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Unit 4: Introduction of Networks

developed by DARPA of the United States Department of Defense. The Internet is also the Notes
communications backbone underlying the World Wide Web (WWW).
Participants in the Internet use a diverse array of methods of several hundred documented, and
often standardized, protocols compatible with the Internet Protocol Suite and an addressing system
(IP addresses) administered by the Internet Assigned Numbers Authority and address registries.
Service providers and large enterprises exchange information about the reachability of their
address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide
mesh of transmission paths.
4.3.11.2 Intranets and Extranets
Intranets and extranets are parts or extensions of a computer network, usually a local area network.
An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web
browsers and file transfer applications, that is under the control of a single administrative entity.
That administrative entity closes the intranet to all but specific, authorized users. Most commonly,
an intranet is the internal network of an organization. A large intranet will typically have at least
one web server to provide users with organizational information.
An extranet is a network that is limited in scope to a single organization or entity and also has
limited connections to the networks of one or more other usually, but not necessarily, trusted
organizations or entities—a company’s customers may be given access to some part of its
intranet-while at the same time the customers may not be considered trusted from a security
standpoint. Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other
type of network, although an extranet cannot consist of a single LAN; it must have at least one
connection with an external network.
4.3.12 Overlay Network

An overlay network is a virtual computer network that is built on top of another network. Nodes
in the overlay are connected by virtual or logical links, each of which corresponds to a path,
perhaps through many physical links, in the underlying network.
A sample overlay network: IP over SONET over Optical.
For example, many peer-to-peer networks are overlay networks because they are organized as
nodes of a virtual system of links run on top of the Internet. The Internet was initially built as an
overlay on the telephone network .
Overlay networks have been around since the invention of networking when computer systems
were connected over telephone lines using modem, before any data network existed.
Nowadays the Internet is the basis for many overlaid networks that can be constructed to permit
routing of messages to destinations specified by an IP address. For example, distributed hash
tables can be used to route messages to a node having a specific logical address, whose IP address
is known in advance.

Overlay networks have also been proposed as a way to improve Internet routing, such as through
quality of service guarantees to achieve higher-quality streaming media. Previous proposals
such as IntServ, DiffServ, and IP Multicast have not seen wide acceptance largely because they
require modification of all routers in the network. On the other hand, an overlay network can be
incrementally deployed on end-hosts running the overlay protocol software, without cooperation
from Internet service providers. The overlay has no control over how packets are routed in the
underlying network between two overlay nodes, but it can control, for example, the sequence of
overlay nodes a message traverses before reaching its destination.
For example, Akamai Technologies manages an overlay network that provides reliable, efficient
content delivery (a kind of multicast). Academic research includes End System Multicast and

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