07-subnetting.pdf

Sub Networking Details by MAZHAR IQBAL BUTT.1


In 1985, RFC 950 defined a standardprocedure for subnetting, means asingle IP class A, B, or C network isfurther divided into a smaller group ofhosts to form an IP sub-network.This technique is used to allow a singleIP network address to span multiplephysical networks. IP hosts shouldsupport subnetting. Subnetworks arecreated for security, manageability andperformance of hosts and networks andto reduce network congestion.


MethodSome of the bits of the host ID

portion of an IP address isfurther divided into a physical

sub-network ID part and a hostID part.

The sub-network ID is used touniquely identify the different

sub-networks within a network.


When is subnetting necessary?Subnetting is required when one network numberneeds to be distributed across multiple LAN segments.This may be the case in instances when:

• A company uses two or more types of LANtechnology (for example, Ethernet, Token Ring) ontheir network.

• Two network segments are restricted by distancelimitations (for example, remote offices linked viapoint-to-point circuit).

• Segments need to be localized for networkmanagement reasons (accounting segment, salessegment, etc.).

• Hosts which dominate most of the LAN bandwidthneed to be isolated.


Subnetting is essentially the modification of asingle IP network to create two or morelogically visible sub-sections.It entails changing the subnet mask of the localnetwork number to produce an even number ofsmaller network numbers, each with acorresponding range of IP addresses.e.g. 255.255.255.0, 255.255.127.0,255.255.0.0.When a host "A" needs to communicate withanother host "B", it must know whether host"B" is in the same network or in a differentnetwork.


If the host "B" is in the same networkthen host "A" can send the datagramdirectly to host "B". But if the host B is ina different network, then host "A" mustsend the datagram to a proper router.By determining the network class of theIP address of host "B", host "A" candetermine whether host "B" is in thesame network or in a different network.By using the subnet mask host "A" canfind out whether host "B" is in the samesubnet or in a different subnet.


Every Network Starts from its NetworkID and ends with its Broadcast ID.

Start : 192.168.1.0 (Network ID)End : 192.168.1.255 (Broadcast ID)

A “classful” network is subnetted bydividing its host ID portion, leaving someof the bits for the host ID while allocatingothers to a new subnet ID.These bits are then used to identifyindividual subnets within the network,into which hosts are assigned.


Advantages and DisadvantagesSubnetting is happened before workstationswere assigned IP addresses. This will allowmore flexibility in segment layout. Subnettingcan provide easier network managementcapabilities and also lends itself to fastertroubleshooting.The routing announcement of a network to theoutside world will still be based on singlenetwork number. That subnetting will decreasethe total amount of IP addresses available butmay require purchasing additional hardwaresuch as a router.


Subnet Mask

Subnet mask is a 4 byte (32 bit) numberused to identify the sub-network ID andthe host ID from an IP address.

All the hosts in a sub-network will havethe same subnet mask.

The subnet mask is used to determinethe bits of the network

identifier. All hosts on the same networkshould have the same subnet mask.


In a subnetting environment, the additionalinformation about which bits are for the subnetID and which for the host ID must becommunicated to devices that interpret IPaddresses.

This information is given in the form of a 32-bitbinary number called a subnet mask.

The term “mask” comes from the binarymathematics concept called bit masking. Thisis a technique where a special pattern of onesand zeroes can be used in combination withboolean functions such as AND and OR toselect or clear certain bits in a number.


The subnet mask is a 32-bit binarynumber that accompanies an IP address.It is has a one bit for eachcorresponding bit of the IP address thatis part of its network ID or subnet ID,and a zero for each bit of the IPaddress’s host ID.

The mask thus tells TCP/IP devices whichbits in that IP address belong to thenetwork ID and subnet ID, and which arepart of the host ID.


How It WorksAn IP address is a 32 bit number dividedinto 4 sections of 8 bits called octets.Each octet is converted from binary todecimal form and separated with a dot.The address is split into network andhost portion.The network portion always remainsfixed for a particular network, remainingbits make up the host portion can bealtered to give the range of addresses toassign to hosts.


To determine where the network portion endsand the host portion begins, a subnet mask (ornetmask, or just mask) is used to fix thenetwork portion and allow the host portion tobe changed.For example, the IP number 38.9.211.0 witha subnet mask of 255.255.255.0. Changingto binary, we get:

So, the netmask as 24 bits, or /24 or 38.9.211.0/24


The fixed bits become the networkportion and the remaining bits becomethe host portion, so there are 8 host bitswhich account for a range from 0-255.IP protocol standards dictate that we useaddresses of all 0's to refer to thenetwork as a whole, and addresses of all1's to refer to the broadcast of all hostson the network, so these host addressesmay not be used.This limits our host range from 1 - 254.


Broadcast address of all hostson the network38.9.211.255

Range of valid hosts38.9.211.1 -38.9.211.254

The entire /24 network38.9.211.0/24


After getting network number(/24, /25, or/27), we have the ability to extend the networkportion further into the host-number field bylengthening the netmask.The number of bits that you extend into theoriginal host portion determines how manysegments, or subnets, will be produced.Lengthening the mask by n bits will produce2^n subnets.If we used 2 subnet bits. The host portionwould be reduced to six bits. There are 4different combinations of arranging the 2subnet bits, so we arrive at 4 unique subnets.


(.192 - .255)11000000 - 11111111Subnet 4

(.128 - .191)10000000 - 10111111Subnet 3

(.64 - .127)01000000 - 01111111Subnet 2

(.0 - .63)00000000 - 00111111Subnet 1

Each has 64 Elements.


38.9.211.25538.9.211.193 -38.9.211.254

38.9.211.192/26

38.9.211.19138.9.211.129 -38.9.211.19038.9.211.128/26

38.9.211.12738.9.211.65 -38.9.211.126

38.9.211.64/26

38.9.211.6338.9.211.1 -38.9.211.6238.9.211.0/26

BroadcastHost RangesNetwork


We have a Network ID of Class “C”:

192.168.1.0

and want to make 6 subnets. Class “C”IP address has pattern:

Net.Net.Net.Host

To break-up host portion into subnets,We will use 2n-2 formula by puttingvalues {n=0, 1, 2, 3, …} to get =6 or>=6.


2n-2

For n=0 20-2=1-2=-1 <6

For n=1 21-2=2-2=0 <6

For n=2 22-2=4-2=2 <6

For n=3 23-2=8-2=6 =6

Value of n for required subnets is 3,so we will act upon 3 left most bitsof host portion.


192

160

128

96

64

224

32

0

BID00000111

00000011

00000101

HOST IDNETORK ID

HOST(5 Remaining Bits)

SUBNET(3 Bits)

Net.Net.Net

0

0

0

0

0

1

0000001

0000110

0000010

0000100

NID0000000

248163264128

192.168.1.

Valid

Ran

ge


62

63

33

32

BID11111100

01111100

HOST IDNETORK ID


SUBNET(3 Bits)

Net.Net.Net

1

0

1

………

0000100

NID0000100

248163264128

Subnet-1

192.168.1.

Valid

Ran

ge


94

95

65

64

BID11111010

01111010

HOST IDNETORK ID


SUBNET(3 Bits)

Net.Net.Net

1

0

1

………

0000010

NID0000010

248163264128

Subnet-2

192.168.1.

Valid

Ran

ge


Valid RangeBIDNIDSubnet StartsNo

193 to 222192.168.1.223192.168.1.192192.168.1.1926

161 to 190192.168.1.191192.168.1.160192.168.1.1605

129 to 158192.168.1.159192.168.1.128192.168.1.1284

97 to 126192.168.1.127192.168.1.96192.168.1.963

65 to 94192.168.1.95192.168.1.64192.168.1.642

33 to 62192.168.1.63192.168.1.32192.168.1.321

6-Subnets of 192.168.1.0 have following details


1. STATIC LENGTH(FLSM-Fixed Length Subnet Mask)

2. VARIABLE LENGTH(VLSM-Variable Length Subnet Mask)


Static Length SubnettingIf all the subnetworks in a single network usethe same subnet mask, it is called as 'StaticLength' subnetting.AdvantageIt is easy to configure a network using 'StaticLength' subnetting. In addition, native IProuting understands only static subnetting.DisadvantageA large number of IP addresses will be wastedbecause of Irrespective number of hosts in thesubnetwork and single subnet mask is used forall the subnetworks.


Some Examples of FLSM values

255.255.255.128

255.255.255.192

255.255.255.224

255.255.255.240

255.255.255.248

255.255.255.252

255.128.0.0

255.192.0.0

255.224.0.0

255.240.0.0

255.248.0.0

255.252.0.0

255.254.0.0

255.255.0.0

255.255.128.0

255.255.192.0

255.255.224.0

255.255.240.0

255.255.248.0

255.255.252.0

255.255.254.0

255.255.255.0


Variable Length SubnettingIf the different subnetworks in a single networkuse different subnet masks, it is called as'Variable Length' subnetting.AdvantageIP addresses are allocated depending on thenumber of hosts present in the subnetwork. Soavailable host IP addresses are efficiently usedand are not wasted.DisadvantageAll the routers in a 'Variable Length' subnettednetwork must understand this type ofsubnetting.


Summary

The subnet addresses in which all the bits arezero or one, are special subnet IP addresses.

The subnet address in which all the bits arezero, represents the local subnetwork in whichthe datagram originated.

The subnet address in which all the bits areone represents a subnet broadcast address.

In the broadcast address, all the other bitsincluding the network and host ID must beone.


Assignments

Class C Network ID

202.125.147.200

Default Subnet Mask

255.255.255.0

Find the Default Network ID, Broadcast ID

Divide Default Network ID into 10 Subnets

07-subnetting.pdf

Documents

different network

network portion

subnetwork id

network segments

network class

network congestion

particular network

classful network