Broadcast adresa

[Petr Kolar]

   Adresa 255.255.255.255 je tzv. "limited broadcast", ktery nesmi zadny
router predat do dalsi site. Cili je to broadcast do podsite, ke ktere je
pripojen odesilatel. Na Ethernetu musi byt zabalen do ramce s adresou
prijemce FF:FF:FF:FF:FF:FF. Protokol BOOTP (a nektere dalsi) jsou ponekud
vyjimecne, protoze jsou pouzivany stroji, ktere o siti nevi vubec nic. Proto
se na vetsine routeru da zaridit, aby BOOTP pakety BYLY predavany pres router
i kdyz maji cilovou adresu 255.255.255.255 (na Cisco routerech prikaz
ip-helper-address xxx.yyy.zzz.uuu na kazdem interface, odkud muze prichazet
BOOTP request a kde neni BOOTP server; na serverech Novell Netware
fungujicich jako IP routery modul BOOTPFWD).

   Dal budu predpokladat sit tridy B rozdelenou na podsite s maskou
255.255.255.0. Pak 128.250.1.255 je "subnet directed broadcast" na podsiti
128.250.1.0, ktery routery nepripojene do teto podsite smeruji jako unicast
ramec spravnym smerem. Router, ktery je primo pripojen k teto siti, pri
prijeti tohoto paketu z podsite 128.250.1.0 neposila nikam nic, pri prijeti
tohoto paketu z jine podsite posle do podsite 128.250.1.0 broadcast (tj na
Ethernetu fyzicka adresa prijemce FF:FF:FF:FF:FF:FF).

   V RFC-1122 (Requirements for Internet Hosts - Communication Layers), je
dal popsany "all-subnets directed broadcast" v nasem pripade 128.250.255.255.

   Aby to nebylo tak jednoduche ;-) , Unixy vychazejici z 4.2 BSD pouzivaji
pri broadcastu na miste jednicek nuly, coz by ostatni mely taktez prijimat.
Pokud se tyto stroje daji presvedcit, nemely by se broadcasty s nulami vubec
pouzivat.

   Sediva je teorie... Pokud vyse popsane nefunguje, musite vyzkouset, co
vlastne funguje a v meznim pripade se s neposlusnym strojem vubec rozloucit.

-------------------------------- Petr Kolar --------------------------------
I      Department of Computer Science, Technical University of Liberec     I
I E-mail: Petr.Kolar at vslib.cz  Phone: +42-48-5227-374  Fax: +42-48-5100865 I
I           Postal: Voronezska 13, 461 17 Liberec, Czech Republic          I
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