TCP IP Illustrated

Htm 3 of 12 12092001 144700 chapter 12 broadcasting

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Unformatted text preview: then used as the destination address for datagrams sent out the interface. The Host Requirements RFC takes no stand on the issue of whether a multihomed host should send a file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/broadcas.htm (3 of 12) [12/09/2001 14.47.00] Chapter 12. Broadcasting and Multicasting limited broadcast out all its interfaces. Net-directed Broadcast The net-directed broadcast address has a host ID of all one bits. A class A net-directed broadcast address is netid.255.255.255, where netid is the class A network ID. A router must forward a net-directed broadcast, but it must also have an option to disable this forwarding. Subnet-directed Broadcast The subnet-directed broadcast address has a host ID of all one bits but a specific subnet ID. Classification of an IP address as a subnet-directed broadcast address requires knowledge of the subnet mask. For example, if a router receives a datagram destined for 128.1.2.255, this is a subnet-directed broadcast if the class B network 128.1 has a subnet mask of 255.255.255.0, but it is not a broadcast if the subnet mask is 255.255.254.0 (0xfffffe00). All-subnets-directed Broadcast An all-subnets-directed broadcast address also requires knowledge of the destination network's subnet mask, to differentiate this broadcast address from a net-directed broadcast address. Both the subnet ID and the host ID are all one bits. For example, if the destination's subnet mask is 255.255.255.0, then the IP address 128.1.255.255 is an allsubnets-directed broadcast. But if the network is not subnetted, then this is a net-directed broadcast. Current feeling [Almquist 1993] is that this type of broadcast is obsolete. It is better to use multicasting than an all-subnets-directed broadcast. [Almquist 1993] notes that RFC 922 requires that an all-subnets-directed broadcast be sent to all subnets, but no current routers do so. This is fortunate since a host that has been misconfigured withou...
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