The Domain Name System (DNS) is a system that
stores information associated with domain names in a distributed database on networks, such as the Internet. The domain name
system associates many types of information with domain names, but most importantly, it provides the IP address associated
with the domain name. It also lists mail exchange servers accepting e-mail for each domain.
DNS is useful for several
reasons. Most well known, the DNS makes it possible to attach hard-to-remember IP addresses (such as 220.127.116.11) to easy-to-remember
domain names (such as "wikipedia.org.") Humans take advantage of this when they recite URLs and e-mail addresses. Less recognized,
the domain name system makes it possible for people to assign authoritative names, without needing to communicate with a central
registrar each time.
Simple Mail Transfer Protocol (SMTP) is the standard for
email transmission across the Internet.
SMTP is a relatively simple, text-based protocol, where one or more recipients
of a message are specified (and in most cases verified to exist) and then the message text is transferred. It is quite easy
to test a SMTP server using the telnet program. SMTP uses TCP port 25. To determine the SMTP server for a given domain name,
the MX (Mail eXchange) DNS record is used.
SMTP started becoming widely used in the early 1980s. At the time, it was
a complement to UUCP which was better suited to handle e-mail transfers between machines that were intermittently connected.
SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time.
Sendmail was one of the first (if not the first) mail transfer agents to implement SMTP. As of 2001 there are at least
50 programs that implement SMTP as a client (sender of messages) or a server (receiver of messages). Some other popular SMTP
server programs include Philip Hazel's exim, IBM's Postfix, D. J. Bernstein's qmail, and Microsoft Exchange Server.
this protocol started out as purely ASCII text-based, it did not deal well with binary files. Standards such as MIME were
developed to encode binary files for transfer through SMTP. Today, most SMTP servers support the 8BITMIME extension, permitting
binary files to be transmitted almost as easily as plain text.
SMTP is a "push" protocol that does not allow one to
"pull" messages from a remote server on demand. To do this a mail client must use POP3 or IMAP. Another SMTP server can trigger
a delivery in SMTP using ETRN.
HyperText Transfer Protocol (HTTP) is the primary
method used to convey information on the World Wide Web. The original purpose was to provide a way to publish and receive
Development of HTTP was co-ordinated by the World Wide Web Consortium and working groups of the Internet
Engineering Task Force, culminating in the publication of a series of RFCs, most notably RFC 2616, which defines HTTP/1.1,
the version of HTTP in common use today.
HTTP is a request/response protocol between clients and servers. An HTTP
client, such as a web browser, typically initiates a request by establishing a TCP connection to a particular port on a remote
host (port 80 by default). An HTTP server listening on that port waits for the client to send a request string, such as "GET
/ HTTP/1.1" (which would request the default page of that web server), followed by an email-like MIME message which has a
number of informational header strings that describe aspects of the request, followed by an optional body of arbitrary data.
Some headers are optional, while others (such as Host) are required by the HTTP/1.1 protocol. Upon receiving the request string
(and message, if any), the server sends back a response string, such as "200 OK", and a message of its own, the body of which
is perhaps the requested file, an error message, or some other information.
Resources used in the HTTP are identified
using Uniform Resource Identifiers (URIs) in the http or https schemes.
file transfer protocol is a protocol used for exchanging files over the Internet. FTP works in the same way as HTTP for transferring
Web pages from a server to a user's browser, and SMTP for transferring electronic mail across the Internet in that FTP uses
the Internet's TCP/IP protocols to enable data transfer. FTP is most commonly used to download a file from a server using
the Internet or to upload a file to a server (e.g., uploading a Web page file to a server). While data is being transferred
across the data stream, the control stream does not do anything. This can cause problems with large data transfers through
a firewall, which will time out sessions after long periods of idleness.
While the file may well be successfully transferred,
the control session can be disconnected by the firewall, causing an error. FTP requires the user to login before data transfer
can occur. However, anonymous access is also popular.
Networking hubs are central
components of local area networks (LANs). To understand the role of networking hubs, a basic understanding of LANs is required.
Whenever one or more computers are networked together, a LAN is created. A LAN can be vital at work, but it can also be useful
at home. The purpose of joining computers together in a LAN is to share resources like files, a printer, a scanner, or Internet
There are four components in a basic wired hub network:
Ethernet wire: This is the physical cable
that links the computers together, enabling them to talk to each other. The Ethernet cable, also called twisted pair, or 10-Base
T, plugs into a network card located in each computer on the LAN.
Network Interface Cards (NICs) : One of these cards
goes into a vacant slot inside each computer. The back of the card features a port for one end of an Ethernet cable. Newer
computers normally have a networking card built-in.
Networking Hubs: The networking hub is a junction box with several
ports in the back for receiving the Ethernet cables that are plugged into each computer on the LAN. With Ethernet cables going
from each NIC to the hub, all computers are connected to the hub.
Networking Software: Most operating systems today
come with networking software built-in, but the software is also available from third parties. The software works with the
hardware to create a networking environment on each computer, allowing the user to see shared files and recourses. It also
allows for administration of the network.
With a group of computers wired to the hub and the software installed, the
computers can talk to each other. Networking hubs will broadcast all traffic that comes through the hub to all machines or
nodes connected to the LAN. Each computer/node on the LAN will have its own address, called a MAC (Medium Access Control)
address. Each node will ignore any traffic not addressed to its MAC address.
Networking hubs are simple devices that
are fine for home use in most cases, but not considered optimal for workplace environments. Networking hubs can only operate
in half-duplex mode -- a computer cannot receive and send transmissions at the same time. Another drawback is that networking
hubs broadcast traffic indiscriminately to all machines on the LAN. This opens the door to security issues, making it easy
to use so-called "packet sniffers" . For instance, to snoop on all network traffic.
A network switch is a computer networking device that connects network segments. It uses the logic of a Network bridge
but allows a physical and logical star topology. It is often used to replace network hubs. A switch is also often referred
to as an intelligent hub
A switch can connect Ethernet, Token Ring, Fibre Channel or other types of packet switched
network segments together to form a heterogeneous network operating at OSI Layer 2.
As a frame comes into a switch,
the switch saves the originating MAC address and the originating port in the switch's MAC address table. The switch then selectively
transmits the frame from specific ports based on the frame's destination MAC address and previous entries in the MAC address
table. If the destination MAC address is unknown, a broadcast address, or a multicast address, the switch simply transmits
the frame out of all of the connected interfaces except the incoming port. If the destination MAC address is known, the frame
is forwarded only to the corresponding port in the MAC address table. If the destination port is the same as the originating
port, the frame is filtered out and not forwarded.
Switches, unlike hubs, use microsegmentation to create collision
domains, one per connected segment. This way, only the NICs which are directly connected via a point-to-point link, or directly
connected hubs are contending for the medium.
By eliminating the possibility of collisions, full-duplex point-to-point
connections on the switch become possible.
Virtual LANs can be used in switches to reduce the size of the broadcast
domains and at the same time increase security.
In redundant architectures, spanning tree protocol can be used in
switches to prevent loops.
A firewall is a piece of hardware and/or software
which functions in a networked environment to prevent some communications forbidden by the security policy, analogous to the
function of firewalls in building construction. A firewall is also called a Border Protection Device (BPD), especially in
NATO contexts, or packet filter in BSD contexts.
A firewall has the basic task of controlling traffic between different
zones of trust. Typical zones of trust include the Internet (a zone with no trust) and an internal network (a zone with high
trust). The ultimate goal is to provide controlled connectivity between zones of differing trust levels through the enforcement
of a security policy and connectivity model based on the least privilege principle.
Proper configuration of firewalls
demands skill from the administrator. It requires considerable understanding of network protocols and of computer security.
Small mistakes can render a firewall worthless as a security tool.
BOOTP is a short for Bootstrap protocol. It is an internet protocol that enables a diskless workstation to discover its
won IP address, the IP address of a BOOTP server on the network, and a file to be loaded into memory to boot the machine.
This enables the workstation to boot without requiring a hard or floppy disk drive. This Protocol was originally defined in
DHCP (Dynamic Host Configuration Protocol) is a more advanced protocol based on BOOTP but it far more complex
to implement. So DHCP servers also offer BOOTP support.
Dynamic Host Configuration Protocol (DHCP)
is a client-server networking protocol. A DHCP server provides configuration parameters specific to the DHCP client host requesting,
generally, information required by the client host to participate on an IP network. DHCP also provides a mechanism for allocation
of IP addresses to client hosts.
DHCP appeared as a standard protocol in October 1993. RFC 2131 provides the latest
(March 1997) DHCP definition.
The latest standard on a protocol describing DHCPv6, DHCP in a IPv6 environment, was
published in July 2003 as RFC 3315.
The Simple Network Management Protocol (SNMP)
forms part of the internet protocol suite as defined by the Internet Engineering Task Force. The protocol can support monitoring
of network-attached devices for any conditions that warrant administrative attention.
The SNMP protocol is extensible
by design. This is achieved through the notion of a management information base or MIB, which specifies the management data
of a specific subsystem of an SNMP-enabled device, using a hierarchical namespace containing object identifiers. This model
permits management across all layers of the OSI reference model, extending into applications such as databases, email, and
the J2EE reference model, as MIBs can be defined for all such area-specific information and operations.