Some Basic Terminology
The following basic terminology is essential in understanding networking and the many mechanisms involved. Terms covered here include the following:
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Network interface card
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MAC address
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Internet protocol
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Packet
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Networking protocol
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Ethernet
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Source host
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Destination host
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Broadcast
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Multicast
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Sniffer
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Protocol Analyzer
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Socket
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Firewall
Network Interface Card
The network interface adapter or card (NIC) connects the computer on which it is installed to the cable medium, converting the output that the computer sends to signals (normally electronic in nature) that are sent over this medium. The receiving NIC mirrors this process by receiving a set of electronic signals and then translating them into packets for the computer to receive.
MAC Address
Each machine connected to the network is identified by an address built in to its NIC. This address, called the Medium Access Control (MAC) address, is a physical address used to uniquely identify every computer on the network.
IP Address
Internet Protocol (IP) addresses are logical (not physical) addresses that are (with one exception—IPv61) always 32 bits in length. These bits are represented in terms of four octets, a format with which nearly everyone who uses the Internet is familiar. Each host address is typically represented as a four-octet IP address; the hypothetical host sunshine.global.net might, for example, have IP address 120.10.20.11. This address in turn corresponds to the particular set of bits shown in Figure 3.1.
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Figure
3.1
Composition of an IP address in terms of octets (above) and bits (below).
An Alternative Addressing Convention
Windows NT networking is largely based on the IP protocol. In contrast, Novell Internetwork Packet Exchange (IPX) addressing is a variant of the Xerox Network Service (XNS) addressing scheme originally developed for Ethernets. In the latter scheme, the host address is always 48 bits in length; a node address is always 32 bits. These bits are represented as four-digit hexadecimal numbers. An example of an IPX node address is 41.0000.od00.12bb. The Novell addressing scheme is not used in the Internet; and given that Novell NetWare 5.0 runs TCP/IP natively, this addressing scheme is rapidly becoming a thing of the past. It is, nevertheless, still used in reaching NetWare 4.x (and earlier versions) hosts over the network and is therefore relevant to Windows NT networking because of interoperability considerations.
Packet
Data sent over the network consists of blocks of characters (properly known as datagrams, but most people call them packets). Packets consist of a header, a small set of data about the type of packet, sending and destin-ation host, and so forth, followed by the contents (data).
Networking Protocol
When networked computers communicate with each other, they must share common methods and meanings to successfully do so. When one computer transmits data, for example, the receiving computer must not only prepare to receive it, but also must determine the context and meaning of this data. Protocols provide the solution by establishing methods of communication between computers: how communications are started, parameters that must be passed between machines, the order in which they are to be sent, and many other considerations.
Ethernet
Ethernet is a very widely used protocol that network adaptors connected to cabling media use. It determines the numeric values given to series of electrical signals to form valid Ethernet packets. Its method of networking is based on carrier sense multiple access collision detect (CSMA/CD) control in which hosts on the network wait for the right time to send network transmissions to each other. If no traffic is going over the part of the network to which these hosts interface, they start to send their transmissions. If more than one host sends a transmission at once, however, the packets collide with each other, necessitating another transmission attempt. Collisions are not so much of a problem in smaller Ethernets, but the opposite is true in larger ones. Both switching, a mechanism that makes a direct path between ports, and routing are frequently used to deal with these types of problems.
Source Host
The source host initiates a network transmission (for example, packets).
Destination Host
The destination host is the host designated to receive a network transmission.
Broadcast
Networked computers often need to obtain data about the status and availability of other computers or to provide information to other computers. In other cases, computers need something (an IP address, an executable, for example) to locate a particular type of server or something else from a server. This is where broadcasts fit in. Broadcast packets are designed to be sent to every machine within a local network. With a destination address of 255.255.255.255, these packets can reach every host on the local network, but by default cannot be sent outside of the local network unless they are encapsulated within another routable protocol.
Multicast
Multicast is an optimized multipoint transmission protocol. It is used in networking contexts, such as multimedia transmission and high volume data transfers.
Sniffer
Sniffer is a trademarked name owned by Network Associates. The Sniffer product that Network Associates makes captures packets transmitted over network cabling, dumps the data contents to storage media, and provides a user interface to view the data. The term sniffer, however, is a widely used term within the computing community to refer to any physical device or program that captures packet data. Because of trademark considerations, packet capture device or packet capture program is the appropriate term.
Protocol Analyzer
Having a device or program that captures data is useful; but having a device or program that dumps and sorts the data in different formats and views is even more useful, given the volume of data normally collected. A protocol analyzer does exactly this—it dumps captured network data in different formats and views to make analysis easier.
Socket
A socket is a convention that enables a user or program to reach a particular program on a destination host.
Firewall
A firewall is a host used as a security barrier between networks. Firewalls selectively screen traffic and sometimes also manage connections between one network and another.
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IPv6, the newest rendition of the IP protocol, is based on 128-bit addressing. There is no IPv6 stack in Windows 2000.