to 78
 netD: 204.15.5.80/28 host address
range 81 to 94
 netC: 204.15.5.96/30 host address
range 97 to 98
MORE EFFICIENT USE OF SPACE.!
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Mixing address classes and CIDR
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A further method to optimise address space
involves utilising two address ranges
e.g. A sub-netted class B for the Local Area
Networks (LANS)
AND a sub-netted class C range (for
example) for the point to point WAN links
VLSM is not fully supported by older
protocols and routers so the network
manager would decide on the optimal
scheme for the WAN design.
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IPv6
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Address depletion in the 1990’s invoked the
design of a replacement protocol for IPv4
IPv6 was approved in 1995 (Elahi, 2001)
However, “NAT”, “CIDR” and “VLSM”
techniques have since fulfilled address space
and route optimisation needs using IPv4
IPv6 has therefore been slow to mature and be
accepted by ISP’s and the general networking
community
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IPv6
“ARIN again warns of IPv4 address depletion”
[Campbell, 2009, gcn.com]
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However, several “scares” from the American
Registry for Internet Numbers and other bodies
were promoting the adoption of IPv6 by 2011
[ARIN, 2010]
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Despite warnings, few seem to be adopting
IPv6 as IPv4 is so well known and understood
Whether it be next year or well beyond that,
IPv4 will not provide our addressing needs
forever
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Summary
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A brief IPv4 and sub-netting re-cap
Limitations of IPv4 and address space
depletion
Extending the design of IPv4 – NAT and CIDR
Optimising IPv4 – VLSM schemes
IPv6
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Lab tasks - IPv4 VLSM design
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In pairs, consider IPv4 VLSM and a multiple
class addressing schemes for your WAN
design
The aim is to design the most optimised and
therefore LEAST wasteful scheme
You will ultimately implement your design in
software and use the hardware the LAB
Further study – VLSM workbook on Moodle
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