  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 IP
  diktyo - klashs A , B kai C wste na leitoyrgei swsta gia diafora
  diasyndedemena diktya.

  11..  PPnneeyymmaattiikkhh IIddiiookktthhssiiaa

  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2..  EEiissaaggwwgghh

  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 epikentrwnete se diktya IP klashs C - alla oi

  arxes efarmozontai epishs kai se diktya A kai B klasewn.


  22..11..  AAlllleess pphhggeess ppllhhrrooffoorriiaass

  Yparxei ena plh8os allwn phgwn plhroforias poy einai sxetikes

  gia leptomereis plhrofories ka8ws kai gia plhrofories ypoba8roy
  sxetika me IP diey8unseis.  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3..  HH aannaattoommiiaa ttwwnn IIPP aarrii88mmwwnn

  Prin proxwrisoyme sthn apolaysh ths ypodiktuwshs (sub-networking) ,
  xreiazetai na 8esoyme ta basika sxetika me tis IP diey8unseis.

  33..11..  OOii ddiieeyy88uunnsseeiiss IIPP aannooiikkooyynn ssee IInntteerrffaacceess -- OOXXII ssee  hhoossttss!!

  Prwta ap'ola, as jeka8arisoyme merikes basikes aities parejhghshs - oi
  ari8moi IP einai ddeenn anati8entai se hosts. Oi ari8moi IP anati8entai
  se interfaces diktuwn poy exoyn oi hosts.


  Eeee - Ti einai ayto?


  Enw polloi (ean oxi oi perissoteroi) ypologistes se ena IP diktyo 8a
  katexoyn ena mono interface diktuoy (kai exoyn ena mono ari8mo IP san
  synepeia), aytos den einai o monos tropos poy ginontai ta pragmata. Oi
  ypologistes ka8ws kai alles syskeyes mporoun na exoyn diafora (ean oxi
  polla) interfaces diktuoy- kai ka8e interface na exei ton diko toy
  ari8mo IP.


  Etsi mia syskeyh me 6 diaforetika interfaces (opws enas dromologhths -
  router) 8a exei 6 IP ari8mous - enan gia ka8e interface gia ka8e
  diktyo sto opoio einai syndedemenos. H aitia

  gi'ayto einai profanhs otan koitajoyme ena diktyo IP!


  Prol'ayta, oi perissoteroi anaferontai se _h_o_s_t _d_i_e_y_8_u_n_s_e_i_s otan
  anaferontai se ari8mous IP. Apla na 8ymaste, ayto einai mia eykolia
  gia _t_o_n _a_r_i_8_m_o _I_P  _t_o_y _s_y_g_k_e_k_r_i_m_e_n_o_y _i_n_t_e_r_f_a_c_e _s_e _a_y_t_o_n _t_o_n _h_o_s_t.
  Polles (ean oxi oi pleiochfia) twn syskeywn sto Internet exoyn
  monadiko interface kai epomenws monadiko ari8mo IP.


  33..22..  OOii aarrii88mmooii IIPP ssaann ""TTeettrraaddeess mmee tteelleeiieess""

  Sthn sygkekrimenh (IPv4) ylopoihsh twn ari8mwn IP, aytoi apotelountai
  apo 4 (8 bit) bytes - dinontas ena sunolo 32 bits 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 diaxorizomenoi apo
  (.) teleies.


  Ka8ws ka8e enas apo toys 4 ari8mous einai h dekadikh anaparastash enos
  8 bit byte, ka8e enas apo toys 4 ari8mous mporei na einai apo 0 ews
  255 (dhladh 256 monadikes times - 8ymh8hte, to 0 einai kai ayto mia
  timh).


  Epipleon, meros toy ari8mou IP enos host prosdiorizei to diktyo sto
  opoio o host edreuei, ta ypoloipa 'bits' toy ari8mou IP prsdiorizoyn
  to idio to host (oyps... -to interface toy diktuoy). Akribws to posa
  bits xrhsimopoiountai to ID toy diktuoy kai posa einai dia8esima gia
  na prosdiorizoyn hosts (interfaces) se ayto to diktyo orizetai apo thn
  'klash' ('class') toy diktuoy.

  33..33..  KKllaasseeiiss ttwwnn DDiikkttuuwwnn IIPP

  Yparxoyn treis klaseis ari8mwn IP


    Oi ari8moi diktuwn IP klashs A xrhsimopoioyn ta 8 aristerotera bits
     (ta aristerotera sthn tetrada) gia na prosdiorizoyn to diktyo,
     afhnontas 24 bits (ta ypoloipomena tria apo thn tetrada) gia na
     prosdiorizoyn host interfaces se ayto to diktyo.
     Oi diey8unseis klashs A ppaannttaa exoyn sto aristerotero bit apo to
     aristerotero byte ena mhdeniko - ayth einai mia dekadikh timh apo 0
     ews 127 gia thn prwto tetrada.  Etsi yparxoyn to megisto 128
     ari8moi 1hs tajhs diktuoy dia8esimoi, me ka8'enan na periexei panw
     apo 33,554,430 dynata interfaces.

     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 _d_i_a_8_e_s_i_m_o_i ari8moi
     diktuwn klashs A.

    Oi ari8moi diktuwn IP klashs B xrhsimopoioun ta aristerotera 16
     bits (tis duo aristeroteres terades) gia na prosdiorizoyn to
     diktyo, afhnontas 16 bits (tis duo teleytaies tetrades) gia na
     prosdiorizoyn host interfaces. Oi diey8unseis klashs B panta exoyn
     ta aristerotera 2 bits toy aristeroteroy byte 1 0. Ayto afhnei 14
     bits 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 interfaces.

    Oi ari8moi diktuwn IP klashs C xrhsimopoioun ta 24 aristerotera
     bits (ta aristerotera tria bytes) gia na prosdiorizoyn to diktyo,
     afhnontas 8 bits (to dejiotero byte) gia na prosdiorizoyn host
     interfaces. Oi diey8unseis 3hs tajhs panta jekinane  me ta
     aristerotera 3 bits 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 IP klashs C diktuwn, opoy to ka8ena
     periexei 254 interfaces. (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 to 126
                       B (2h)  128 to 191
                       C (3h)  192 to 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
  hosts.


  33..44..  OOii aarrii88mmooii ddiikkttuuwwnn,, ddiieeyy88uunnsseeiiss iinntteerrffaaccee,, ddiieeyy88uunnsseeiiss aannaakkooiinn
  wwsshhss ((bbrrooaaddccaasstt aaddddrreesssseess))

  Oi ari8moi IP mporei na exoyn treis pi8anes shmasies:-


    H dieu8ynsh enos IP diktuoy (mia omada apo IP syskeyes poy exoyn
     koinh prosbash se ena meso metadoshs - opws an p.x. briskontai oles
     sto ido tmhma enos diktuoy Ethernet).  Enas ari8mos diktuoy panta
     8a exei ta interface (host) bits toy xwroy ths dieu8ynshs 0 (ektos
     kai an to diktyo exei ypodiktya - opws 8a doume);

    h dieu8ynsh koinhs ekpomphs/anakoinwshs (broadcast) enos IP diktuoy
     (h dieu8ynsh poy xrhsimopoieitai gia na 'milame',  taytoxrona, se
     oles tis syskeyes se ena IP diktyo). Oi diey8unseis anakoinwshs gia
     ena diktyo panta exoyn ta interface (host) bits 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 bits
     eekkttooss ola mhdenika h ola 1 - giati me ta host bits ola mhdenika,

     h dieu8ynsh einai mia dieu8ynsh diktuoy kai me ta host bits 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

  bytes toy xwroy dieu8ynshs toy host)

          10.0.0.0 einai enas ari8mos, diktuoy klashs A giati ola ta host
                  bits toy xwroy dieu8ynshs, einai 0
          10.0.1.0 einai h dieu8hynsh enos host se ayto to diktyo
          10.255.255.255.255 einai h dieu8ynsh anakoinwshs/ekpomphs (broadcast) aytou toy diktuoy
                  giati ola ta host bits toy xwroy dieu8ynshs einai 1

  Gia ena diktyo klashs B ...
  (duo bytes toy xwroy dieu8ynshs diktuoy akoloy8oumena apo duo bytes 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 bytes xwros dieu8ynshs diktuoy akoloy8oumenos 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 IP ari8moi diktuoy poy paramenoyn dia8esimoi gia
  desmeysh

  pros to paron einai diey8unseis klashs C.


  33..55..  HH mmaasskkaa ddiikkttuuooyy ((nneettwwoorrkk mmaasskk))

  H maska diktuoy legetai swstotera maska ypodiktuoy (subnetwork mask).
  Omws, genika anaferomaste s'ayth ws h maska diktuoy.


  H maska diktuoy kai oi epiptwseis ths sthn metafrash twn IP
  diey8unsewn, _t_o_p_i_k_a, se ena kommati IP diktuoy poy kyriws mas afora
  edw, ka8ws ayto ka8orizei poia (ean yparxei kapoia) ypo-diktuwsh
  lambanei xwra.


  H klassikh maska (upo-) diktuoy einai ola ta bits toy diktuoy mias
  dieu8ynshs '1' kai ola ta host bits '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 epirreazei mono thn ttooppiikkhh metafrash ttooppiikkwwnn IP
     ari8mwn (opoy topiko shmainei, se ayto to sygkekrimeno kommati toy
     diktuoy);

    H maska diktuoy ddeenn einai IP ari8mos - xrhsimopoieite gia na
     tropopoiei ton tropo topikhs metafrashs twn IP ari8mwn.


  44..  TTii eeiinnaaii ttaa yyppooddiikkttyyaa??

  Ena ypodiktyo einai enas tropos na paroyme mia IP dieu8ynsh diktuoy
  kai ttooppiikkaa na thn spasoyme etsi wste ayth h aplh IP dieu8ynsh diktyoy
  na mporei sthn pragmatikothta na xrhsimopoih8ei se diafora
  diasyndedemena topika diktya. 8ymh8hte, enas aplos ari8mos IP mporei
  na xrhsimopoih8ei mono se ena aplo diktyo.


  To shmantiko edw einai to ttooppiikkaa: oson afora ton kosmo ejw apo ths
  mhxanes

  kai ta fysika diktya poy kaluptontai apo to ypo-diktyomeno diktyo IP,

  tipota den exei allajei - paramenei akoma ena aplo IP diktyo.  Ayto
  einai shmantiko - h ypo-dktuwsh einai mia ttooppiikkhh ru8mish

  kai einai aorath stn ypoloipo kosmo.


  55..  GGiiaattii yyppooddiikkttyyoo??

  Oi aities pisw apo thn ypo-diktuwsh xronologountai pisw stis prwtes
  prodiagrafes toy IP - opoy mono ligosta sites etrexan se ari8mous
  diktuoy klashs A, ta opoia epitrepoyn ekatomuria syndedemenoys hosts.


  Prokaleitai profanws megalo kykloforiako kai diaxeiristiko problhma,
  ean oloi oi IP computers 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 diaxeirhsh ka8e tetoioy
  jexwristou diktuoy mporei epishs na metabibastei.


  Ayto epitrepei th dhmioyrgia mikrwn, diaxeirisimwn diktuwn - polu
  pi8anon xrhsimopoiwntas diaforetikes texnologies diktuwshs. 8ymh8hte,
  den mporeite na anamijete Ethernet, Token Ring, FDDI, ATM klp. sto
  idio fysiko diktyo - mporoun parol'ayta na diasynde8oun!


  lloi logoi ypodiktuwshs einai:-


    H fysikh katonomh 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 IP ari8mo diktuoy.
     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 (xwritikothta)


    Oi prodiagrafes asfaleis mporei epishs na ypagoreuoyn pws
     diaforetikes klaseis xrhstwn den moirazontai to ido 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)!

    Yparjh ejoplismou poy xrhsimopoiei asumbates diktyakes texnologies,
     kai h anagkh na tis diasyndesete kai aytes (opws anafer8hke kai
     parapanw).


  66..  PPwwss nnaa yyppooddiikkttyywwsseettaaii eennaann aarrii88mmoo ddiikkttuuooyy IIPP

  Afou apofasisate pws xreiazetai na ypodiktywsete ton ari8mo toy IP
  diktuoy 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
     diasundeseis - 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 IP dieu8ynsh 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 interfaces (hosts), syn ton
  ypoxrewtiko ari8mo diktuoy (192.168.1.0) kai thn dieu8ynsh ekpomphs
  (broadcast address) (192.168.1.255).


  66..11..  KKaannttee tthhnn ffyyssiikkhh ssuunnddeesshh


  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 leptomereis 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6..22..  MMeeggee88ooss yyppooddiikkttuuwwnn

  Yparxei ena pazari metaju toy ari8mou twn ypodiktuwn poy dhmioyrgeite
  kai twn 'xaramismenwn' IP ari8mwn.


  To ka8e diktyo IP exei dyo diey8unseis poy einai axrhstes gia
  diey8unseis interface (host)- ton idio ton ari8mo toy IP diktuoy kai
  thn dieu8ynsh ekpomphs/anakoinwshs. Otan ypodiktywnete, ka8e ypodiktyo
  apaitei apo mono toy, monadiko IP ari8mo diktuoy kai diey8ynsh
  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 network se duo jexwrista
  ypodiktya, dhmioyrgountai dduuoo diey8unseis diktuwn kai dduuoo diey8unseis
  ekpompwn- ayjanontas tis 'mh xrhsimopoioumenes' diey8unseis interface
  (host); dhmioyrgwntas 4 ypodiktya dhmioyrgei ookkttww axrhste diey8unseis
  gia interface (host) k.o.k.


  Sthn oysia, to mikrotero xrhsimo ypodiktyo apoteleitai apo 4 IP
  ari8mous:-

    Dyo xrhsimoys ari8mous IP gia interfaces - 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 IP ari8mo diktuoy sas, to polu
  se 2^n (opoy n einai ena ligotero apo ton ari8mo twn bits ston IP
  ari8mo diktyoy sas) idioy mege8oys ypodiktya.


  Kai etsi, as eisaste realistes kata to sxediasmo toy diktuoy sas -
  8elete ton eellaaxxiissttoo ari8mo jexwristwn topikwn diktuwn poy
  antapokrinetai stoys ylikous periorismous kai stoys periorismous
  diaxeirishs, ejoplismou, asfaleias poy exete!


  66..33..  YYppoollooggiizzoonnttaass tthhnn mmaasskkaa yyppooddiikkttuuooyy kkaaii ttooyyss aarrii88mmoouuss ddiikkttuuwwnn

  H maska diktuoy einai ayth poy kanei ola ta ttooppiikkaa magika ths
  diaireshs enos diktuoy IP se ypodiktya.


  H maska diktuoy gia ena mh-ypodiaremeno IP ari8mo diktyoy einai apla
  mia diaxwrizomenh me teleies terada ari8mwn poy exei ola ta 'bits
  diktuoy' toy ari8mou diktuoy ws '1' kai ola ta host bits ws '0'.


  Etsi, gia tis treis klaseis twn diktuwn IP, oi klassikes maskes
  diktuoy einai:-

    Klash A (8  bits diktuoy) : 255.0.0.0

    Klash B (16 bits diktuoy): 255.255.0.0

    Klash C (24 bits diktuoy): 255.255.255.0


  O tropos me ton opoio h ypo-diktywsh dra, einai na _d_a_n_e_i_z_e_t_a_i ena h
  perissotera apo ta dia8esima bits kai meta na kanei ta interfaces
  ttooppiikkaa na metafrazoyn ta daneika bits ws meros twn network bits. Etsi,
  gia na diairesoyme enan ari8mo diktuoy se duo ypodiktya, 8a
  daneizomastan ena host bit 8etwntas 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 akoloy8eisete
  ton parapanw tropo ypodiktuwshs, kata ton opoio ta bits 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 _P_O_L_Y periergh akoloy8ia!
  Ayto kanei polu duskolo gia emas toys an8rwpoys, to na apofasisoyme se
  pio ypodiktyo anhkei ka8e 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 IP ari8mo diktuo klashs C
  kai apeikonizontas mono to _t_e_l_e_y_t_a_i_o 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 eidw8ei, 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 thm plhroforhsh, eisaste pleon se 8esh na ana8esete ari8mous
  IP se hosts kai diktya ka8ws kai maskes diktuoy.


  77..  DDrroommoolloogghhsshh

  An xrhsimopoieite ena H/Y Linux me duo kartes diktuoy gia na
  dromologeite metaju duo (h perissoterwn)ypodikuwn, xreiazetai na exete
  energopoihsei thn IP Prow8hsh (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 IP Prow8hsh (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
  thn maska diktuoy) einai giati sthn pragmatikothta einai ena 'yper-
  diktyo' duo ypodiktuwn. Oi Hosts sta alla duo diktya 8a metafrazoyn to
  192.168.1.192 san thn _d_i_k_t_y_a_k_h dieu8ynsh toy 'mh-yparktou' ypodiktuoy.
  Paromoia, 8a metafrazoyn to 192.168.1.191 san thn dieu8ynsh
  ekpomhs/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 katadyknuei ena shmantiko 8ema me ta ypodiktya - oi
  xrhsimopoihsimes diey8unseis ka8orizontai apo to MMIIKKRROOTTEERROO ypodiktyo
  ston xwro diey8unsewn sas.


  77..11..  OOii ppiinnaakkeess ddrroommoolloogghhsseewwnn ((rroouuttiinngg ttaabblleess))

  As ypo8esoyme oti enas ypologisths poy trexei Linux dra san router gia
  ayto to diktyo. 8a exei tria interfaces 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 IP
  dieu8ynsh se ka8e ypodiktyo sto antistoixo interface gia to diktyo
  ayto. 8a

  ru8mize ta diktyaka toy interfaces 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 hosts 8a ry8mizontan me to proswpiko
  toys IP ari8mo 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 IP dieu8hnsh 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.




























