User’s Manual
Appendix 5
Basics of Internetworking
Communication networks can be divided into two basic types:
connection-oriented
(also
circuit-
switched
) and
connectionless
(sometimes called
packet-switched
). Connection-oriented networks
operate by forming a dedicated connection or circuit between two points. Typical example of
connection-oriented network is a telephone system. The advantage of connection-oriented
networking lies in its guaranteed capacity. That is once a circuit is established; no other network
activity will decrease the capacity of that circuit. However, the disadvantages arise from cost.
In a connectionless network, that is often used to connect computers, data to be transmitted across
a network is divided into small pieces called packets that are multiplexed onto high capacity inter
machine connections. A packet, which usually contains only a few hundred bytes of data, carries
identification that enables the network hardware to know how to send it to the specified destination.
For example, a large file to be transmitted between two machines must be broken into many
packets that are sent across a network one at a time. The network hardware delivers the packets to
the specified destination, where software reassembles them into a single file again. The main
advantage of packet-switching is that multiple communications among computers can proceed
concurrently, with inter machine connections shared by all pairs of computer that are
communicating. The disadvantage is that as activity increases, a given pair of communicating
computers receive less of the network capacity. That is, whenever a packet switched network
becomes overloaded, computers using the network must wait before they can send additional
packets.
Each network hardware technology defines an
addressing mechanism
that computers use to
specify the destination for a packet. Every computer attached to a network is assigned a unique
address, usually an integer. A packet sent across a network includes a
destination address field
that contains the address of the intended recipient. The destination address appears in the same
location in all packets, making it possible for the network hardware to examine the destination
address easily. A sender must know the address of the intended recipient, and must place the
recipient’s address in the destination address field of a packet before transmitting the packet. Each
hardware technology specifies how computers are assigned addresses. The hardware specifies, for
example, the number of bits in the address as well as the location of the destination address field in
a packet.
Ethernet
is the name given to a popular packet-switched LAN technology invented at Xerox PARC
in the early 1970s. Xerox Corporation, Intel Corporation, and Digital Equipment Corporation
standardized Ethernet in 1978; IEEE released a compatible version of the standard using the
standard number 802.3. Ethernet has become the most popular LAN technology; it now appears in
virtually all corporate networks as well as many small installations.
A
twisted pair Ethernet
allows a computer to access an Ethernet using conventional unshielded
copper wires. Formally known as
10Base-T
, the twisted pair Ethernet operates at 10 Mbps.
Ethernet defines a 48-bit addressing scheme. Each computer attached to an Ethernet network is
assigned a unique 48-bit number known as its
Ethernet address
or
Medium Access Control
(
MAC
)
address
. The Ethernet address contains a fixed manufacturer dependent part and device specific
part. Thus, no two hardware interfaces have the same Ethernet address. Note that physical
addresses are associated with the Ethernet interface hardware. Therefore, moving the hardware
interface to a new machine or replacing a hardware interface that has failed changes the machine’s
physical address.
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