1.
Network links
The communication media used to
link devices to form a computer network include:
1)
Electrical cable : Home PNA,
power line communication, optical fiber (Wired
technology), and
2)
Radio waves (wireless technology).
In the OSI model, these are
defined at layers 1 and 2 — the physical layer and the data link layer.
A widely adopted family of communication media used in
local area network (LAN) technology is collectively known as Ethernet. The
media and protocol standards that enable communication between networked
devices over Ethernet are defined by IEEE 802.3. Ethernet transmit data over
both copper and fiber cables. Wireless LAN standards (e.g. those defined by IEEE
802.11) use radio waves, or others use infrared signals as a transmission
medium. Power line communication uses a building's power cabling to transmit
data.
3.1 Wired
technologies
The orders of the following
wired technologies are, roughly, from slowest to fastest transmission speed.
- Twisted pair wire is the most widely used medium for all telecommunication.
Twisted-pair cabling consist of copper wires that are twisted into pairs.
Ordinary telephone wires consist of two insulated copper wires twisted
into pairs. Computer network cabling (wired Ethernet as defined by IEEE
802.3) consists of 4 pairs of copper cabling that can be utilized for both
voice and data transmission. The use of two wires twisted together helps
to reduce crosstalk and electromagnetic induction. The transmission speed
ranges from 2 million bits per second to 10 billion bits per second.
Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and
shielded twisted-pair (STP). Each form comes in several category ratings,
designed for use in various scenarios.
- Coaxial cable is widely used for cable television systems, office buildings,
and other work-sites for local area networks. The cables consist of copper
or aluminium wire surrounded by an insulating layer (typically a flexible
material with a high dielectric constant), which itself is surrounded by a
conductive layer. The insulation helps minimize interference and
distortion. Transmission speed ranges from 200 million bits per second to
more than 500 million bits per second.
- An optical
fiber is a glass fiber. It uses pulses of light to transmit
data. Some advantages of optical fibers over metal wires are very low
transmission loss and immunity from electrical interference. Optical
fibers can simultaneously carry multiple wavelengths of light which
greatly increases the rate that data can be sent, and helps enable data
rates of up to trillions of bits per second. Optic fibers can be used for
long runs of cable carrying very high data rates, and are used for undersea
cables to interconnect continents.
3.2 Wireless technologies
- Terrestrial microwave – Terrestrial microwave communication uses Earth-based
transmitters and receivers resembling satellite dishes. Terrestrial
microwaves are in the low-gigahertz range, which limits all communications
to line-of-sight. Relay stations are spaced approximately 48 km
(30 mi) apart.
- Communications satellites – Satellites communicate via microwave radio waves, which are
not deflected by the Earth's atmosphere. The satellites are stationed in
space, typically in geosynchronous orbit 35,400 km (22,000 mi)
above the equator. These Earth-orbiting systems are capable of receiving
and relaying voice, data, and TV signals.
- Cellular and PCS systems use several radio communications
technologies. The systems divide the region covered into multiple
geographic areas. Each area has a low-power transmitter or radio relay
antenna device to relay calls from one area to the next area.
- Radio and spread
spectrum technologies – Wireless
local area networks use a high-frequency radio technology similar to
digital cellular and a low-frequency radio technology. Wireless LANs use
spread spectrum technology to enable communication between multiple
devices in a limited area. IEEE
802.11 defines a common flavor of open-standards wireless radio-wave
technology known as Wifi.
2.
Network
nodes and Network interfaces
Ø In data communication, a physical network node may either be a data
communication equipment (DCE) such as a modem, hub, bridge
or switch;
or a data terminal equipment (DTE) such as a
digital telephone handset, a printer or a host
computer, for example a router,
a workstation or a server.
If the network
in question is a LAN or WAN,
every LAN or WAN node (that are at least data
link layer devices) must have a MAC address,
typically one for each network interface controller it
possesses. Examples are computers, packet switches, xDSL modems (with
Ethernet interface) and wireless LAN access points. Note that a hub constitutes
a physical network node, but does not constitute a LAN network node, since a
hubed network logically is a bus network. Analogously, a repeater or PSTN modem
(with serial interface) is a physical network node but not a LAN node in this
sense.
If the network
in question is the Internet or an Intranet, many physical network nodes are host
computers, also known as Internet nodes, identified by an IP address, and
all hosts are physical network nodes. However, some data link
layer devices such as switches, bridges and WLAN access points do not have an IP host address
(except sometimes for administrative purposes), and are not considered to be
Internet nodes or hosts, but as physical network nodes and LAN nodes.
Ø A network interface controller (NIC) is computer hardware that
provides a computer with the ability to access the transmission media, and has
the ability to process low-level network information. For example the NIC may
have a connector for accepting a cable, or an aerial for wireless transmission
and reception, and the associated circuitry.
In addition, the NIC will have
access to an identifier for either the NIC or the computer as a whole.
v In Ethernet networks, each network interface controller has a unique
Media Access Control (MAC) address which is usually stored in the controller's
permanent memory. MAC address uniqueness is maintained and administered by the
Institute of Electrical and Electronics Engineers (IEEE) in order to avoid
address conflicts between devices on a network. The size of an Ethernet MAC
address is six octets. The 3 most significant octets are reserved to identify
NIC manufacturers. These manufacturers, using only their assigned prefixes,
uniquely assign the 3 least-significant octets of every Ethernet interface they
produce.
4.1 Repeaters and hubs
A
repeater is an electronic device that receives a network signal, cleans it of
unnecessary noise, and regenerates it. The signal is retransmitted at a higher
power level, or to the other side of an obstruction, so that the signal can
cover longer distances without degradation. In most twisted pair Ethernet
configurations, repeaters are required for cable that runs longer than 100
meters. With fiber optics, repeaters can be tens of even hundreds of kilo-meters
apart.
A repeater with multiple ports
is known as a hub. Repeaters work on the physical layer of the OSI model.
Repeaters require a small amount of time to regenerate the signal. This can
cause a propagation delay which can affect network performance. As a result,
many network architectures limit the number of repeaters that can be used in a
row, e.g., the Ethernet 5-4-3 rule.
Hubs have been mostly obsoleted
by modern switches; but repeaters are used for long distance links, notably
undersea cabling.
4.2 Bridges
A network bridge connects and
filters traffic between two network segments at the data link layer (layer 2)
of the OSI model to form a single network. This breaks the network's collision
domain but maintains a unified broadcast domain. Network segmentation breaks
down a large, congested network into an aggregation of smaller, more efficient
networks.
Bridges come in three basic
types:
- Local bridges: Directly connect LANs
- Remote bridges: Can be used to create a wide area network (WAN)
link between LANs. Remote bridges, where the connecting link is slower
than the end networks, largely have been replaced with routers.
- Wireless bridges: Can be used to join LANs or connect remote
devices to LANs.
4.3 Switches
A network switch is a device
that forwards and filters OSI layer 2 data grams between ports based on the MAC
addresses in the packets. A switch is
distinct from a hub in that it only forwards the frames to the physical ports
involved in the communication rather than all ports connected. It can be
thought of as a multi-port bridge. It
learns to associate physical ports to MAC addresses by examining the source
addresses of received frames. If an unknown destination is targeted, the switch
broadcasts to all ports but the source. Switches normally have numerous ports,
facilitating a star topology for devices, and cascading additional switches.
Multi-layer switches are capable
of routing based on layer 3 addressing or additional logical levels. The term switch
is often used loosely to include devices such as routers and bridges, as well
as devices that may distribute traffic based on load or based on application
content (e.g., a Web URL identifier).
4.4 Modems
A modem (modulator-demodulator)
is a device that modulates an analog carrier signal to encode digital
information and demodulates the signal to decode the transmitted information.
The goal is to produce a signal that can be transmitted easily and
decoded to reproduce the original digital data. Modems can be used with any
means of transmitting analog signals, from light emitting diodes to radio. The most
familiar type is a voice band modem that turns the digital
data of a computer
into modulated electrical signals in the voice frequency range
of a telephone
channel. These signals can be transmitted over telephone
lines and demodulated by another modem at the receiver side to recover the
digital data.
Modems are generally classified
by the amount of data they can send in a given unit of time,
usually expressed in bits per second (bit/s or bps), or bytes
per second (B/s). Modems can also be classified by their symbol rate,
measured in baud. The baud unit denotes symbols per second, or the number of
times per second the modem sends a new signal. For example, the ITU V.21
standard used audio frequency shift keying with two possible frequencies,
corresponding to two distinct symbols (or one bit per symbol), to carry 300
bits per second using 300 baud. By contrast, the original ITU V.22 standard,
which could transmit and receive four distinct symbols (two bits per symbol),
transmitted 1,200 bits by sending 600 symbols per second (600 baud) using phase
shift keying.
4.5 Routers
A router is an
internetworking device that forwards packets between networks by processing the
routing information included in the packet or datagram (Internet protocol
information from layer 3). The routing information is often processed in
conjunction with the routing table (or forwarding table). A router uses its
routing table to determine where to forward packets. (A destination in a
routing table can include a "null" interface, also known as the
"black hole" interface because data can go into it, however, no
further processing is done for said data.)
It is often said that if anyone were to send a
postcard with minimal address information such as “Mahatma Gandhi, India” or
“Albert Einstein, USA,” it would be routed to them due to their fame; no
listing of the street address or the city name would be necessary. The postal system can do such routing to
famous personalities usually on a case-by-case basis, relying on the name
alone. In an electronic
communication network, a similar phenomenon is possible to reach any website
or to contact person by telephone anywhere in the world without knowing where
the site or the person is currently located. Not only that, it is possible to
do so very efficiently, within a matter of a few seconds. How is this possible
in a communication network, and how can it be done so quickly? At the heart of
the answer to this question lies network
routing . Network routing refers to the ability of an electronic
communication network to send a unit of information from point A to point B by
determining a path through the network, and by doing so efficiently and
quickly. The determination of an efficient path depends on a number of factors,
as we will be discussing in detail throughout this book.
First, we start with a key and necessary factor,
known as addressing. In a communication network, addressing and how
it is structured and used plays a critical role. In many ways, addressing in a
communication network has similarities to postal addressing in the postal
system. Thus, we will start with a brief discussion of the postal addressing
system to provide an analogy.
A typical postal address that we write on a postcard
has several components the name of the person, followed by the street address
with the house number (“house address”), followed by the city, the state name,
and the postal code. If we, on the other hand, take the processing view to
route the postcard to the right person, we essentially need to consider this
address in the reverse order of listing,
i.e., start with the postal code, then the city or the state name, then the
house address, and finally the name of the person. You may notice that we can
reduce this information somewhat; that is, you can just use the postal code and
leave out the name of the city or the name of the state, since this is
redundant information. This means that the information needed in a postal
address consists of three main parts: the postal code, the street address (with
the house number), and the name.
A basic routing problem in the postal network,
then, is as follows: the postcard is first routed to the city or the
geographical region where the postal code is located. Once the card reaches the
postal code, the appropriate delivery post office for the address specified is
identified and delivered to. Next, the postman or post-woman delivers the
postcard at the address, without
giving much consideration to the name listed on the card. Rather, once the card
arrives at the destination address, the residents at this address take the
responsibility of handing it to the person addressed.
You may note that at a very high-level view, the
routing process in the postal system is broken down to three components: how to get the card to the specific
postal code (and sub-sequently the post office), how the card is delivered to
the destination address, and finally, how
it is delivered to the actual person at the address. If we look at it in
another way, the place where the
postcard originated in fact does not need to know the detailed information
of the street or the name to start with;
the postal code is sufficient to determine to which geographical area or city to send the card. Thus,
we can see that postal routing uses address for
routing decisions. An advantage of this approach is the decoupling of the
routing decision to multiple levels such as the postal code at the top, then
the street address, and so on. An important requirement of this hierarchical
view is that there must be a way to
divide the complete address into multiple distinguishable parts to help with
the routing decision.
Now
consider an electronic communication network; for example, a critical
communication network of the modern age is the Internet. Naturally, the first
question that arises is: how does addressing work for routing a unit of
information from one point to another, and is there any relation to the postal addressing
hierarchy that we have just discussed? Second, how is service delivery provided?
4.5.1
Addressing and Internet Service
In
many ways, Internet addressing has similarities to the postal addressing
system. The ad dressing in the Internet is referred to as Internet
Protocol (IP) addressing. An IP
address defines two parts: one part
that is similar to the postal code and the other part that is similar to the
house address; in Internet terminology, they are known as the net-id and the host-id, to identify a network and a host address, respectively. Thus,
a host is the end point of communication in the Internet and where a
communication starts. A host is a generic term used for indicating many
different entities; the most common ones are a web-server, an email server, and
certainly the desktop, laptop, or any computer we use for accessing the
Internet. A net-id identifies a
contiguous block of addresses Like any service delivery system, we also need a
delivery model for the Internet. For example,
in the postal system, one can request guaranteed delivery for an additional
fee. The Internet’s conceptual
framework, known as TCP/IP Transmission Control Protocol /Internet Protocol), relies on a delivery model in which TCP is in charge of the reliable
delivery of information, while IP is in charge of routing, using the IP
addressing mechanism. IP, however, does
not worry about whether the information is reliably delivered to the address or
is lost during transit. This is
somewhat similar to saying that the postal system will route a postcard to the house address, while residents at this
address (not the postal authority) are responsible for ensuring that the person named on the card
receives it. While this may seem odd at first, this paradigm has been found to work well in practice, as the success
of the Internet shows. A key
difference in the Internet as opposed to the postal system is that the sending
host first sends a beacon to the
destination address (host) to see if it is reachable, and waits for an
acknowledgment before sending the actual message. Since the beacon also
uses the same transmission
mechanism, i.e., IP, it is possible that it may not reach the destination. In
order to allow for this uncertainty
to be factored in, another mechanism known as a timer is used. That
is, the sending host sends the beacon, then waits for a certain amount of time
to see if it receives any response.
If it does not hear back, it tries to send the beacon a few more times, waiting for a certain amount of time
before each attempt, until it stops trying after reaching the limit on the maximum number of attempts. The basic idea,
then, requires that the receiving
host should also know the address of the sender so that it can acknowledge the
receipt of the beacon. As you can see, this
means that when the sending host sends its beacon, it must also include its source IP address.
Once the connectivity is established through the
beacon process, the actual transmission of the content transpires. This is
where a good analogy is not available in the postal system; rather, the road transportation network is a
better fit to describe an analogy. If we imagine a group of 100 friends wanting to go to a game, then we can easily see
that not all can fit in one car. If
we consider that a car can hold five people, we will need twenty cars to
transport this entire group. The
Internet transfer model also operates in this fashion. Suppose that a document that we want to download from a host
(web-server) is 2 MB. Actually, it cannot be accommodated entirely into a single fundamental unit of IP, known as
packet datagram or datagram
due to a limitation imposed by
the underlying transmission system. This limitation is known as the Maximum
Transmission Unit (MTU). MTU is similar to the limitation on how many
people
can fit into a single car. Thus, the document would need to be broken down into smaller units that fit into
packets. Each packet is then labelled with both the destination and the source address, which is then routed through
the Internet toward the destination. Since the IP delivery mechanism is assumed
to be unreliable, any such packet can possibly get lost during transit, and thus would need to be
retransmitted if the timer associated with this packet expires. Thus another important component is that content that
has been broken down into smaller
packets, once it arrives at the destination, needs to be reassembled in the
proper order before delivering the
document. We conclude this section
by pointing out that the acknowledgment and retransmission mechanism is used for most well-known
applications on the Internet such as web or email. A slightly different model is used for applications
that do not require reliable delivery; this will be discussed later in the
chapter.
4.5.2
Network Routing
In the
previous section, we provided a broad overview of addressing and transfer
mechanisms for data in Internet communication services. we can see that
eventually packets are
to be routed from a source to a destination. Such packets may need to traverse
many cross-points, similar to
traffic intersections in a road transportation network. Cross-points in the Internet are known as router. A router’s
functions are to read the destination address marked in an incoming IP packet,
to consult its internal information to identify an outgoing link to which the packet is to be forwarded, and
then to forward the packet. Similar to the number of lanes and the speed limit on a road, a network link that
connects two routers is limited by
how much data it can transfer per unit of time, commonly referred to as the bandwidth
or capacity of a link; it is generally represented by a data rate, such as
1.54 megabits per second (Mbps). A
network then carries TRAFFIC on its links and through its routers to the
eventual destination; traffic in a network
refers to packets generated by different applications, such as web or email. Suppose that traffic suddenly increases, for
example, because of many users trying to download from the same website; then,
packets that are generated can possibly be queued at routers or even dropped. Since a router
maintains a finite amount of space, known as a buffer, to temporarily store
backlogged packets, it is possible to reach the buffer limit. Since the basic
principle of TCP/IP allows the possibility of an IP packet not being delivered
or being dropped enroute, the finite
buffer at a router is not a problem. On the other hand, from an efficient delivery point of view, it is
desirable not to have any packet loss (or at least, minimize it) during transit. This is because the
reliable delivery notion works on the principle of retransmission and acknowledgment and any drop would mean an
increase in delay due to the need
for retransmission. In addition, during transit, it is also possible that the
content enclosed in a data packet is
possibly corrupted due to, for example, an electrical signalling problem on a communication link. This then
results in garbling of a packet. From an end-to- end communication point of view, a garbled packet is the same as a lost
packet.
Thus, for efficient delivery of packets, there
are several key factors to consider: (1) routers with a reasonable amount of buffer space, (2)
links with adequate bandwidth, (3) actual transmission with minimal error (to minimize packets being garbled),
and (4) the routers’ efficiency in
switching a packet to the appropriate outgoing link. We have already briey
discussed why the first two factors are important. The third factor, an
important issue, is outside the
scope of this book since encoding or development of an error-free transmission
system is an enormous subject by
itself; interested readers may consult books such as [666]. Thus, we next move to the fourth factor.
Why is the fourth factor important? A packet is
to be routed based on the IP address of the destination host; however, much like street
address information in a postal address, there are far too many possible hosts; it is impossible and
impractical to store all host addresses at any router. For example, for a 32-bit address, theoretically a maximum of
2^32 hosts are possible a very large number (more about IP addressing in the
next section). Rather, a router needs to consider a coarser level of address information, i.e., the net-id
associated with a host, so that an
outgoing link can be identified quickly just by looking up the net-id. Recall
that a net-id is very much like a
postal code. There is, however, a key difference—net-ids do not have any
geographical proximity association as with
postal codes. For example, postal codes in the Assam States are Six digits long and are known as PIN (Postal Index
Number) codes. Consider now Guwahati
City, Silchar, where a PIN code starts with 78 such as 781015, 788001, and so on. Thus, a postcard can be routed to Guwahati
City, AS (“78”) which in turn then can take
care of routing to the specific PIN code. This idea is not possible with IP
addressing since net-Ids do not have
any geographical proximity. In fact, an IP net-id address such 134.193.0.0
can be geographically far away from the
immediately preceding IP net-id address 134.192.0.0. Thus, at the net-id level, IP addressing is at;
there is no hierarchy.
You might be wondering why IP
address numbering is not geographic. To give a short answer, an advantage of a non-geographic address
is that an organization that has been as- signed an IP address block can keep its address block even if it moves
to a different location or if it
wants to use a different provider for connectivity to the Internet. A
geographically based address system
usually has limitations in regard to providing location-independent flexibility.
In order to provide the flexibility that two net-ids
that appear close in terms of their actual numbering can be geographically far
away, core routers in the Internet need to maintain an explicit list of all
valid net-ids along with an identified outgoing link so that when a packet arrives the router knows which way to
direct the packet. The list of valid net-ids is quite large, currently at 196,000 entries. Thus, to minimize switching
time at a router, efficient mechanisms
are needed that can look up an address, identify the appropriate outgoing link
(direction), and process the packet quickly
so that the processing delay can be as minimal as possible.
There is, however, another important phase that
works in tandem with the lookup process
at a router. This is the updating of a table in the router, known as the
routing table
that contains the identifier for the next
router, known as the next hop , for a given destination net-id. The routing table is in fact updated
ahead of time. In order to update such a table, the router would need to store all net-ids it has
learned about so far; second, if a link downstream is down or congested or a net-id is not reachable
for some reason, it needs to know so that an alternate path can be determined as soon as possible. This means
that a mechanism is required for communicating congestion or a failure of a link or non-reach ability of a net-id.
This mechanism is known as the
routing protocol
mechanism. The information learned through
a routing protocol is used for
generating the routing table ahead of time.
If new information is learned about the status of
links or nodes, or the reach ability of a net id through a routing protocol, a routing algorithm is then invoked at a router to determine the best possible next hop for each destination
net id in order to update the routing table. For efficient packet processing, another table, known as the forwarding table, is derived from the routing table that
identifies the outgoing link interfaces. The forwarding table is also known
as the Forwarding Information Base (FIB).
We will use the terms forwarding table and FIB interchangeably.
It should be noted that a routing algorithm may
need to take into account one or more factors
about a link, such as the delay incurred to traverse the link, or its available
bandwidth, in order to determine the
best possible path among a number of possible paths. If a link along a path does not have adequate bandwidth,
congestion or delay might occur. To minimize delay, an important function, called traffic engineering, is performed. Traffic engineering is concerned with ways to improve the operational
performance of a network and identifies procedures or controls to be put in place ahead of time to obtain good
network performance. Finally, there
is another important term associated with networking in general and network
routing in particular, labeled as architecture. There are two broad ways the term architecture from the architecture
of a building is applicable here: (1) a
floor inside a building may
be organized so that it can be partitioned efficiently for creating office
spaces of different sizes by putting
in flexible partitions without having to tear down any concrete walls, (2) it
provides standardized interfaces, such as
electrical sockets, so that equipment that requires power can be easily connected using a
standardized socket without requiring modification to the building or the floor or the equipment. Similarly,
there are several ways we use the architecting a network: for example, from the protocol point of view,
various functions are divided so
that each function can be done separately, and one function can depend on
another through a well-defined relationship.
From a router’s perspective, architecting a network refers to how it is
organized internally for a variety of functions, from routing protocol handling
to packet processing. From a network perspective, this means how the network
topology architecture should be organized,
where routers are to be located and bandwidth of links determined for efficient traffic engineering, and so on. Later,
we will elaborate more on architectures.
To summarize, we can say that the broad scope of
network routing is to address routing algorithms,
routing protocols, and architectures, with architectures encompassing several
different aspects for efficient routing. In this book, we will delve into these
aspects in depth. With the above
overview, we now present IP addressing in detail.
4.5.3
IP Addressing
If one
has to send data to any host in the Internet, there is a need to uniquely
identify all the hosts
in the Internet. Thus, there is a need for a global addressing scheme in which
no two hosts have the same address.
Global uniqueness is the first property that should be provided in an addressing scheme.
Ø Class full Addressing Scheme: An IP address assigned to a
host is 32 bits long and should be unique. This addressing, known as IPv4 addressing, is written in the bit format,
from left to right, where the left-most bit is considered the most significant bit. The hierarchy in IP addressing,
similar to the postal code and the
street address, is reflected through two parts, a network part and a host part
referred as the pair (net-id, host-id). Thus, we can think of the Internet as the
interconnection of networks identified through net-ids where each netid has a collection of hosts.
The network part (netid) identifies
the network to which the host is attached, and the host part (host-id)
identifies a host on that network.
The network part is also referred as the IP prefix. All hosts
attached to the same network share
the network part of their IP addresses but must have a unique host parts. To
support different sizes for the (netid, hostid) part, a good rule on how to
partition the total IP address space of 2^32 addresses was needed, i.e., how
many network addresses will be
allowed and how many hosts each of them will support. Thus, the IP address
space was originally divided into
three different classes, Class A, Class B, and Class C, as shown in Figure 1.1 for networks and hosts. Each class was
distinguished by the first few initial bits of a 32-bit address.
For readability, IP addresses are expressed as
four decimal numbers, with a dot between them. This format is called the dotted decimal
notation. The notation divides the
32-bit IP ad- dress into 4 groups of
8 bits and specifies the value of each group independently as a decimal number separated by dots. Because of 8-bit
breakpoints, there can be at most 256 (= 2^8) decimal values in each part.
Since 0 is an assignable value, no decimal values can be more than 255. Thus,
an example of an IP address is 10.5.21.90 consisting of the four decimal
values, separated by a dot or period.
Each Class A address has the first bit set to 0
and is followed by 7 bits for the network part, resulting in a maximum of 128 (= 2^7) ) networks; this is then followed
by a 24-bit host part. Thus, Class A
supports a maximum of 2^24-2 hosts per network. This calculation subtracts 2
because 0s and 1s in the host part of a Class A address may not be assigned to
individual hosts; rather, all 0s
that follows a net-id such as 10.0.0.0 identify the network, while all 1s that
follow a net-id such as 10.255.255.255 are
used as the broadcast address for this network. Each Class B network address has the first two bits
set to “10,” followed by a 14-bit network part, which is then followed by a 16-bit host part. A maximum of 2^14
networks can be defined with UP TO 2^16-2 hosts per network. Finally, a Class C
network address has the first three bits set as “110” and followed by a 21-bit
network part, with the last 8 bits to identify the host part. Class C provides support for a maximum of 2^21(-2,
097, 152) part. Class C provides support
for a maximum of 254 (2^8-2) hosts. In each class, a set of network addresses
is reserved for a variety of purposes;
Three address classes discussed so far are used
for unicasting in the Internet, that is, for a host-to-host communication. There is another class of IP
addresses, known as Class D\ addressing,
that is used for multicasting
in the Internet; in this case, the first
four bits of the 32-bit address are
set to “1110” to indicate that it is a multicast address. A host can use a
multicast address as the destination
address for a packet generated to indicate that the packet is meant for any hosts on the Internet; in order
for any hosts to avail this feature, they must use another mechanism to tune into this address. Multicast address on
the Internet can be thought of as
similar to a radio station frequency; a radio station transmits on a particular
frequency any listener who wants to listen
to this radio station must tune the radio dial to this frequency.
Ø Sub-netting/ Net-mask : Consider the IP address
192.168.40.3 that is part of Class C network 192.168.40.0. A subnet or sub-network is defined through a network mask
boundary using the specified number of
significant bits as 1s. Since Class C defines networks with a 24-bit boundary,
we can then consider that the most
significant 24 bits are 1s, and the lower 8 bits are 0s. This translates
to the dotted decimal notation
255.255.255.0, which is also compactly written as “/24” to indicate how many most significant bits are 1s.
We can then do a bit-wise logical “AND” operation between the host address and the net mask to obtain the Class
C network address as shown below:
11000000 10101000 00101000 00000011 >192.168.40.3
AND 11111111
11111111 11111111 00000000 > net mask (/24)
11000000 10101000 00101000 00000000 >192.168.40.0
As you
can see, both the host address and the net mask have 1s in the first two
positions from the
left; thus, the “AND” operation results in 1s for these two positions. For the
third position from left, the host
has 0 while the net mask has 1; thus, the result of the “AND” operation is zero; and so on. Note that for network
addresses such as Class C address, the net-mask is implicit and it is on a /24 subnet boundary. Now consider that we
want to change the net-mask explicitly to /21 to identify a network larger than a 24-bit subnet boundary. If
we now do the bit-wise operation
11000000
10101000 00101000 00000011 > 192.168.40.3
AND 11111111
11111111 11111000 00000000 >
net mask (/21)
11000000
10101000 00101000 00000000 >192.168.40.0
we
note that the network address is again 192.168.40.0. However, in the latter
case, the network boundary is 21 bits. Thus, to be able to clearly distinguish
between the first and the second one, it is necessary to explicitly mention the net-mask. This is
commonly written for the second
example as 192.168.40.0/21, where the first part is the net-id and the second
part is the mask boundary indicator.
In this notation, we could write the original Class C address as 192.168.40.0/24 and thus, there is no ambiguity
with 192.168.40.0/21.
4.5.4
Classless Inter-domain Routing
Classless
Inter-domain Routing (CIDR) uses an explicit net-mask with an IPv4 address
block to identify a
network, such as 192.168.40.0/21. An advantage of explicit masking is that an
address block can be assigned at any bit
boundaries, be it /15 or /20; most important, the assignment of Class C level
addresses for networks that can show up in the global routing table can be
avoided or minimized. For example, a contiguous address block can be assigned
at the /21 boundary which can be
thought of as an aggregation of subnets at the /24 boundary. Because of this, the term Super netting or
variable-length subnet masking (VLSM)
is also used in reference to the
explicit announcement of the net mask.
Through such a process, and because of address
block assignment at boundaries such as 21,
the routing table growth at core routers can be delayed. In the above example,
only then net-id 192.168.40.0/21
needs to be listed in the routing table entry, instead of listing eight entries from 192.168.40.0/24 to 192.168.47.0/24.
Thus, you can see how the routing table growth can be curtailed. CIDR was
introduced around the mid-1990s; the current global routing table size, as of
this writing, is about 196,000 entries. The routing table growth over time,
along with projection, is shown
later in Figure 9.10. In order for CIDR to take effect, any network address reach ability announcement that is
communicated with a routing protocol such as the Border Gateway Protocol must also carry the mask information explicitly.
