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NETWORK
TOPOLOGIES
This document is maintained by Dr. Benedict Hung. Comments should be
sent to [email protected].
Adapted from various publications and online resources. Please refer to the
REFERENCES section for details.
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INTRODUCTION
Network topology refers to layout of a network and the arrangement of the
nodes in the network. The topological arrangement can either be physical
(i.e. the physical layout of the devices on a network) or logical (i.e. the way
the signals act on a network and the way the data is transmitted).
1.1
POINT-TO-POINT LINK
In a point-to-point link, two devices monopolize a single communication
medium. Since the communication medium is not shared, the devices are not
addressed. Point-to-point links can be simplex, half-duplex, or full-duplex.
When devices are engaged in a bi-directional communication on a half-
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duplex point-to-point link, there is a mechanism to switch the roles of the
devices between being a sender and a receiver.
1.1.1
Simplex Communication
There are two stations in the network. The signals in the network flow in one
direction from one sender to one receiver. The sender and the receiver
operate on the same frequency. Examples of simplex communication include
radio broadcasting, television broadcasting, computer to printer
communication, and keyboard to computer connections.
1.1.2
Half-Duplex Communication
The two stations can both transmit and receive data on the network but not
simultaneously. For example, one workstation on the network can send data
on the line and then immediately receive data on the line from the same
direction in which data was just transmitted. A walkie-talkie is a half-duplex
device, because only one party can talk at a time.
1.1.3
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Full-Duplex Communication
The two stations can transmit and receive data simultaneously. The link
capacity is shared between the two devices or by two separate transmission
paths. An example of a full-duplex communication is a telephone where the
two parties at both ends of a call can speak and be heard by the other party
simultaneously. Full-duplex has no collisions so time is not wasted by having
a retransmit frames or by waiting for an acknowledgement from the
receiver.
1.2
MULTI-POINT LINK
There are three or more devices connected through a single communication
medium. For sharing a common channel, each device needs a way to identify
itself and the device to which it wants to send the information. The method
used to identify the senders and the receivers is called addressing.
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NETWORK ADDRESS
A network address is an identifier for a node or network interface within a
network. All network addressed are theoretically designed to be unique
across the network. However, it is possible for more than one type of
network address to be used in a single network.
One of the best known form of network addressing is the Internet Protocol
(IP) address. IP addresses consist of four bytes (32 bits) that uniquely identify
all computers on the public internet. Another popular form of address is the
Media Access Control (MAC) address. MAC addresses are six bytes (48
bits)that manufacturers of network adapters burn into the products to
uniquely identify them.
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2.1
PUBLIC VS. PRIVATE ADDRESSING
An IP address is a unique numerical value that is used to identify a computer
on a network. There are two kinds of IP addresses, public (also called
globally unique IP addresses) and private.
2.1.1
Public IP Addresses
Public IP addresses are assigned by the Internet Assigned Numbers Authority
(IANA). The addresses are guaranteed to be globally unique and reachable
on the Internet. IANA assures that multiple computers do not have the same
IP address. An Internet service provider (ISP) obtains a range of public IP
addresses from IANA, and then the ISP assigns the addresses to customers
to use when they connect to the Internet through the ISP. Public IP
addresses are routable on the Internet, which means that a computer with a
public IP address is visible to other computers on the Internet.
2.1.2
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Private IP Address
Private IP addresses cannot be used on the Internet. IANA has set aside
three blocks of IP addresses that cannot be used on the global Internet.
These three blocks of addresses are private IP addresses, and they are used
for networks that do not directly connect to the Internet.
A private IP address is within one of the following blocks or range of
addresses:



192.168.0.0/16:
This block allows valid IP addresses within the range 192.168.0.1 to
192.168.255.254.
172.16.0.0/12:
This block allows valid IP addresses within the range 172.16.0.1 to
172.31.255.254.
10.0.0.0/8:
This block allows valid IP addresses within the range 10.0.0.1 to
10.255.255.254.
Most small businesses and private networks prefer to use private IP
addresses for the local network, because ISPs generally charge a fee for each
public IP address. As a result, using public IP addresses on a local network is
costly. Hence, rather than purchasing a globally unique IP address for each
client computer on a local network, the network administrator can purchase
one globally unique IP address and use it for the router interface that
connects to the ISP.
2.2 IPV4 VS. IPV6 ADDRESSES
The version of the IP that is commonly used is version 4 (IPv4), which has not
changed substantially since RFC 791 was published in 1981. IPv4 is robust,
easily implemented, interoperable, and capable of scaling to a global utility
that can function with the Internet.
The Internet continues to grow exponentially, and the adoption of
broadband technologies, such as cable modems, mobile information
appliances, such as personal data assistants or PDAs, and cellular phones,
means that many more addresses are needed.
IPv6 significantly increases the number of addresses that are available. The
most obvious difference between IPv6 and IPv4 is the size of the addresses.
An IPv4 address is 32 bits long, and an IPv6 address is 128 bits long, which is
four times longer than an IPv4 address.
2.3 DYNAMIC VS. STATIC IP ADDRESSES
A local area network can have static and dynamic IP
addresses.
2.3.1
Dynamic IP addresses
Dynamic IP addresses are acquired from a DHCP server, and they may
change from time to time. The DHCP server must be assigned a static IP
address.
2.3.2
Static IP addresses
A static IP address does not change. It is assigned by the network
administrator, and it is manually entered into the properties for the network
adapter that is on a server or on a client computer. A static IP address does
not require that a DHCP server is running on the network.
Certain types of servers must have a static IP address. These servers include
DHCP servers, DNS servers, WINS servers, and any server that is providing
access to users who are using the Internet.
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2.4 NETMASK
IP addresses are broken into 4 octets (IPv4) separated by dots called dotted
decimal notation. An octet is a byte consisting of 8 bits. The IPv4 addresses
are in the following form:
192.168.10.1
There are two parts of an IP address:

Network ID

Host ID
The various classes of networks specify additional or fewer octets to
designate the network ID versus the host ID.
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CLASS
1ST OCTET
A
Net Id
B
C
2ND OCTET
3RD OCTET
4TH OCTET
Host ID
Net ID
Host ID
Net ID
Host ID
When a network is set up, a netmask is also specified. The netmask
determines the class of the network. When the netmask is setup, it specifies
some number of most significant bits with a 1's value and the rest have
values of 0. The most significant part of the netmask with bits set to 1's
specifies the network address, and the lower part of the address will specify
the host address.
2.4.1
Class A-E networks
The addressing scheme for class A through E networks is shown below.
NETWORK TYPE
ADDRESS
RANGE
NORMAL
NETMASK
CLASS A
001.x.x.x to 126.x.x.x
255.0.0.0
For very large
networks
CLASS B
128.1.x.x to
191.254.x.x
255.255.0.0
For medium size
networks
CLASS C
192.0.1.x to
223.255.254.x
255.255.255.0
For small networks
CLASS D
224.x.x.x to
239.255.255.255
CLASS E
240.x.x.x to
247.255.255.255
COMMENTS
Used to support
multicasting
There are some network addresses reserved for private use by the IANA
which can be hidden behind a computer which uses IP masquerading to
connect the private network to the internet. There are three sets of
addresses reserved.

10.x.x.x

172.16.x.x - 172.31.x.x

192.168.x.x
Other reserved or commonly used addresses:


127.0.0.1 - The loopback interface address. All 127.x.x.x addresses are
used by the loopback interface which copies data from the transmit
buffer to the receive buffer of the NIC when used.
0.0.0.0 - This is reserved for hosts that don't know their address and
use BOOTP or DHCP protocols to determine their addresses.
255 - The value of 255 is never used as an address for any part of the IP
address. It is reserved for broadcast addressing. Please remember, this is
exclusive of CIDR. When using CIDR, all bits of the address can never be all
ones.
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3
LAN TRANSMISSIONS
The application in use, such as multimedia, database updates, e-mail, or file
and print sharing, generally determines the type of data transmission.
LAN transmissions fit into one of three categories:

unicast

multicast

broadcast
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3.1
UNICAST
With unicast transmissions, a single packet is sent from the source to a
destination on a network. The source-node addresses the packet by using
the network address of the destination node. The packet is then forwarded
to the destination network and the network passes the packet to its final
destination.
Figure 1 Unicast Network
server
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

client
client
client
3.2 MULTICAST
With a multicast transmission, a single data packet is copied and forwarded
to a specific subset of nodes on the network. The source node addresses the
packet by using a multicast address. For example, the TCP/IP suite uses
224.0.0.0 to 239.255.255.255. The packet is then sent to the network, which
makes copies of the packet and sends a copy to each segment with a node
that is part of the multicast address.
Figure 2 Multicast Network
server



client
client
client
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3.3 BROADCAST
Broadcasts are found in LAN environments. Broadcasts do not traverse a
WAN unless the Layer 3 edge-routing device is configured with a helper
address to direct these broadcasts to a specified network address. This Layer
3 routing device acts as an interface between the local-area network (LAN)
and the wide-area network (WAN).
Figure 3 Broadcast Network
server


12

client
client
client
4
LAN TOPOLOGIES
There are five basic LAN topologies:

Star

Ring

Bus

Tree

Mesh
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4.1
STAR TOPOLOGY
All stations are attached by cable to a central point, usually a wiring hub or
other device operating in a similar function.
Figure 4 Star Topology

  

node
server
node
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node
node
Several different cable types can be used for this point-to-point link, such as
shielded twisted-pair (STP), unshielded twisted-pair (UTP), and fiber-optic
cabling. Wireless media can also be used for communications links.
The advantage of the star topology is that no cable segment is a single point
of failure impacting the entire network. This allows for better management
of the LAN. If one of the cables develops a problem, only that LAN-attached
station is affected; all other stations remain operational. The centralized
networking equipment can reduce costs in the long run by making network
management much easier.
The disadvantage of a star topology is the central hub device. This central
hub is a single point-of-failure in that if it fails, every attached station is out of
service.
4.2 RING TOPOLOGY
All stations in a ring topology are considered repeaters and are enclosed in a
loop. Unlike the star topology, a ring topology has no end points. The
repeater in this case is a function of the LAN-attached station’s network
interface card (NIC).Since each NIC in a LAN-attached station is a repeater,
each LAN station will repeat any signal that is on the network, regardless of
whether it is destined for that particular station.
Figure 5 Ring Topology

node


node
node

node
The major advantage of the ring topology is that no single device
monopolizes the network. Moreover, the network function continues in
slower speed after the full capacity is exceeded.
However, if a LAN-attached station fails to perform its repeater function, the
entire network could come down. When a station fails to operate, it is often
difficult to troubleshoot. Furthermore, it is necessary to disrupt the network
when adding or removing devices.
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4.3 BUS TOPOLOGY
Bus topology is a simple design that utilizes a single length of cable, also
known as the medium, with directly attached LAN stations. All stations share
this cable segment. Every station on this segment sees transmissions from
every other station on the cable segment; this is known as a broadcast
medium. The LAN attachment stations are definite endpoints to the cable
segment and are known as bus network termination points.
Figure 6 Bus Topology
server
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node
 
 
node
node
It is a very economical topological model and it is easy to extent the network
by adding cables with a repeater that boosts the signal and allows it to travel
a longer distance. Network management is straightforward and inexpensive.
There is a risk of a single point of failure. If the cable is broken, no LAN
station will have connectivity or the ability to transmit and receive. The
network becomes slow by heavy network traffic with a lot of devices
because networks do not coordinate with each other to reserve times to
transmit.
4.4 TREE TOPOLOGY
The tree topology is a logical extension of the bus topology and could be
described as multiple interconnected bus networks. The physical (cable)
plant is known as a branching tree with all stations attached to it. The tree
begins at the root, the pinnacle point, and expands to the network
endpoints. This topology allows a network to expand dynamically with only
one active data path between any two network endpoints.
Figure 7 Tree Topology
server

 

 
node
server
server
node
node
node
node
Tree topology is an extension of star and bus topologies, so in the networks
where these topologies cannot be implemented individually for reasons
related scalability, tree topology is the best alternative. With the nature of
this topology, expansion is possible and easy, furthermore, network
management, error detection, and correct are relatively easy.
The critical factor to ensure the operation of the network depends on a main
bus cable. If the main bus cable breaks, the entire network is crippled.
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4.5 MESH TOPOLOGY
In a mesh topology, each of the network node, computer, and other devices,
are interconnected with one another. Every node not only sends its own
signals but also relays data from other nodes. A true mesh topology is the
one where every node is connected to every other node in the network. This
type of topology is very expensive as there are many redundant connections.
Figure 8 Mesh Topology
server



node
node
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 
node
node
The first advantage for a mesh topology is where data can be transmitted
from different devices simultaneously. This topology can withstand high
traffic. Due to the number of redundant connections, if one of the
components fails there is always an alternative.
With the number of redundant connections, maintenance and
troubleshooting problems on a mesh topology is difficult and expensive.
5
NETWORK DEVICES
The four primary devices used in LANs are:

Hubs

Bridges

Switches

Routers
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5.1
HUBS
Hubs operate at the physical layer (Layer 1) of the Open Systems
Interconnection (OSI) model. A hub is used to connect devices so that they
are on one shared LAN. If two devices are directly connected with LAN
cables, a hub is needed to interconnect two or more devices on a single LAN.
The cable termination points are the hub and the LAN device (host).
Ethernet hubs send all the data from a network device on one port to all
other hub ports. When network devices are connected via a hub, LANattached devices will hear all conversations across the LAN. Each station
then examines the message header to determine if it is the intended
recipient. If more than one LAN station transmits at the same time, a
collision occurs and both stations initiate a backoff algorithm before
attempting retransmission.
5.2 BRIDGES
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This section focuses on transparent bridges, which can also be referred to as
learning or Ethernet bridges. Bridges have a physical layer (Layer 1), but are
said to operate at the data link layer (Layer 2) of the OSI model. Bridges
forward data frames based on the destination MAC address. Bridges also
forward frames based on frame header information. Bridges create multiple
collision domains and are generally deployed to provide more useable
bandwidth. Bridges do not stop broadcast traffic, instead they forward
broadcast traffic out every port of each bridge device. Each port on a bridge
has a separate bandwidth (collision) domain, but all ports are on the same
broadcast domain.
5.3 SWITCHES
LAN switches are used to connect a common broadcast domain (a hub).
They are also used to provide frame-level filtering as well as dedicated port
speed to specific end users. Some switches have limited routing capabilities
and can provide Layer 3 routing functions at the most basic level. Some of
the major benefits of using switches in a network are higher bandwidth to
the desktop and ease of configuration. Switches are being deployed more
often to replace hubs and bridges as more bandwidth-intensive applications
are being implemented at all levels of an organization.
5.4 ROUTERS
Routers are not usually active in simple LAN environments because routers
are (Wide-Area Network) WAN devices. Routers are typically found at the
edge of a LAN, interfacing with a WAN. Routers operate at the network layer
(Layer 3) of the OSI model. Broadcast containment and security are needed
in more complex environments.
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REFERENCES

Basic Addressing
http://comptechdoc.org/independent/networking/guide/netaddress
ing.html

Network Basics
http://comptechdoc.org/independent/networking/guide/netaddress
ing.html

Data Communications & Networking
Chan, R. W. N., ISBN: 962-7548-44-8

LAN Topoogies
http://media.techtarget.com/searchNetworking/Downloads/NetCons
ultantsch2.pdf
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