15/12/2015 FdSc Appplied Computing with MMT - Wide Area Networks 1 FdSc Computing Technologies Wide Area Networks and Security IPv4, CIDR, VLSM Last update

21/04/23FdSc Appplied Computing with

MMT - Wide Area Networks 1

FdSc Computing TechnologiesWide Area Networks and Security IPv4, CIDR, VLSM

Last update : 15/04/2013



Objectives

This lecture aims to cover:- IPv4 re-cap Limitations of IPv4 and address space depletion Extending the design of IPv4 - CIDR Optimising IPv4 - VLSM Introduction to IPv6



Introduction…

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)



Limitations of IPv4

Classful addressing: 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



LANS, NAT and PAT

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



Address Translation, NAT and PAT

Source: Cisco.com

NAT is essentially a 1 to 1 mapping between private and public addressesPAT extends this to a “1 to many” mapping using dynamic port allocation



NAT

Adapted from: Cisco.com

NAT provides one to one mapping i.e. 192.168.1.20 to 171.69.68.10



Port Address Translation (PAT)

PAT provides “many to 1” or “many to few” Internet access for larger organisations

Source: Cisco.com



Address space depletion and CIDR 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



Variable Length Subnet Masks 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



Variable Length Subnet Masking exampleadapted from Cisco, (2008-09) 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)



Variable Length Subnet Masking examplePROBLEM

netA: must support 14 hostsnetB: must support 28 hostsnetC: must support 2 hostsnetD: must support 7 hostsnetE: must support 28 hosts

VLSM METHOD (provides an individual scheme for each network) Determine what mask allows the required number of hosts.

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)



Variable Length Subnet Masking example 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.!



Mixing address classes and CIDR 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.



IPv6

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



IPv6

“ARIN again warns of IPv4 address depletion” [Campbell, 2009, gcn.com]

However, several “scares” from the American Registry for Internet Numbers and other bodies were promoting the adoption of IPv6 by 2011 [ARIN, 2010]

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



Summary

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



Lab tasks - IPv4 VLSM design 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

Documents

15/12/2015 FdSc Appplied Computing with MMT - Wide Area Networks 1 FdSc Computing Technologies Wide Area Networks and Security IPv4, CIDR, VLSM Last update