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Appendix A.  Tutorial: Working with Binary and Hex

Appendix A.  Tutorial: Working with Binary and Hex

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100=1

101=1x10=10

102=10x10=100

103=100x10=1000

104=1000x10=10,000

So,thefirstfiveplacevaluesofthebase10numberingsystem

are

10,0001000100101

Readinganumbersuchas57,258intermsofplacevalues

meanstherearefivequantitiesof10,000,sevenquantitiesof

1000,twoquantitiesof100,fivequantitiesof10,andeight

quantitiesof1.Thatis,

5x10,000=50,000

7x1000=7000

2x100=200

5x10=50

8x1=8

Addingtheseindividualresultstogether,theresultis50,000+

7000+200+50+8=57,258.

Allofusaresoacquaintedwithworkinginbase10thatwe

seldomthinkofbreakinganumberdownintoitsplacevalues.

However,thistechniqueisessentialtodeciphernumbersin

otherbases.







WorkingwithBinaryNumbers

Computersare,atthemostfundamentallevel,simplya

collectionofelectricalswitches.Numbersandcharactersare

representedbythepositionsoftheseswitches.Becausea

switchhasonlytwopositions,onoroff,itusesabinary,orbase

2,numberingsystem.(Therootbimeanstwo.)Abase2

systemhasonlytwodigits:0and1.

Computersusuallygroupthesedigitsintoeightplacevalues,

knownasabyteoranoctet.Theeightplacevaluesare

2726252423222120

Theplacevaluesarecalculatedasfollows:

20=1

21=1x2=2

22=2x2=4

23=4x2=8

24=8x2=16

25=16x2=32

26=32x2=64

27=64x2=128

So,theplacevaluesofabinaryoctetare

1286432168421

Thus,thebinaryoctet10010111canbereadasfollows:



1x128=128

0x64=0

0x32=0

1x16=16

0x8=0

1x4=4

1x2=2

1x1=1

or128+16+4+2+1=151

Workinginbinaryiseasybecauseforeveryplacevaluethereis

eitheronequantityofthatvalueornoneofthatvalue.For

anotherexample,11101001=128+64+32+8+1=233.

Whereconvertingbinarytodecimalisamatterofaddingthe

placevalues,convertingfromdecimaltobinaryisamatterof

subtractingplacevalues.Toconvertthedecimalnumber178to

binary,forinstance,beginbysubtractingthehighestbase2

placevaluepossiblefromthenumber:

1. 178isgreaterthan128,soweknowthereisa1atthat

placevalue:178128=50.

2. 50islessthan64,sothereisa0atthatplacevalue.

3. 50isgreaterthan32,sothereisa1atthatplacevalue:50

32=18.

4. 18isgreaterthan16,sothereisa1atthatplacevalue:18

16=2.

5. 2islessthan8,sothereisa0atthatplacevalue.

6. 2islessthan4,sothereisa0atthatplacevalue.

7. 2isequalto2,sothereisa1atthatplacevalue:22=0.

8. 0islessthan1,sothereisa0atthatplacevalue.



Puttingtheresultsofallthesestepstogether,178is10110010

inbinary.

Anotherexamplemightbehelpful.Given110,

1. 110islessthan128,sothereisa0atthatplacevalue.

2. 110isgreaterthan64,sothereisa1atthatplacevalue:

11064=46.

3. 46isgreaterthan32,sothereisa1atthatplacevalue:46

32=14.

4. 14islessthan16,sothereisa0atthatplacevalue.

5. 14isgreaterthan8,sothereisa1atthatplacevalue:14

8=6.

6. 6isgreaterthan4,sothereisa1atthatplacevalue:64

=2.

7. Thereisa1atthe2placevalue:22=0.

8. 0islessthan1,sothereisa0atthatplacevalue.

Therefore,110is01101110inbinary.



WorkingwithHexadecimalNumbers

Writingoutbinaryoctetsisn'tmuchfun.Forpeoplewhomust

workwithsuchnumbersfrequently,abriefernotationis

welcome.Onepossiblenotationistohaveasinglecharacterfor

everypossibleoctet;however,thereare28=256different

combinationsofeightbits,soasingle-characterrepresentation

ofalloctetswouldrequire256digits,orabase256numbering

system.

Lifeismucheasierifanoctetisviewedastwogroupsoffour

bits.Forinstance,11010011canbeviewedas1101and0011.

Thereare24=16possiblecombinationsoffourbits,sowitha

base16,orhexadecimal,numberingsystem,anoctetcanbe

representedwithtwodigits.(Theroothexmeanssix,anddeci

meansten.)TableA-1showsthehexadecimaldigitsandtheir

decimalandbinaryequivalents.

TableA-1.Hex,decimal,andbinaryequivalents.

Hex



Decimal



Binary



0



0



0000



1



1



0001



2



2



0010



3



3



0011



4



4



0100



5



5



0101



6



6



0110



7



7



0111



8



8



1000



9



9



1001



A



10



1010



B



11



1011



C



12



1100



D



13



1101



E



14



1110



F



15



1111



Becausethefirst10charactersofthedecimalandthe

hexadecimalnumberingsystemarethesame,itiscustomaryto

precedeahexnumberwitha0x,orfollowitwithanh,to

distinguishitfromadecimalnumber.Forexample,thehex

number25wouldbewrittenas0x25oras25h.Thisbookuses

the0xconvention.

Afterworkingwithbinaryforonlyashortwhile,itiseasyto

determineadecimalequivalentofa4-bitbinarynumberinyour

head.Itisalsoeasytoconvertadecimaldigittoahexdigitin

yourhead.Therefore,convertingabinaryoctettohexiseasily

doneinthreesteps:

1. Dividetheoctetintotwo4-bitbinarynumbers.

2. Converteach4-bitnumbertodecimal.



3. Writeeachdecimalnumberinitshexequivalent.



Forexample,toconvert11010011tohex,

1. 11010011becomes1101and0011.

2. 1101=8+4+1=13,and0011=2+1=3.

3. 13=0xD,and3=0x3.

Therefore,11010011inhexis0xD3.

Convertingfromhextobinaryisasimplematterofworkingthe

threestepsbackward.Forexample,toconvert0x7Btobinary,

1. 0x7=7,and0xB=11.

2. 7=0111,and11=1011.

3. Puttingthe4-bitnumberstogether,0x7B=01111011,

whichisdecimal123.



AppendixB.Tutorial:AccessLists

Accesslistsareprobablymisnamedthesedays.Asthename

implies,theoriginalintentionofanaccesslistwastopermitor

denyaccessofpacketsinto,outof,orthrougharouter.Access

listshavebecomepowerfultoolsforcontrollingthebehaviorof

packetsandframes.Theirusefallsintothreecategories(see

FigureB-1):

Securityfiltersprotecttheintegrityoftherouterandthe

networkstowhichtheyarepassingtraffic.Typically,

securityfilterspermitthepassageofafew,well-understood

packetsanddenythepassageofeverythingelse.

Trafficfilterspreventunnecessarypacketsfrompassing

ontolimited-bandwidthlinks.Thesefilterslookandbehave

muchlikesecurityfilters,butthelogicisgenerallyinverse:

Trafficfiltersdenythepassageofafewunwantedpackets

andpermiteverythingelse.

Packetidentificationisrequiredformanytoolsavailableon

Ciscorouters,suchasdialerlists,routefilters,routemaps,

andqueuinglists.Thetoolsmustbeabletoidentifycertain

packetstofunctionproperly.Accesslistsmightbelinkedto

theseandothertoolstoprovidethispacketidentification

function.



FigureB-1.Accesslistsareusedassecurity

filters,astrafficfilters,andforpacket

identification.



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Appendix A.  Tutorial: Working with Binary and Hex

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