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

Internet Addressing
TCP/IP Protocols and Functions
through this address. This mapping is normally performed through a translation
table. If a match cannot be found for a destination internet address, ARP is
invoked to determine this address.
Indirect routing is used when the source and destination nodes are on different
networks within an internet. The source node sends packets to a gateway or router
on the same network using direct routing. From there, the packets are forwarded
through intermediate gateways or routers, as required, until they arrive at the
destination network. Direct routing is then used to forward the packets to the
destination host on that network. Each gateway, router, and host in an internet has
a routing table that defines the address of the next gateway or router to other
networks (as well as other nodes on other networks) in an internet.
Each internet host is assigned at least one unique internet address. This address is
used by the IP and other higher-level protocols. When gateway hosts are used,
more than one address may be required. Each interface to an internet is assigned
its own unique address. Internet addresses are used to route packets through the
network.
Addresses within an internet consist of a network number and a local address. The
unique network number is assigned to each network when it connects to another
internet. If a local network is not going to connect to other internets, any
convenient network number is assigned.
Hosts that exchange packets on the same physical network should have the same
network number. Hosts on different physical networks might also have the same
network number. If hosts have the same network number, part of the local address
is used as a subnetwork number. All host interfaces to the same physical network
are given the same subnetwork number.
An internet can provide standards for assigning addresses to networks, broadcasts,
and subnetworks. Examples of these standard formats are described in the
following sections.
Network Address Format
A standard internet address uses a two-part, 32-bit address field. The first part of
the address field contains the network address; the second part contains the local
address. The four different types of address fields are classified as A, B, C, or D,
depending on the bit allocation.
Figure 2 represents a class A address. A class A address has a 7-bit network
number and a 24-bit local address. The highest order bit is set to 0.
0 1234567
1 2 3
890123456789012345678901
0 Network
Local Address
Figure 2. Class A Address
Chapter 1. Introducing Computer Networks and Protocols 11