IP Sub-Networking Mini-Howto
Robert Hart, hartr@interweft.com.au
v1.0, 31 Martioy 1997
To keimeno ayto perigrafei to giati kai pws na ypodiktywsete ena dik
tyo IP - klashs A , B kai C wste na leitoyrgei swsta gia diafora
diasyndedemena diktya.
1. Pneymatikh Idiokthsia
To keimeno ayto dianeimetai katw apo toys oroys ths GNU Public License
(GPL).
To keimeno ayto yposthrizetai amesa apo toys IT sumboyloys ths
InterWeft (Melbournh, Aystralia).
H teleytaia ekdosh aytou toy keimenoy einai dia8esimh apo to InterWeft
WWW site sto InterWeft IT Consultants <http://www.interweft.com.au/>
kai apo to The Linux Documentation Project
<http://sunsite.unc.edu/LDP>.
2. Eisagwgh
Me toys dia8esimoys ari8mous diktuwn IP na ginontai suntoma eidos ypo
ejafanish, h apotelesmatikh xrhsh aytou toy oloena kai spanioteroy
poroy,einai shmantikh.
Ayto to keimeno perigrafei pws na spasete ena aplo ari8mo diktuoy IP
etsi wste na mporei na xrhsimopoih8ei se diafora diaforetika diktya.
To keimeno ayto epikentrwnetai se diktya IP klashs C - alla oi arxes
efarmozontai epishs kai se diktya A kai B klasewn.
2.1. Alles phges plhroforias
Yparxei ena plh8os allwn phgwn plhroforias poy einai sxetikes me
leptomereis plhrofories ka8ws kai me plhrofories ypoba8roy sxetika me
diey8unseis IP. Aytes poy proteinontai apo ton syggrafea einai:
· The Linux Network Administrators Guide
<http://sunsite.unc.edu/LDP/LDP/nag/nag.html>.
· The Linux System Administration Guide
<http://linuxwww.db.erau.edu/SAG/>.
· TCP/IP Network Administration toy Craig Hunt, ekdoseis O'Reilly and
Associates <http://www.ora.com/catalog/tcp/noframes.html>.
3. H anatomia twn ari8mwn IP
Prin proxwrhsoyme sthn apolaysh ths ypodiktuwshs (sub-networking) ,
xreiazetai na 8esoyme ta basika sxetika me tis diey8unseis IP.
3.1. Oi diey8unseis IP anhkoyn se interface - OXI se host!
Prwta ap'ola, as jeka8arisoyme merikes basikes aities parejhghshs - oi
ari8moi IP den anati8entai se host. Oi ari8moi IP anati8entai se
interface diktuwn poy exoyn oi host.
Eeee - Ti einai ayto?
Enw polloi (an oxi oi perissoteroi) ypologistes se ena diktyo IP 8a
katexoyn ena mono interface diktuoy (kai exoyn ena mono ari8mo IP sa
synepeia), aytos den einai o monos tropos poy ginontai ta pragmata. Oi
ypologistes ka8ws kai alles syskeyes mporoun na exoyn diafora (an oxi
polla) interface diktuoy -kai ka8e interface na exei ton diko toy
ari8mo IP.
Etsi mia syskeyh me 6 diaforetika interface (opws enas dromologhths -
router) 8a exei 6 ari8mous IP- enan gia ka8e interface gia ka8e diktyo
sto opoio einai syndedemenos. H aitia gi' ayto einai profanhs otan
koitajoyme ena diktyo IP!
Parol' ayta, oi perissoteroi anaferontai se host diey8unseis otan
anaferontai se ari8mous IP. Apla na 8ymaste, ayto einai mia eykolia
gia ton ari8mo IP toy sygkekrimenoy interface se ayton ton host.
Polles (an oxi oi pleiochfia) twn syskeywn sto Internet exoyn monadiko
interface kai epomenws monadiko ari8mo IP.
3.2. Oi ari8moi IP san "Tetrades me teleies"
Sthn sygkekrimenh (IPv4) ylopoihsh twn ari8mwn IP, aytoi apotelountai
apo 4 byte (1 byte=8 bit) - dinontas ena sunolo 32 bit dia8esimhs
plhroforias. Ayto katalhgei se ari8mous poy einai mallon megaloi
(akoma kai otan einai grammenoi se dekadikh morfh). Etsi gia
anagnwsimothta (ka8ws kai gia logoys organwshs) oi ari8moi IP einai
synh8ws grammenoi se morfh 'tetradas me teleies' ('dotted quad'). O IP
ari8mos
192.168.1.24
einai ena paradeigma aytou- 4 (dekadikoi) ari8moi diaxwrizomenoi apo
(.) teleies.
Ka8ws ka8e enas apo toys 4 ari8mous einai h dekadikh anaparastash enos
byte twn 8 bit, ka8e enas apo toys 4 ari8mous mporei na einai apo 0
ews 255 (dhladh 256 monadikes times - 8ymh8eite, to 0 einai kai ayto
mia timh).
Epipleon, meros toy ari8mou IP enos host prosdiorizei to diktyo sto
opoio o host edreuei, ta ypoloipa 'bit' toy ari8mou IP prosdiorizoyn
to idio to host (oyps... -to interface toy diktuoy). Akribws to posa
bit xrhsimopoiountai gia to ID toy diktuoy kai posa einai dia8esima
gia na prosdiorizoyn host (interface) se ayto to diktyo orizetai apo
thn 'klash' ('class') toy diktuoy.
3.3. Klaseis twn Diktuwn IP
Yparxoyn treis klaseis ari8mwn IP
· Oi ari8moi diktuwn IP klashs A xrhsimopoioun ta 8 aristerotera bit
(ta aristerotera sthn tetrada) gia na prosdiorizoyn to diktyo,
afhnontas 24 bit (ta ypoleipomena tria apo thn tetrada) gia na
prosdiorizoyn host interface se ayto to diktyo.
Oi diey8unseis klashs A panta exoyn sto aristerotero bit apo to
aristerotero byte ena mhdeniko - ayth einai mia dekadikh timh apo 0
ews 127 gia thn prwth tetrada. Etsi yparxoyn to megisto 128
ari8moi 1hs tajhs diktuoy dia8esimoi, me ka8enan na periexei panw
apo 33,554,430 dynata interface.
Omws, ta diktya 0.0.0.0 (gnwsto san proka8orismenh diadromh default
route) kai 127.0.0.0 (to diktyo anadrashs - loop back network)
exoyn eidikh shmasia kai den einai dia8esima gia thn
diey8ynsiodothsh diktuwn. Etsi yparxoyn mono 126 dia8esimoi ari8moi
diktuwn klashs A.
· Oi ari8moi diktuwn IP klashs B xrhsimopoioun ta aristerotera 16 bit
(tis duo aristeroteres tetrades) gia na prosdiorizoyn to diktyo,
afhnontas 16 bit (tis duo teleytaies tetrades) gia na prosdiorizoyn
host interface. Oi diey8unseis klashs B panta exoyn ta aristerotera
2 bit toy aristeroteroy byte 1 0. Ayto afhnei 14 bit ypoloipo gia
ton ka8orismo ths dieu8ynshs toy diktuoy dinontas 32767 dia8esima
diktya klashs B. Gi'ayto ta diktya 2hs tajhs exoyn ari8mous apo 128
ews 191 gia to prwto meros ths tetradas, me ka8e diktyo na periexei
mexri 32,766 pi8ana interface.
· Oi ari8moi diktuwn IP klashs C xrhsimopoioun ta 24 aristerotera bit
(ta aristerotera tria bytes) gia na prosdiorizoyn to diktyo,
afhnontas 8 bit (to dejiotero byte) gia na prosdiorizoyn host
interface. Oi diey8unseis klashs C panta jekinane me ta
aristerotera 3 bit ry8mismena se 1 1 0 h se ena euros apo 192 ews
255 gia to aristerotero melos ths tetradas. Yparxoyn epomenws
4,194,303 dia8esimoi ari8moi diktuwn IP klashs C, opoy to ka8ena
periexei 254 interface. (Ta diktya klashs C poy exoyn to prwto byte
megalutero apo 223 einai desmeymena kai mh dia8esimo gia xrhsh).
Anakefalaiwsh:
Tajh diktuoy Xrhsimo euros timwn toy 1oy byte (dekadiko)
A (1h) 1 ews 126
B (2h) 128 ews 191
C (3h) 192 ews 254
Yparxoyn epishs eidikes diey8unseis poy einai desmeymenes gia 'mh
syndedemena' diktya - ayta einai diktya poy xrhsimopoioun IP alla den
einai syndedemena sto Internet, oi diey8unseis aytes einai:
· Ena diktyo klashs A
10.0.0.0
· 16 diktya klashs B
172.16.0.0 - 172.31.0.0
· 256 diktya klashs C
192.168.0.0 - 192.168.255.0
8a parathrhsete oti to keimeno ayto xrhsimopoiei aytes tis allhloyxies
diey8unsewn gia na apofeyx8ei to mperdema me 'pragmatika' diktya kai
host.
3.4. Oi ari8moi diktuwn, diey8unseis interface, diey8unseis anakoin
wshs (broadcast addresses)
Oi ari8moi IP mporei na exoyn treis pi8anes shmasies:
· H dieu8ynsh enos diktuoy IP (mia omada apo syskeyes IP poy exoyn
koinh prosbash se ena meso metadoshs - opws an p.x. briskontai oles
sto idio tmhma enos diktuoy Ethernet). Enas ari8mos diktuoy panta
8a exei ta interface (host) bit toy xwroy ths dieu8ynshs 0 (ektos
kai an to diktyo exei ypodiktya - opws 8a doume);
· h dieu8ynsh koinhs ekpomphs/anakoinwshs (broadcast) enos diktuoy IP
(h dieu8ynsh poy xrhsimopoieitai gia na 'milame', taytoxrona, se
oles tis syskeyes se ena diktyo IP). Oi diey8unseis anakoinwshs gia
ena diktyo panta exoyn ta interface (host) bit toy xwroy dieu8ynshs
1 (ektos kai an to diktyo exei ypodiktyo - pali, opws 8a doume).
· h dieu8ynsh enos interface (opws mia karta Ethernet h ena PPP
interface se ena host, dromologhth, diakomisth ektypwsewn klp.).
Aytes oi diey8unseis mporei na exoyn opoiadhpote timh sta host bit
ektos ola mhdenika h ola 1 - giati me ta host bit ola mhdenika, h
dieu8ynsh einai mia dieu8ynsh diktuoy kai me ta host bit ola 1 h
dieu8ynsh einai h dieu8ynsh ekpomphs/anakoinwshs.
Gia anakefalaiwsh kai gia na jeka8arisoyme ta pragmata
Gia ena diktyo klashs A ...
(ena byte toy xwroy dieu8ynshs toy diktuoy akoloy8oumeno apo tria
byte toy xwroy dieu8ynshs toy host)
10.0.0.0 einai enas ari8mos, diktuoy klashs A giati ola ta host
bit toy xwroy dieu8ynshs, einai 0
10.0.1.0 einai h dieu8ynsh enos host se ayto to diktyo
10.255.255.255.255 einai h dieu8ynsh anakoinwshs/ekpomphs (broadcast) aytou toy diktuoy
giati ola ta host bit toy xwroy dieu8ynshs einai 1
Gia ena diktyo klashs B ...
(duo byte toy xwroy dieu8ynshs diktuoy akoloy8oumena apo duo byte toy
xwroy dieu8ynshs toy host)
172.17.0.0 einai enas ari8mos diktuoy klashs B
172.17.0.1 einai h dieu8ynsh enos host se ayto to diktyo
172.17.255.255 einai h dieu8ynsh anakoinwshs/ekpomphs aytou toy diktuoy
Gia ena diktyo klashs C...
(tria byte toy xwroy dieu8ynshs diktuoy akoloy8oumena apo ena byte toy
xwroy dieu8ynshs toy host)
192.168.3.0 einai enas ari8mos diktuoy klashs C
192.168.3.42 einai h dieu8ynsh enos host se ayto to diktyo
192.168.3.255 einai h dieu8ynsh ekpomphs/anakoinwshs aytou toy diktuoy
sxedon oloi oi ari8moi diktuoy IP poy paramenoyn dia8esimoi gia
desmeysh pros to paron einai diey8unseis klashs C.
3.5. H maska diktuoy (network mask)
H maska diktuoy legetai swstotera maska ypodiktuoy (subnetwork mask).
Omws, genika anaferomaste se ayth ws h maska diktuoy.
H maska diktuoy kai oi epiptwseis ths sth metafrash twn diey8unsewn
IP, topika, se ena kommati diktuoy IP, poy kyriws mas afora edw, ka8ws
ayto ka8orizei poia (an yparxei kapoia) ypo-diktuwsh lambanei xwra.
H klassikh maska (ypo-) diktuoy einai ola ta bit toy diktuoy mias
dieu8ynshs '1' kai ola ta host bit '0'. Ayto shmainei oti oi klassikes
maskes diktuoy gia tis treis klaseis diktuwn einai:
· Maska klashs diktuoy A: 255.0.0.0
· Maska klashs diktuoy B: 255.255.0.0
· Maska klashs diktuoy C: 255.255.255.0
Yparxoyn duo shmantika pragmata poy prepei na 8ymatai kaneis gia thn
maska diktuoy:
· H maska diktuoy ephreazei mono thn topikh metafrash topikwn ari8mwn
IP (opoy topiko shmainei, se ayto to sygkekrimeno kommati toy
diktuoy);
· H maska diktuoy den einai ari8mos IP - xrhsimopoieite gia na
tropopoiei ton tropo topikhs metafrashs twn ari8mwn IP.
4. Ti einai ta ypodiktya;
Ena ypodiktyo einai enas tropos na paroyme mia dieu8ynsh diktuoy IP
kai topika na th spasoyme etsi wste ayth h aplh dieu8ynsh IP diktuoy
na mporei sthn pragmatikothta na xrhsimopoih8ei se diafora
diasyndedemena topika diktya. 8ymh8eite, enas aplos ari8mos IP mporei
na xrhsimopoih8ei mono se ena aplo diktyo.
To shmantiko edw einai to topika: oson afora ton kosmo ejw apo tis
mhxanes kai ta fysika diktya poy kaluptontai apo to ypo-diktywmeno
diktyo IP, tipota den exei allajei - paramenei akoma ena aplo diktyo
IP. Ayto einai shmantiko - h ypo-diktuwsh einai mia topikh ru8mish
kai einai aorath ston ypoloipo kosmo.
5. Giati ypodiktyo;
Oi aities pisw apo thn ypo-diktuwsh xronologountai pisw stis prwtes
prodiagrafes toy IP - opoy mono ligosta site etrexan se ari8mous
diktuoy klashs A, ta opoia epitrepoyn ekatommuria syndedemenoys host.
Prokaleitai profanws megalo kykloforiako kai diaxeiristiko problhma,
an oloi oi IP computer se ena megalo site prepei na einai syndedemenoi
sto idio diktyo: prospa8wntas na elegjoyme ena tetoio terastio kthnos
8a htan efialths kai to diktyo (sxedon sigoyra) 8a swriastei katw apo
to fortio ths idias toy ths kinhshs (koresmos diktuoy).
Kai etsi mpainoyme sthn ypo-diktuwsh: h dieu8ynsh toy diktuoy IP
klashs A, mporei na katatmh8ei, wste na epitrecei ton diamoirasmo ths
se merika (an oxi polla) jexwrista diktya. H diaxeirish ka8e tetoioy
jexwristou diktuoy mporei epishs na metabibastei.
Ayto epitrepei th dhmioyrgia mikrwn, diaxeirisimwn diktuwn - polu
pi8anon xrhsimopoiwntas diaforetikes texnologies diktuwshs. 8ymh8eite,
de mporeite na anamijete Ethernet, Token Ring, FDDI, ATM klp. sto idio
fysiko diktyo - mporoun parol' ayta na diasynde8oun!
¢lloi logoi ypodiktuwshs einai:-
· H fysikh katanomh toy site einai dynaton na dhmioyrgei periorismous
(mhkos kalwdiwn) sxetikous me to pws h fysikh ypodomh mporei na
synde8ei, apaitwntas pollapla diktya. H ypo -diktuwsh epitrepei thn
ylopoihsh toy parapanw, se ena periballon IP, xrhsimopoiwntas ena
mono ari8mo diktuoy IP.
Oysiastika to parapanw ginetai polu syxna apo toys ISPs, poy
epi8ymoun na dinoyn stoys, monima syndedemenoys pelates toys me
topika diktya, statikous ari8mous IP.
· O fortos toy diktuoy einai arketa ychlos, wste na prokalei
shmantikes epibradunseis. Me to na xwrizoyme to diktyo,
xrhsimopoiwntas ypodiktya, h kinhsh poy einai topikh se kapoio
diktyako tmhma, kratietai topikh - meiwnontas thn synolikh kinhsh
kai epitaxunontas thn syndesimothta me to diktyo xwris thn apaithsh
perissoteroy pragmatikou diktyakou bandwidth (xwrhtikothta)
· Oi prodiagrafes asfaleias mporei epishs na ypagoreuoyn pws
diaforetikes klaseis xrhstwn de moirazontai to idio diktyo - ka8ws
h kinhsh se ena diktyo mporei panta na ypoklapei apo enan "sxetiko"
xrhsth. H ypo-diktuwsh parexei ena tropo na krathsoyme to Tmhma
Prow8hshs apo to na parakoloy8ei/ypokleptei thn diktyakh kinhsh toy
Tmhmatos Ereynas kai Anaptyjhs (R & D) (h toys foithtes apo to na
ypokleptoyn to diktyo toy Tmhmatos Diaxeirishs)!
· Uparjh ejoplismou poy xrhsimopoiei asumbates diktyakes texnologies,
kai h anagkh na tis diasyndesete kai aytes (opws anafer8hke kai
parapanw).
6. Pws na ypodiktywsete enan ari8mo diktuoy IP
Afou apofasisate pws xreiazetai na ypodiktywsete ton ari8mo toy
diktuoy IP sas, pws 8a proxwrhsete; Ta parakatw einai mia sunach twn
bhmatwn, ta opoia kai 8a ejhgh8oun argotera me leptomereia:-
· Kante thn fysikh sundesh (kalwdiwsh diktuoy kai diktyakes
diasyndeseis - opws dromologhtes.
· Apofasiste poso megalo/mikro xreiazetai na einai ka8e ypodiktyo, me
bash ton ari8mo twn syskeywn poy 8a einai syndedemenes se ayto -
p.x. posoi xrhsimopoioumenoi ari8moi IP apaitountai gia ka8e
epimeroys tmhma toy diktuoy.
· Ypologiste thn katallhlh maska diktuoy kai dieu8ynsh diktuoy.
· Dwste se ka8e interface ka8e diktuoy thn dikia toy dieu8ynsh IP kai
thn katallhlh maska diktuoy.
· Ry8miste tis dromologhseis (routes) stoys dromologhtes kai tis
katallhles pules (gateways), tis dromologhseis kai tis
proka8orismenes dromologhseis stis diktyakes syskeyes.
· Elegjte to susthma, luste ta problhmata kai meta xalarwste!
Gia toys skopous aytou toy paradeigmatos, 8a ypo8esoyme pws ypo-
diktywnoyme ena mono ari8mo diktuoy klashs C: 192.168.1.0
Aytos arkei gia to polu 254 syndedemena interface (host), syn ton
ypoxrewtiko ari8mo diktuoy (192.168.1.0) kai thn dieu8ynsh ekpomphs
(broadcast address) (192.168.1.255).
6.1. Kante thn fysikh sundesh
8a xreiastei na egkatasthsete thn katallhlh ypodomh kalwdiwshs gia
oles tis syskeyes poy epi8ymeite na diasyndesete, wste na
antapokrinontai sthn fysikh sas organwsh.
8a xreiasteite epishs ena mhxanismo gia na diasyndeete metaju toys ta
diafora tmhmata (dromologhtes, metatropeis meswn (media converters)
klp.).
Einai profanws adunath mia leptomerhs syzhthsh aytwn twn pragmatwn
edw. Se periptwsh poy xreiasteite boh8eia yparxoyn gurw sumboyloi
sxediasmou/egkatastashs diktuwn poy parexoyn tetoioy tupoy yphresies.
Dwrean symboyles einai dia8esimes epishs se ena plh8os Usenet news
groups (opws to comp.os.linux.networking).
6.2. Mege8os ypodiktuwn
Yparxei ena pazari metaju toy ari8mou twn ypodiktuwn poy dhmioyrgeite
kai twn 'xaramismenwn' ari8mwn IP.
To ka8e diktyo IP exei dyo diey8unseis poy einai axrhstes gia
diey8unseis interface (host)- ton idio ton ari8mo toy diktuoy IP kai
thn dieu8ynsh ekpomphs/anakoinwshs. Otan ypodiktywnete, ka8e ypodiktyo
apaitei apo mono toy, monadiko ari8mo diktuoy IP kai dieu8ynsh
anakoinwsewn - kai 8a prepei na einai egkyres diey8unseis apo thn
gkama twn diey8unsewn poy parexetai apo to diktyo IP poy
ypodiktywnete.
Etsi, ypodiktywnontas, ena diktyo IP se duo jexwrista ypodiktya,
dhmioyrgountai duo diey8unseis diktuwn kai duo diey8unseis ekpompwn-
ayjanontas tis 'mh xrhsimopoioumenes' diey8unseis interface (host);
dhmioyrgwntas 4 ypodiktya dhmioyrgei oktw axrhstes diey8unseis gia
interface (host) k.o.k.
Sthn oysia, to mikrotero xrhsimo ypodiktyo apoteleitai apo 4 ari8mous
IP:
· Dyo xrhsimoys ari8mous IP gia interface - enas gia to interface toy
dromologhth aytou toy diktuoy kai ena gia ton monadiko host sto
diktyo ayto.
· Enas ari8mos diktuoy.
· Mia dieu8ynsh ekpomphs/anakoinwshs.
To giati kapoios bebaia 8a h8ele na dhmioyrghsei ena toso mikro diktyo
einai mia allh erwthsh! Me mono ena host sto diktyo, ka8e diktyakh
epikoinwnia 8a ginetai pros alla diktya kai mono. Parola ayta, to
paradeigma ejyphretei sto na epideijei ton nomo poy isxuei sthn ypo-
diktuwsh.
Oysiastika, mporeite na diairesete ton ari8mo diktuoy IP sas, to polu
se 2^n (opoy n einai ena ligotero apo ton ari8mo twn bit ston ari8mo
diktuoy IP sas) idioy mege8oys ypodiktya.
Kai etsi, as eisaste realistes kata to sxediasmo toy diktuoy sas -
8elete ton elaxisto ari8mo jexwristwn topikwn diktuwn poy
antapokrinetai stoys ylikous periorismous kai stoys periorismous
diaxeirishs, ejoplismou kai asfaleias poy exete!
6.3. Ypologizontas thn maska ypodiktuoy kai toys ari8mous diktuwn
H maska diktuoy einai ayth poy kanei ola ta topika magika ths
diaireshs enos diktuoy IP se ypodiktya.
H maska diktuoy gia ena mh-ypodiairemeno ari8mo diktuoy IP einai apla
mia diaxwrizomenh me teleies tetrada ari8mwn poy exei ola ta 'bit
diktuoy' toy ari8mou diktuoy ws '1' kai ola ta host bit ws '0'.
Etsi, gia tis treis klaseis twn diktuwn IP, oi klassikes maskes
diktuoy einai:
· Klash A (8 bit diktuoy): 255.0.0.0
· Klash B (16 bit diktuoy): 255.255.0.0
· Klash C (24 bit diktuoy): 255.255.255.0
O tropos me ton opoio h ypo-diktuwsh dra, einai na daneizetai ena h
perissotera apo ta dia8esima bit kai meta na kanei ta interface topika
na metafrazoyn ta daneika bit ws meros twn network bit. Etsi, gia na
diairesoyme enan ari8mo diktuoy se duo ypodiktya, 8a daneizomastan ena
host bit 8etontas to katallhlo bit sthn maska diktuoy toy prwtoy
(kanonikou) host bit se '1'.
Gia mia dieu8ynsh klashs C, ayto 8a katelhge se mia maska diktuoy san
thn
11111111.11111111.11111111.10000000
h 255.255.255.128
Gia ton ari8mo diktuoy mas ths klashs C 192.168.1.0, aytes einai
merikes apo tis epiloges ypo-diktuwshs poy exete:-
______________________________________________________________________
No of No of
subnets Hosts/net netmask
2 126 255.255.255.128 (11111111.11111111.11111111.10000000)
4 62 255.255.255.192 (11111111.11111111.11111111.11000000)
8 30 255.255.255.224 (11111111.11111111.11111111.11100000)
16 14 255.255.255.240 (11111111.11111111.11111111.11110000)
32 6 255.255.255.248 (11111111.11111111.11111111.11111000)
64 2 255.255.255.252 (11111111.11111111.11111111.11111100)
______________________________________________________________________
Oysiastika, den yparxei apolutws kanenas logos gia na akoloy8eiste ton
parapanw tropo ypodiktuwshs, kata ton opoio ta bit ths maskas diktuoy
prosti8entai apo to pio shmantiko bit toy host pros to pio ashmanto.
Parola ayta, an den to kanete kat'ayton ton tropo, oi ari8moi IP poy
8a prokucoyn 8a einai se mia POLY periergh akoloy8ia! Ayto kanei polu
duskolo gia emas toys an8rwpoys, to na apofasisoyme se poio ypodiktyo
anhkei ka8e ari8mos IP, ka8ws den eimaste polu kaloi sto na
skeftomaste sto dyadiko susthma (oi ypologistes apo thn allh einai kai
8a xrhsimopoihsoyn opoiodhpote sxhma toys peite, me thn idia panta
eykolia).
Afou apofasisete gia thn katallhlh maska diktuoy, xreiazetai na
katalabete poies einai oi diafores diey8unseis Diktuoy kai
ekpomphs/anakoinwshs - kai thn klimaka twn ari8mwn IP gia ka8e ena apo
ayta ta diktya. Pali, 8ewroume mono enan ari8mo diktuoy IP klashs C
kai apeikonizontas mono to teleytaio meros (toy host) exoyme:
______________________________________________________________________
Netmask Subnets Network B'cast MinIP MaxIP Hosts Total Hosts
--------------------------------------------------------------------------
128 2 0 127 1 126 126
128 255 129 254 126 252
192 4 0 63 1 62 62
64 127 65 126 62
128 191 129 190 62
192 255 193 254 62 248
224 8 0 31 1 30 30
32 63 33 62 30
64 95 65 94 30
96 127 97 126 30
128 159 129 158 30
160 191 161 190 30
192 223 193 222 30
224 255 225 254 30 240
______________________________________________________________________
Opws mporei na idw8ei, yparxei mia polu kala ka8orismenh akoloy8ia se
aytous toys ari8mous, h opoia toys kanei ton elegxo toys sxetika aplo.
H 'arnhtikh pleyra' ths ypo-diktuwshs einai epishs emfanhs, oson afora
thn meiwsh toy olikou ari8mou twn dia8esimwn host diey8unsewn, oso to
plh8os twn ypodiktuwn ayjanei.
Me aythn thn plhroforhsh, eisaste pleon se 8esh na ana8esete ari8mous
IP se host kai diktya ka8ws kai maskes diktuoy.
7. Dromologhsh
An xrhsimopoieite ena H/Y Linux me duo kartes diktuoy gia na
dromologeite metaju duo (h perissoterwn)ypodiktuwn, xreiazetai na
exete energopoihsei thn Prow8hsh IP (IP Forwarding) ston pyrhna sas.
Kante ena
______________________________________________________________________
cat /proc/ksyms | grep ip_forward
______________________________________________________________________
8a prepei na sas epistrecei kati san...
______________________________________________________________________
00141364 ip_forward_Rf71ac834
______________________________________________________________________
An oxi, tote den exete energopoihsei thn Prow8hsh IP (IP-Forwarding)
ston pyrhna sas kai xreiazetai na epanametaglwtissete kai na
egkatasthsete enan neo.
Gia xarh toy paradeigmatos, as ypo8esoyme pws exete apofasisei na
ypodiktywsete ton C klashs ari8mo diktuoy sas 192.168.1.0 se 4
ypodiktya (to ka8ena apo 62 xrhsimopoihsimoys interface/host ari8mous
IP). Parola ayta, duo apo ayta ta diktya syndyazontai se ena
megalutero eniaia diktyo, dinontas tria fysika diktya.
ayta einai :-
______________________________________________________________________
Network Broadcast Netmask Hosts
192.168.1.0 192.168.1.63 255.255.255.192 62
192.168.1.64 192.168.1.127 255.255.255.192 62
182.168.1.128 192.168.1.255 255.255.255.126 124 (des shmeiwsh)
______________________________________________________________________
Shmeiwsh: o logos poy to teleytaio diktyo exei mono 124
xrhsimopoihsimes diey8unseis diktuoy (oxi 126 opws 8a perimename apo
th maska diktuoy) einai giati sthn pragmatikothta einai ena 'yper-
diktyo' duo ypodiktuwn. Oi Host sta alla duo diktya 8a metafrazoyn to
192.168.1.192 san th diktyakh dieu8ynsh toy 'mh-yparktou' ypodiktuoy.
Paromoia, 8a metafrazoyn to 192.168.1.191 san th dieu8ynsh
ekpomphs/anakoinwshs toy 'mh-yparktou' ypodiktuoy.
Etsi, an xrhsimopoieite to 192.168.1.191 h to 192 san host diey8unseis
toy tritoy diktuoy, tote oi mhxanes twn duo mikroterwn ypodiktuwn den
8a einai ikanes na epikoinwnoun mazi toys.
Ayto katadeiknuei ena shmantiko 8ema me ta ypodiktya - oi
xrhsimopoihsimes diey8unseis ka8orizontai apo to MIKROTERO ypodiktyo
ston xwro diey8unsewn sas.
7.1. Oi pinakes dromologhsewn (routing tables)
As ypo8esoyme oti enas ypologisths poy trexei Linux dra san router gia
ayto to diktyo. 8a exei tria interface diktuoy sta topika LANs kai
pi8anon ena tetarto interface sto Internet (poy 8a apotelei kai thn
proka8orismenh toy dromologhsh).
As ypo8esoyme oti to Linux xrhsimopoiei thn xamhloterh dia8esimh
dieu8ynsh IP se ka8e ypodiktyo sto antistoixo interface gia to diktyo
ayto. 8a ru8mize ta diktyaka toy interface ws:
______________________________________________________________________
Interface IP Address Netmask
eth0 192.168.1.1 255.255.255.192
eth1 192.168.1.65 255.255.255.192
eth2 192.168.1.129 255.255.255.128
______________________________________________________________________
H dromologhsh poy 8a egka8idrye 8a htan:
______________________________________________________________________
Destination Gateway Genmask Interface
192.168.1.0 0.0.0.0 255.255.255.192 eth0
192.168.1.64 0.0.0.0 255.255.255.192 eth1
192.168.1.128 0.0.0.0 255.255.255.128 eth2
______________________________________________________________________
Se ka8'ena apo ta ypodiktya, oi host 8a ry8mizontan me to proswpiko
toys ari8mo IP kai maska diktuoy (katallhlh gia to sygkekrimeno
diktyo). Ka8e host 8a dhlwne to Linux PC ws th dikia toy
pulh/dromologhth (gateway/router), 8etontas thn dieu8ynsh IP toy
interface toy Linux gi'ayto to sygkekrimeno diktyo.
Gia sxolia, parathrhseis, dior8wseis, enhmerwseis peri ths Ellhnikhs
metafrashs aytou toy HOWTO, steilte email sthn Boula Sanida :
voulariba@hellug.gr h ston Eyripidh Papakwsta : evris@hellug.gr kai
enallaktika : ep@rocketmail.com
Robert Hart Melbournh, Aystralia Martios 1997.