Networking: A Beginner's Guide
An Internet connection for a network consists of a telecommunications network
connection to an ISP, using a physical connection such as a leased DSL line, an ISDN
line, or a fractional or full DS1 (T-1) connection. This line comes into the building and
connects to a box called a channel service unit/data service unit (CSU/DSU), which
converts the data from the form carried by the local telephone company to one usable
on the LAN. The CSU/DSU is connected to a router that routes data packets between
the local network and the Internet. (Sometimes both the CSU/DSU and the router are
built into the same device.) Internet security is provided either by filtering the packets
going through the router or, more commonly, by adding a firewall system. A firewall
system runs on a computer (or has a computer built into it, if it's an appliance device)
and helps you secure your network against various threats.
An intranet, as its name suggests, is an internally focused network that mimics
the Internet itself. For example, a company might deploy an intranet that hosts a web
server, which stores documents such as employee handbooks, purchasing forms, and
other information that the company publishes for internal use. Intranets can also host
other Internet-type services, such as FTP servers or Usenet servers, or these services
can be provided by other tools that offer the same functionality. Intranets usually are
not accessible from outside the LAN (although they can be) and are just a much smaller
version of the Internet that a company maintains for its own use.
Understanding the technologies, services, and features of the Internet is complex.
You can learn much more about some of the hardware that makes the Internet work
in Chapter 6.
Any time you share important and confidential information on a network, you need
to carefully consider the security of those resources. Users and management must help
set the level of security required for the network and the different information it stores,
and they need to participate in deciding who has access to which resources.
Network security is provided by a combination of factors, including features of the
network operating system, the physical cabling plant, the network connection to other
networks, the features of the client workstations, the actions of the users, the security
policies of management, and how well the security features are implemented and
administered. All these factors form a chain, and any single weak link in the chain can
cause it to fail. Security failures can have severe consequences, so network security is
usually an extremely important part of any network. For a more detailed discussion
of network security, see Chapter 11.
Understanding the OSI Networking Model
The Open Systems Interconnection (OSI) model defines all the methods and protocols
needed to connect one computer to any other over a network. It is a conceptual
model, used most often in network design and in engineering network solutions.
Generally, real-world networks conform to the OSI model, although differences exist