HUB
Networks using a Star
topology require a central point for the devices to connect. Originally this
device was called a concentrator since it consolidated the cable runs from all
network devices. The basic form of concentrator is the hub.
The hub is a hardware device that contains multiple, independent ports that
match the cable type of the network. Most common hubs interconnect Category 3
or 5 twisted-pair cable with RJ-45 ends, although Coax BNC and Fiber Optic BNC
hubs also exist. The hub is considered the least common denominator in device
concentrators. Hubs offer an inexpensive option for transporting data between
devices, but hubs don't offer any form of intelligence. Hubs can be active or
passive.
An active
hub strengthens and regenerates the incoming signals before sending
the data on to its destination.
Passive hubs do nothing with the signal.
Ethernet Hubs
An Ethernet hub is
also called a multiport repeater. A repeater is a device that amplifies a
signal as it passes through it, to counteract the effects of attenuation. If,
for example, you have a thin Ethernet network with a cable segment longer than
the prescribed maximum of 185 meters, you can install a repeater at some point
in the segment to strengthen the signals and increase the maximum segment
length. This type of repeater only has two BNC connectors, and is rarely seen
these days.
8 Port mini Ethernet Hub
The hubs used on UTP
Ethernet networks are repeaters as well, but they can have many RJ45 ports instead
of just two BNC connectors. When data enters the hub through any of its ports,
the hub amplifies the signal and transmits it out through all of the other
ports. This enables a star network to have a shared medium, even though each
computer has its own separate cable. The hub relays every packet transmitted by
any computer on the network to all of the other computers, and also amplifies
the signals.
The maximum segment
length for a UTP cable on an Ethernet network is 100 meters. A segment is
defined as the distance between two communicating computers. However, because
the hub also functions as a repeater, each of the cables connecting a computer
to a hub port can be up to 100 meters long, allowing a segment length of up to
200 meters when one hub is inserted in the network.
Multistation Access Unit
A Multistation
Access Unit (MAU) is a special type of hub used for token ring
networks. The word "hub" is used most often in
relation to Ethernet networks, and MAU only refers to token ring networks. On
the outside, the MAU looks like a hub. It connects to multiple network devices,
each with a separate cable.
Unlike a hub that uses
a logical bus topology over a physical star, the MAU uses a logical ring
topology over a physical star.
When the MAU detects a
problem with a connection, the ring will beacon. Because it uses a physical
star topology, the MAU can easily detect which port the problem exists on and
close the port, or "wrap" it. The MAU does actively regenerate
signals as it transmits data around the ring.
Switches
Switches
are a special type of hub that offers an additional layer of intelligence to
basic, physical-layer repeater hubs. A switch must be able to read the MAC
address of each frame it receives. This information allows switches to repeat
incoming data frames only to the computer or computers to which a frame is
addressed. This speeds up the network and reduces congestion.
Switches
operate at both the physical layer and the data link layer of the OSI Model.
Bridges
A bridge is
used to join two network segments together, it allows computers on either
segment to access resources on the other. They can also be used to divide large
networks into smaller segments. Bridges have all the features of repeaters, but
can have more nodes, and since the network is divided, there is fewer computers
competing for resources on each segment thus improving network performance.
Bridges
can also connect networks that run at different speeds, different topologies,
or different protocols. But they cannot, join an Ethernet segment with a Token
Ring segment, because these use different networking standards. Bridges operate
at both the Physical Layer and the MAC sublayer of the Data Link layer. Bridges
read the MAC header of each frame to determine on which side of the bridge the
destination device is located, the bridge then repeats the transmission to the
segment where the device is located.
Routers
Routers
Are networking devices used to extend or segment networks by forwarding packets
from one logical network to another. Routers are most often used in large
internetworks that use the TCP/IP protocol suite and for connecting TCP/IP
hosts and local area networks (LANs) to the Internet using dedicated leased
lines.
Routers
work at the network layer (layer 3) of the Open Systems Interconnection (OSI)
reference model for networking to move packets between networks using their
logical addresses (which, in the case of TCP/IP, are the IP addresses of
destination hosts on the network). Because routers operate at a higher OSI
level than bridges do, they have better packet-routing and filtering
capabilities and greater processing power, which results in routers costing
more than bridges.
Routing tables
Routers
contain internal tables of information called routing tables that keep track of
all known network addresses and possible paths throughout the internetwork,
along with cost of reaching each network. Routers route packets based on the
available paths and their costs, thus taking advantage of redundant paths that
can exist in a mesh topology network.
Because
routers use destination network addresses of packets, they work only if the
configured network protocol is a routable protocol such as TCP/IP or IPX/SPX.
This is different from bridges, which are protocol independent. The routing
tables are the heart of a router; without them, there's no way for the router
to know where to send the packets it receives.
Unlike
bridges and switches, routers cannot compile routing tables from the
information in the data packets they process. This is because the routing table
contains more detailed information than is found in a data packet, and also
because the router needs the information in the table to process the first
packets it receives after being activated. A router can't forward a packet to
all possible destinations in the way that a bridge can.
·
Static routers: These must have
their routing tables configured manually with all network addresses and paths
in the internetwork.
·
Dynamic routers: These
automatically create their routing tables by listening to network traffic.
·
Routing tables are the means by which
a router selects the fastest or nearest path to the next "hop" on the
way to a data packet's final destination. This process is done through the use
of routing metrics.
·
Routing metrics which are the means of
determining how much distance or time a packet will require to reach the final
destination. Routing metrics are provided in different forms.
·
hop is simply a router that the packet must
travel through.
·
Ticks measure the time it takes to traverse a
link. Each tick is 1/18 of a second. When the router selects a route based on
tick and hop metrics, it chooses the one with the lowest number of ticks first.
You
can use routers, to segment a large network, and to connect local area segments
to a single network backbone that uses a different physical layer and data link
layer standard. They can also be used to connect LAN's to a WAN's.
Brouters
Brouters
are a combination of router and bridge. This is a special type of equipment
used for networks that can be either bridged or routed, based on the protocols
being forwarded. Brouters are complex, fairly expensive pieces of equipment and
as such are rarely used.
A
Brouter transmits two types of traffic at the exact same time: bridged traffic
and routed traffic. For bridged traffic, the Brouter handles the traffic the
same way a bridge or switch would, forwarding data based on the physical
address of the packet. This makes the bridged traffic fairly fast, but slower
than if it were sent directly through a bridge because the Brouter has to
determine whether the data packet should be bridged or routed.
Gateways
A
gateway is a device used to connect networks using different protocols.
Gateways operate at the network layer of the OSI model. In order to communicate
with a host on another network, an IP host must be configured with a route to
the destination network. If a configuration route is not found, the host uses
the gateway (default IP router) to transmit the traffic to the destination
host. The default t gateway is where the IP sends packets that are destined for
remote networks. If no default gateway is specified, communication is limited
to the local network. Gateways receive data from a network using one type of
protocol stack, removes that protocol stack and repackages it with the protocol
stack that the other network can use.
Examples
·
E-mail
gateways-for example, a gateway that receives Simple Mail Transfer Protocol
(SMTP) e-mail, translates it into a standard X.400 format, and forwards it to
its destination
·
Gateway
Service for NetWare (GSNW), which enables a machine running Microsoft Windows
NT Server or Windows Server to be a gateway for Windows clients so that they
can access file and print resources on a NetWare server
·
Gateways
between a Systems Network Architecture (SNA) host and computers on a TCP/IP
network, such as the one provided by Microsoft SNA Server
·
A
packet assembler/disassembler (PAD) that provides connectivity between a local
area network (LAN) and an X.25 packet-switching network
CSU / DSU (Channel Service Unit / Data Service Unit)
A CSU/DSU is
a device that combines the functionality of a channel service unit (CSU) and a
data service unit (DSU). These devices are used to connect a LAN to a WAN, and
they take care of all the translation required to convert a data stream between
these two methods of communication.
A DSU provides
all the handshaking and error correction required to maintain a connection
across a wide area link, similar to a modem. The DSU will accept a serial data
stream from a device on the LAN and translate this into a useable data stream
for the digital WAN network. It will also take care of converting any inbound
data streams from the WAN back to a serial communication.
A CSU is
similar to a DSU except it does not have the ability to provide handshaking or
error correction. It is strictly an interface between the LAN and the WAN and
relies on some other device to provide handshaking and error correction.
NICs (Network Interface Card)
Network
Interface Card, or NIC is a hardware card installed in a computer so it can
communicate on a network. The network adapter provides one or more ports for
the network cable to connect to, and it transmits and receives data onto the
network cable.
Wireless
Lan card
Every
networked computer must also have a network adapter driver, which controls the
network adapter. Each network adapter driver is configured to run with a certain
type of network adapter.
Network
card
Network
Interface Adapter Functions
Network interface adapters perform a variety of functions that are crucial to
getting data to and from the computer over the network.
These
functions are as follows:
Data
encapsulation
The network interface adapter and its driver are responsible for building the
frame around the data generated by the network layer protocol, in preparation
for transmission. The network interface adapter also reads the contents of
incoming frames and passes the data to the appropriate network layer protocol.
Signal
encoding and decoding
The network interface adapter implements the physical layer encoding scheme
that converts the binary data generated by the network layer-now encapsulated
in the frame-into electrical voltages, light pulses, or whatever other signal
type the network medium uses, and converts received signals to binary data for
use by the network layer.
transmission
and reception
The primary function of the network interface adapter is to generate and
transmit signals of the appropriate type over the network and to receive
incoming signals. The nature of the signals depends on the network medium and
the data-link layer protocol. On a typical LAN, every computer receives all of the
packets transmitted over the network, and the network interface adapter
examines the destination address in each packet, to see if it is intended for
that computer. If so, the network interface adapter passes the packet to the
computer for processing by the next layer in the protocol stack; if not, the
network interface adapter discards the packet.
Data
buffering
Network interface adapters transmit and receive data one frame at a time, so
they have built-in buffers that enable them to store data arriving either from
the computer or from the network until a frame is complete and ready for
processing.
Serial/parallel
conversion
The communication between the computer and the network interface adapter runs
in parallel, that is, either 16 or 32 bits at a time, depending on the bus the
adapter uses. Network communications, however, are serial (running one bit at a
time), so the network interface adapter is responsible for performing the
conversion between the two types of transmissions.
Media
access control
The network interface adapter also implements the MAC mechanism that the
data-link layer protocol uses to regulate access to the network medium. The
nature of the MAC mechanism depends on the protocol used.
Network protocols
A
networked computer must also have one or more protocol drivers (sometimes
called a transport protocol or just a protocol). The protocol driver works
between the upper-level network software and the network adapter to package
data to be sent on the network.
In
most cases, for two computers to communicate on a network, they must use
identical protocols. Sometimes, a computer is configured to use multiple
protocols. In this case, two computers need only one protocol in common to
communicate. For example, a computer running File and Printer Sharing for
Microsoft Networks that uses both NetBEUI and TCP/IP can communicate with
computers using only NetBEUI or TCP/IP.
ISDN (Integrated Services Digital Network) adapters
Integrated
Services Digital Network adapters can be used to send voice, data, audio, or
video over standard telephone cabling. ISDN adapters must be connected directly
to a digital telephone network. ISDN adapters are not actually modems, since
they neither modulate nor demodulate the digital ISDN signal.
Like
standard modems, ISDN adapters are available both as internal devices that
connect directly to a computer's expansion bus and as external devices that
connect to one of a computer's serial or parallel ports. ISDN can provide data
throughput rates from 56 Kbps to 1.544 Mbps (using a T1 carrier service).
ISDN
hardware requires a NT (network termination) device, which converts network
data signals into the signaling protocols used by ISDN. Some times, the NT
interface is included, or integrated, with ISDN adapters and ISDN-compatible
routers. In other cases, an NT device separate from the adapter or router must
be implemented. ISDN works at the physical, data link, network, and transport
layers of the OSI Model.
WAPs (Wireless Access Point)
A
wireless network adapter card with a transceiver sometimes called an access
point, broadcasts and receives signals to and from the surrounding computers
and passes back and forth between the wireless computers and the cabled
network.
Access
points act as wireless hubs to link multiple wireless NICs into a single
subnet. Access points also have at least one fixed Ethernet port to allow the
wireless network to be bridged to a traditional wired Ethernet network.
Modems
A
modem is a device that makes it possible for computers to communicate over telephone
lines. The word modem comes from Modulate and Demodulate. Because standard
telephone lines use analog signals, and computers digital signals, a sending
modem must modulate its digital signals into analog signals. The computers
modem on the receiving end must then demodulate the analog signals into digital
signals.
Modems
can be external, connected to the computers serial port by an RS-232 cable or
internal in one of the computers expansion slots. Modems connect to the phone
line using standard telephone RJ-11 connectors.
Transceivers (media converters)
Transceiver
short for transmitter-receiver, a device that both transmits and receives
analog or digital signals. The term is used most frequently to describe the
component in local-area networks (LANs) that actually applies signals onto the
network wire and detects signals passing through the wire. For many LANs, the
transceiver is built into the network interface card (NIC). Some types of
networks, however, require an external transceiver.
In
Ethernet networks, a transceiver is also called a Medium Access Unit (MAU).
Media converters interconnect different cable types twisted pair, fiber, and
Thin or thick coax, within an existing network. They are often used to connect
newer 100-Mbps, Gigabit Ethernet, or ATM equipment to existing networks, which
are generally 10BASE-T, 100BASE-T, or a mixture of both. They can also be used
in pairs to insert a fiber segment into copper networks to increase cabling
distances and enhance immunity to electromagnetic interference (EMI).
Firewalls
In
computing, 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 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.
There
are three basic types of firewalls depending on:
·
whether
the communication is being done between a single node and the network, or
between two or more networks
·
whether
the communication is intercepted at the network layer, or at the application
layer
·
whether
the communication state is being tracked at the firewall or not
With
regard to the scope of filtered communication these firewalls are exist:
·
Personal
firewalls, a software application which normally filters traffic entering or
leaving a single computer through the Internet.
·
Network
firewalls, normally running on a dedicated network device or computer
positioned on the boundary of two or more networks or DMZs (demilitarized
zones). Such a firewall filters all traffic entering or leaving the connected
networks.
In
reference to the layers where the traffic can be intercepted, three main
categories of firewalls exist:
·
network
layer firewalls An example would be iptables.
·
application
layer firewalls An example would be TCP Wrapper.
·
application
firewalls An example would be restricting ftp services through /etc/ftpaccess
file
These
network-layer and application-layer types of firewall may overlap, even though
the personal firewall does not serve a network; indeed, single systems have
implemented both together.
There's
also the notion of application firewalls which are sometimes used during wide
area network (WAN) networking on the world-wide web and govern the system
software. An extended description would place them lower than application layer
firewalls, indeed at the Operating System layer, and could alternately be
called operating system firewalls.
Lastly,
depending on whether the firewalls track packet states, two additional
categories of firewalls exist:
·
stateful
firewalls
·
stateless
firewalls
Network layer
firewalls
Network
layer firewalls operate at a (relatively low) level of the TCP/IP protocol
stack as IP-packet filters, not allowing packets to pass through the firewall
unless they match the rules. The firewall administrator may define the rules;
or default built-in rules may apply (as in some inflexible firewall systems).
A
more permissive setup could allow any packet to pass the filter as long as it
does not match one or more "negative-rules", or "deny
rules". Today network firewalls are built into most computer operating
system and network appliances.
Modern
firewalls can filter traffic based on many packet attributes like source IP
address, source port, destination IP address or port, destination service like
WWW or FTP. They can filter based on protocols, TTL values, netblock of
originator, domain name of the source, and many other attributes.
Application-layer
firewalls
Application-layer
firewalls work on the application level of the TCP/IP stack (i.e., all browser
traffic, or all telnet or ftp traffic), and may intercept all packets traveling
to or from an application. They block other packets (usually dropping them
without acknowledgement to the sender). In principle, application firewalls can
prevent all unwanted outside traffic from reaching protected machines.
By
inspecting all packets for improper content, firewalls can even prevent the
spread of the likes of viruses. In practice, however, this becomes so complex
and so difficult to attempt (given the variety of applications and the diversity
of content each may allow in its packet traffic) that comprehensive firewall
design does not generally attempt this approach.
Proxies
A
proxy device (running either on dedicated hardware or as software on a
general-purpose machine) may act as a firewall by responding to input packets
(connection requests, for example) in the manner of an application, whilst
blocking other packets.
Proxies
make tampering with an internal system from the external network more
difficult, and misuse of one internal system would not necessarily cause a
security breach exploitable from outside the firewall (as long as the
application proxy remains intact and properly configured). Conversely,
intruders may hijack a publicly-reachable system and use it as a proxy for their
own purposes; the proxy then masquerades as that system to other internal
machines. While use of internal address spaces enhances security, crackers may
still employ methods such as IP spoofing to attempt to pass packets to a target
network.
