z/VM: TCP/IP User's Guide - z/VM - IBM

all the hosts on the network and processed as if the datagram was sent directly to
the host’s IP address. The IP broadcast address is formed by setting all the host
bits to ones.
For more information about IP broadcasting, see RFCs 919 and 922.
Multicast Address Format
TCP/IP uses IP multicasting to send datagrams to all the TCP/IP hosts on a
network or subnetwork. The multicast datagrams are only received by those
TCP/IP hosts that have signed up to listen for the particular IP multicast address
(joined the multicast group). If a TCP/IP host has not joined the multicast group,
then the datagram is discarded.
For more information on IP multicasting, see RFC 1112.
Subnetwork Address Format
The subnetwork capability of TCP/IP divides a single network into multiple
logical networks (subnets). For example, an organization can have a single internet
network address that is known to users outside the organization, yet configure its
internal network into different departmental subnets. Subnetwork addresses
enhance local routing capabilities, while reducing the number of network numbers
required. For a subnet, the local address part of an internet address is divided into
a subnet number and a host number, for example:
network_number subnet_number host_number
TCP/IP Protocols and Functions
where:
network_number
subnet_number
host_number
Is the network portion of the internet address.
Is a field of a constant width for a given network.
Is a field that is at least 1-bit wide.
If the width of the subnet_number field is 0, the network is not organized into
subnets, and addressing to the network is done with an internet network address
(network_number).
Figure 6 represents a class B address with a 6-bit wide subnet field.
1 2 3
01 23456789012345 6789012345678901
10
Network
Subnet
Host
Figure 6. Class B Address with Subnet
The bits that identify the subnet are specified by a bit mask. A bit mask is a
pattern of characters used to assign subnet addresses. The subnet bits are not
required to be adjacent in the address. However, the subnet bits generally are
contiguous and are the most significant bits of the local address.
For more information about subnetwork addresses, see RFC 950.
Chapter 1. Introducing Computer Networks and Protocols 13