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Lesson#3
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HOW MANY CLASS A B C NETWORKS AND HOSTS ARE
POSSIBLE
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HOW MANY CLASS A, B, C NETWORKS AND HOSTS ARE POSSIBLE?
Note that 126 class A networks are possible (0 and 127 are not
used for general purposes). Each class A
network can accommodate a maximum of 16 million hosts on it.
There are 16384 class B networks possible
and each can host upto 65000 host machines. There are 2 million
class C networks possible each having the
capacity to accommodate upto 256 host machines on it.
Subnet mask
An IP address is meaningless for the computers unless it is
accompanied by a subnet mask. It tells to the
computer machines that what part of the accompanying IP address
corresponds to the network address and
what part corresponds to the host machine on that network. A
subnet mask is also represented by four
decimal numbers separated by a period. Default subnet masks for
class A, B and C types of IP addresses are
as under:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
IP version
The existing IP scheme (version 4) is likely to be replaced by
IP version 6. It would provide 128 bits IP
addresses in hexadecimal format. According to an estimate a
total of 3.4 x 1038
addresses would then be
available.
Domain name system
A Domain Name is a user friendly name used to locate a web site
on the internet. For example, vu.edu,
bestcomputers.com etc. Domain Name System (DNS) provides the
structure and the strategy that is used to
refer to computers on the internet by these user friendly names.
Domain Names are Unique. They are
assigned as one has to pay and register for them. With the help
of DNS a domain name is translated into its
corresponding IP address
(see Fig. 1 below).
A fully qualified domain name is processed from right to left
for its translation into the corresponding IP address. A fully
qualified domain name can be made up of a top
level domain (TLD), second level domain (SLD) and subdomains,
as shown in Fig. 2 below.
Seven
popular TLDs are “.com,.edu,.org,.gov,.net,.countryname,.int”.
SLDs represent the name of a
company/institution/entity. Subdomains represent the
geographical or functional units of a
company/institution etc.
Name resolution
The translation or resolution of a fully qualified domain name
into its IP address takes place using the
hierarchy of special computer machines called Domain Name
Servers. A DNS server is a server on the
network that maintains a database/table that contains the list
of domain names and their corresponding IP
addresses. The name servers for TLDs are also called Root Name
Servers. There is a program called
‘Resolver’ built into the browser, which sends the request for
resolution of the domain name to the machine
called Local Name Server. Local name server then contacts the
root name server, which looks into its
database and where possible provides the IP address of the name
server below in the Hierarchy. Hence in a
similar number of steps the fully qualified domain name is
completely translated into the IP address of the
web server where the requested web page is stored. The process
of name resolution has been shown in an
example in
Name Resolution
Getting domain names and IP addresses
Domain names are administered in a hierarchy. At the global
level the task of registration/administration of
domain names is supervised by the organization called Internet
Corporation for Assigned Names and
Numbers (ICAAN). There are organizations working under ICAAN in
different regions. For example,
APNIC is for Asia and Pacific Rim, ARIN for America and South
Africa, RIPE-NCC for Europe and
North Africa etc. Each of the above are further connected to
Internet Service Providers (ISPs) at the local
level. One can register one’s domain name through an ISP also.
Similarly, one can lease IP addresses from a
local ISP apart from the direct sources of IP addresses, that
is, APNIC, ARIN or RIPE-NCC. ISPs can use
a server to dynamically supply the IP addresses to their clients
for a session. Such a server is called DHCP
(Dynamic Host Configuration Protocol) server.
Media access control (MAC) address
Data to be delivered on a network has to be converted first into
serial transmission. This is done through a
device called Network Interface Card (NIC) that must be
installed in a computer on the network. NIC has
also got printed on it a 48 bits unique address called the MAC
or hardware address of the computer
machine. In other words, it consists of 12 hexadecimal
characters and can have different format as shown
below:
090017A9B2EF
09:00:17:A9:B2:EF
09-00-17-A9-B2-EF
Whereas the IP address of a computer may change from time to
time, its MAC address would remain the
same unless the existing NIC is replaced with another. It can
easily be inferred that there are 248 unique
MAC addresses possible (which is an unimaginably high number).
An organization called Institute of
Electrical and Electronic Engineers (IEEE) administers the
allocation of MAC addresses, worldwide. It
ensures that there is no duplication of MAC addresses by the
manufacturers of NICs.
8
ecom.cs.vu.edu ecom.cs.vu.edu
vu.edu, 128.196.128.233
Client
1
2
3
4
5
6
7
192.12.69.60
Local
Name
server
Root
Name
Server
VU
Name
Server
CS
Name
Server
ecom.cs.vu.edu
cs.vu.edu, 192.12.69.5
ecom.cs.vu.edu
ecom.cs.vu.edu
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