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About network and Internet
Network is an aggregation of computers interlinking with cable, line, or wireless communication. With network, you can share or change network data and information with other network users online.
Generally, we separate network as LAN (Local Area Network) and WAN (Wide Area Network) according to the area size of computer's linking. The network in a house, a building or a campus is so called LAN, and the network spanning regions and countries is called WAN. Internet is the biggest typical WAN which originated from ARPANET (A Program by Advanced Research Project Agency of US Department of Defense) in 1969. In 1986, NSF( US National Science Foundation) created NSFNET with TCP/IP sponsored by the government and opened it to public as Internet in 1989.
With the commercialization and popularization of network in global scope, Internet has been becoming a name of information modernizing. We can say Internet is a data library covering nearly everything, or Internet is a hinge central of the whole world through which you can gain international news, chat with friends, write or read emails, shop online, view films, listen concert and do many other things.
About Ethernet
Ethernet was originally developed by XEROX Corporation. Due to the using
of DEC, INTEL and XEROX together, Ethernet was configured in 1980 and
adapted by IEEE as 802.3 standard afterwards. 
The basic feature of Ethernet is to adopt a kind of shared access plan
called CSMA/CD (Carrier Sense Multiple Access/Collision Detection), that
is, multi workstations connect on a central line and all workstations
send monitoring signals ceaselessly to the central line. However, only
one workstation can transmit signals on the line at one time and the
others must wait for starting their own transmission until that
workstation finishes.
- Ethernet and LAN
Only a kind of LAN, Ethernet is in parallel with Token-Ring and FDDI and
involved in LAN system.
Nowadays, 90% network is Ethernet for it is the main technique applied
in LAN at present so that Ethernet almost has been another name of LAN.
- Types of Ethernet
- Classified by speed, Ethernet has 10Mbps Ethernet, 100Mbps
Ethernet, 1000Mbps Ethernet and 10Gbps Ethernet;
- Classified by the usage type of wideband, Ethernet has shared
Ethernet and switched Ethernet two kinds.
What is a packet?
Packet is a piece of a message transmitted over a packet-switching network. When any file (e-mail message, HTML file, Graphics Interchange Format file, Uniform Resource Locator request, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into "chunks" of an efficient size for routing. Each of these packets is separately numbered and includes the Internet address of the destination. The individual packets for a given file may travel different routes through the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end).
Packet header
| 6bytes | 6 bytes | 2 bytes | 20 bytes | 20 bytes | 1460 bytes | 4 bytes |
| Destination Address | Source Address | Ethernet Type | IP Datagram Header | TCP Segment Header | Application Data | CRC |
| Data Link Layer | Network Layer | Transport Layer |
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Packets are created at the machine sending the information. The
application builds the data on the sending machine and passes the data
to a protocol stack running on that machine. The data is sent down the
protocol stack through each layer, wrapped in one or more chunks to be
reassembled in the correct order at the destination. And then, the
protocol stack on the sending machine passes the packets to the Ethernet
hardware: the NIC (Network Interface Card). The Ethernet hardware adds
information to the data by pretending headers to the data that it
receives.
The Ethernet headers added by the sending machine will point to a router
or a switch accord with the packet's final destination address. The
router will open the packet, separate the Ethernet wrapper from the
packet, find the final destination address and rewrap the packet with a
new header, send it to the ISP internet of the destination address.
What is network protocol?
Computers need to abide by certain regulations together to communicate normally via network (involving internet), these regulations are so called network protocols. Network protocols are usually sorted into several layers and each layer has its own functions.
A computer must comply with network protocols in order to communicate with others, and only those on a same protocol layer may contact each other.
TCP/IP and IPX/SPX are two most popular protocols, the prior is used widely on internet, and the latter is used normally on intranet.
What is network monitoring?
Do you know why network monitoring is so important in internet management? According to a report, 30%-40% of employees' network activities are irrelevant to their work, 37% employees state that they often surf online during working time, and 70% visits to erotic websites happen at working time. These are also the reasons why there are 80% American and European corporations have taken steps to monitor the internet activities of employees.
Network monitoring, in nature, is to read and parse the contents of packets transferred on network. The operation principal of network transmission is that a host sends packets to another host, each packet involving data and frame which covers the information of packet like source and destination. The data of a packet may be email, web page, registration information, password, e-business records, and other information of network transactions. |