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 anakoinwshs (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.