Network Tutorial: Internet Protocol

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LAN:

A LAN supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home. LANs are useful for sharing resources like files, printers, games or other applications. A LAN in turn often connects to other LANs, and to the Internet or other WAN.

Most LANs are built with relatively inexpensive hardware such as Ethernet cables, network adapters, and hubs.  Wireless LAN and other more advanced LAN hardware options also exist.

Specialized operating system software may be used to configure a LAN. For example, most flavors of Microsoft Windows provide a software package called Internet Connection Sharing (ICS) that supports controlled access to LAN resources.

The term LAN party refers to a multiplayer gaming event where participants bring their own computers and build a temporary LAN.

Also Known As: local area network

Examples: The most common type of LAN is an Ethernet LAN. The smallest home LAN can have exactly two computers; a large LAN can accommodate many thousands of computers. Many LANs are divided into logical groups called subnets. An Internet Protocol (IP) "Class A" LAN can in theory accommodate more than 16 million devices organized into subnets.

WLAN:

WLANs provide wireless network communication over short distances using radio or infrared signals instead of traditional network cabling.

A WLAN typically extends an existing wired local area network. WLANs are built by attaching a device called the access point (AP) to the edge of the wired network.  Clients communicate with the AP using a wireless network adapter similar in function to a traditional Ethernet adapter.

Network security remains an important issue for WLANs. Random wireless clients must usually be prohibited from joining the WLAN. Technologies like WEP raise the level of security on wireless networks to rival that of traditional wired networks.

Also Known As: wireless LAN

Examples: For WLANs that connect to the Internet, Wireless Application Protocol (WAP) technology allows Web content to be more easily downloaded to a WLAN and rendered on wireless clients like cell phones and PDAs.

Intranet:

Intranet is the generic term for a collection of private computer networks within an organization. Intranets are communication tools designed to enable easy information sharing within workgroups.

Intranets utilize standard network hardware and software technologies like Ethernet, TCP/IP, Web browsers and Web servers.  An organization's intranet often features Internet access but is firewalled so that its computers cannot be reached directly from the outside.

A common extension to intranets, called extranets, open holes in this firewall to provide controlled access to outsiders.

Many schools and non-profit groups have deployed intranets, but an intranet is still seen primarily as a corporate productivity tool. Besides email and groupware applications, an intranet generally incorporates internal Web sites, documents, and/or databases to disseminate information.

The business value of intranet solutions is generally accepted in larger corporations, but their worth has proven very difficult to quantify in terms of time saved or return on investment.

Also Known As: corporate portal, private business network

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Peer-to-peer is a type of network design where all devices support roughly equivalent capabilities. Peer-to-peer networking (also known simply as peer networking) is in contrast to client/server networking, where certain devices have responsibility for providing or "serving" network information and other devices consume or otherwise act as "clients" of those servers.

Peer-to-peer networking is most common on small LANs, particularly Windows home networks. Peer networking on the Internet gained widespread popularity thanks to file sharing services like Napster. However, many of these file sharing services, including Napster, actually integrate both peer and client/server networking design. Technically, these are called hybrid networks.

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Lesson 5

Subnet

A subnet is a logical grouping of connected network devices. Nodes on a subnet tend to be located in close physical proximity to each other on a LAN.

Network designers employ subnets as a way to partition networks into logical segments for greater ease of administration. When subnets are properly implemented, both the performance and security of networks can be improved.

In IP networking, nodes on a subnet share a contiguous range of IP address numbers. A mask (known as the subnet mask or network mask) defines the boundaries of an IP subnet.

Also Known As: subnetwork

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A subnet allows the flow of network traffic between hosts to be segregated based on a network configuration. By organizing hosts into logical groups, subnetting can improve network security and performance.

Subnet Mask

Perhaps the most recognizable aspect of subnetting is the subnet mask. Like IP addresses, a subnet mask contains four bytes (32 bits) and is often written using the same "dotted-decimal" notation. For example, a very common subnet mask in its binary representation

        11111111 11111111 11111111 00000000
       
is typically shown in the equivalent, more readable form

        255.255.255.0

Applying a Subnet Mask

A subnet mask neither works like an IP address, nor does it exist independently from them. Instead, subnet masks accompany an IP address and the two values work together. Applying the subnet mask to an IP address splits the address into two parts, an "extended network address" and a host address.

For a subnet mask to be valid, its leftmost bits must be set to '1'. For example,

        00000000 00000000 00000000 00000000

is an invalid subnet mask because the leftmost bit is set to '0'.

Conversely, the rightmost bits in a valid subnet mask must be set to '0', not '1'. Therefore,

        11111111 11111111 11111111 11111111

is invalid.

All valid subnet masks contain two parts: the left side with all mask bits set to '1' (the extended network portion) and the right side with all bits set to '0' (the host portion), such as the first example above.

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[iframe]http://sarah72.free.fr/Internet1.htm[/iframe]

[ Last edited by Remy_3D on 25-2-2004 at 09:06 AM ]

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Router:

A router is a physical device that joins multiple wired or wireless networks together. Technically, a wired or wireless router is a Layer 3 gateway, meaning that it connects networks (as gateways do), and that it operates at the network layer of the OSI model.

Home networkers often use an Internet Protocol (IP) wired or wireless router, IP being the most common OSI network layer protocol. An IP router such as a DSL or cable modem broadband router joins the home's local area network (LAN) to the wide-area network (WAN) of the Internet.

By maintaining configuration information in a piece of storage called the "routing table," wired or wireless routers also have the ability to filter traffic, either incoming or outgoing, based on the IP addresses of senders and receivers. Some routers allow the home networker to update the routing table from a Web browser interface. Broadband routers combine the functions of a router with those of a network switch and a firewall in a single unit.

CIDR:

CIDR is an efficient method for specifying IP addresses to Internet routers. CIDR was developed to cope with the surge in demand for IPv4 Internet addresses in the 1990s.

Before CIDR, Internet routers used an inefficient IP addressing scheme based on classes.  Organizations like ISPs reserved address blocks in large "Class A," "Class B," or "Class C" chunks that wasted much of the IP address range.

In contrast, CIDR makes the IP addressing space classless. CIDR associates network masks with IP network numbers independent of their traditional class. Routers that support CIDR recognize these networks as individual routes, even though they may represent an aggregation of several traditional subnets.

Also Known As: Classless Inter-Domain Routing, Classless Internet Domain Routing, supernetting

Examples: CIDR shorthand notation writes an IP address and its associated network mask in the form xxx.xxx.xxx.xxx/n, where 'n' is a number between 1 and 31 that is the number of '1' bits in the mask.

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What is CIDR?

CIDR stands for Classless Inter-Domain Routing. CIDR was developed in the 1990s as a standard scheme for routing IP addresses.

Before CIDR, Internet routers managed IP traffic based solely on the class of IP addresses and their associated subnet masks. This scheme utilized IP address space inefficiently as explained earlier. CIDR allows a more flexible way to associate groups of IP addresses without relying on the original class system. CIDR is also known as supernetting.

CIDR Notation

CIDR specifies an IP address range by the combination of an IP address and its associated network mask. CIDR notation uses the following format -

    xxx.xxx.xxx.xxx/n

where n is the number of (leftmost) '1' bits in the mask. For example,

    192.168.12.0/23

applies the network mask 255.255.254.0 to the 192.168 network, starting at 192.168.12.0. This notation represents the address range 192.168.12.0 - 192.168.13.255. Compared to traditional class-based networking, 192.168.12.0/23 represents an aggregation of the two Class C networks 192.168.12.0 and 192.168.13.0 each using the default network mask 255.255.255.0.

CIDR supports Internet address allocation and message routing independent of the traditional class of a given IP address range. For example,

    10.4.12.0/22

represents the address range 10.4.12.0 - 10.4.15.255 by employing the network mask 255.255.252.0. This effectively represents an apportioning of four Class C networks within the much larger Class A space.

CIDR notation is sometimes adopted even on non-CIDR networks. In non-CIDR IP subnetting, however, the value of n is restricted to either 8 (Class A), 16 (Class B) or 24 (Class C) from the Internet address allocation and routing perspective.

How CIDR Works

The flexibility of CIDR derives from the ability of routers to work with subnet masks other than the traditional Class A, B, or C masks (values of n other than 8, 16, or 24). For CIDR to work, Internet routing protocols must be implemented that support the CIDR conventions. Popular routing protocols like BGP (Border Gateway Protocol) and OSPF (Open Shortest Path First) were updated to support CIDR years ago, but some less popular protocols still do not support CIDR today.

Routers on the Internet backbone (WAN network between ISPs) all generally support CIDR. Backbone support of CIDR is essential to achieve conservation of IP address space. Private networks and small public LANs have much less need to conserve addresses, however, and therefore may not utilize CIDR.

For aggregation to work, the subnets involved must be contiguous (numerically adjacent) in the address space. CIDR cannot, for example, aggregate 192.168.12.0 and 192.168.15.0 into a single route unless the intermediate .13 and .14 address ranges are included. The 192.168.12.0/24 route does exactly this.

CIDR and IPv6

IPv6 utilizes CIDR routing technology and CIDR notation in the same way as IPv4. IPv6 is designed for fully classless addressing.

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Lesson 6

An Introduction to MAC Addressing

In computer networking, the Media Access Control (MAC) address is every bit as important as an IP address. Learn in this article how MAC addresses work and how to find the MAC addresses being used by a computer.

What Is a MAC Address?

The MAC address is a unique value associated with a network adapter. MAC addresses are also known as hardware addresses or physical addresses. They uniquely identify an adapter on a LAN.

MAC addresses are 12-digit hexadecimal numbers (48 bits in length). By convention, MAC addresses are usually written in one of the following two formats:

    MM:MM:MM:SS:SS:SS


    MM-MM-MM-SS-SS-SS

The first half of a MAC address contains the ID number of the adapter manufacturer. These IDs are regulated by an Internet standards body (see sidebar). The second half of a MAC address represents the serial number assigned to the adapter by the manufacturer. In the example,

    00:A0:C9:14:C8:29

The prefix

    00A0C9

indicates the manufacturer is Intel Corporation.

Why MAC Addresses?

Recall that TCP/IP and other mainstream networking architectures generally adopt the OSI model. In this model, network functionality is subdivided into layers. MAC addresses function at the data link layer (layer 2 in the OSI model). They allow computers to uniquely identify themselves on a network at this relatively low level.

MAC vs. IP Addressing

Whereas MAC addressing works at the data link layer, IP addressing functions at the network layer (layer 3). It's a slight oversimplification, but one can think of IP addressing as supporting the software implementation and MAC addresses as supporting the hardware implementation of the network stack. The MAC address generally remains fixed and follows the network device, but the IP address changes as the network device moves from one network to another.

IP networks maintain a mapping between the IP address of a device and its MAC address. This mapping is known as the ARP cache or ARP table. ARP, the Address Resolution Protocol, supports the logic for obtaining this mapping and keeping the cache up to date.

DHCP also usually relies on MAC addresses to manage the unique assignment of IP addresses to devices.

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The method used to find a MAC address depends on the type of network device involved. All popular network operating systems contain utility programs that allow one to find (and sometimes change) MAC address settings.

Find a MAC Address in Windows

In Windows 95, Windows 98 and Windows ME, the winipcfg utility displays MAC addresses. In Windows NT and any newer versions of Windows, the ipconfig utility (using the /all option) can also be used.

Both winipcfg and ipconfig can display multiple MAC addresses. First, one MAC address is shown for each network adapter. Then, one or more additional MAC addresses are shown for other network adapters.

Windows, for example, utilizes built-in MAC addressing to support Windows dial-up connections. Some Windows VPN clients likewise use their own MAC address. Because these other adapters are really software constructs that do not involve unique hardware, these are often referred to as virtual adapters.

Find a MAC Address in Unix or Linux

The specific command used in Unix to find a MAC address varies depending on the "flavor" of the operating system. In Linux and in some forms of Unix, the command ifconfig -a returns MAC addresses.

It's also possible to find MAC addresses in Unix and Linux by reading the boot message sequence, either on-screen as the system boots or from the startup message file. The log file for boot messages is usually /var/log/messages or /var/adm/messages.

Find a MAC Address on the Macintosh

MAC addresses on the Macintosh are generally found on the TCP/IP Control Panel. If the system is running Open Transport, the MAC address can be found under the Info or User Mode/Advanced screens. If the system is running MacTCP, the MAC address can be found under the Ethernet icon.
Finding a MAC Address Summary

The table below summarizes options for finding a computer's MAC address.
Operating system        Method
Windows 95 and newer        winipcfg
Windows NT and newer        ipconfig /all
Linux and some Unix        ifconfig -a
Macintosh with Open Transport        TCP/IP Control Panel - Info or User Mode/Advanced
Macintosh with MacTCP        TCP/IP Control Panel - Ethernet icon

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MAC addresses were designed to be fixed numbers that cannot be changed. However, there are some valid reasons to want to change your MAC address.

Changing MAC Addresses to Support Your ISP

Some ISPs, typically cable modem providers, sell Internet subscriptions on a per-address basis. Occasionally the ISP manages these subscriptions by assigning a static (fixed) IP address to the customer. However, this approach is an inefficient use of IP addresses that are currently in short supply.

More typically, ISPs manage single-address subscriptions by registering the MAC address of the device that connects to the ISP. This device could be a broadband modem, for example. The customer is free to build a home or small business network behind this modem, but the ISP expects the MAC address to match the registered value at all times.

Whenever a customer replaces their modem or adds a broadband router, the MAC address will no longer match that registered at the ISP, and the ISP will disable the customer's Internet connection.

Cloning MAC Addresses

One way to solve this problem is to call the ISP and ask them to update the registered MAC address to match the new hardware. A more efficient way to solve this problem is to configure the device so that it advertises the original MAC address, even though it's hardware is built to utilize a different MAC address. This process is called cloning.

Many broadband routers today support MAC address cloning as an advanced configuration option. The exact procedure varies depending on the type of router.

MAC Addresses and Cable Modems

Note that in addition to MAC addresses stored at the ISP, some broadband modems also store the MAC address of the host computer's network adapter. However, in this case, cloning is not required. It's true that changing network adapters ususally causes the cable modem connection to fail. To remedy this problem, though, requires only that the cable modem and computer be reset (and perhaps a waiting period for the ISP to release the old IP address).

Changing MAC Addresses through the Operating System

Starting with Windows 2000, users can change their MAC address through the Windows My Network Places interface. This feature relies on software support built into the adapter driver program and thus does not work for all adapters.

Likewise, the ifconfig command available in Linux and other flavors of Unix supports changing MAC addresses with the necessary network card and driver support.

The MAC address is an important element of computer networking. MAC addresses uniquely identify a computer on the LAN. MAC is an essential component required for network protocols like TCP/IP to function.

Computer operating systems and broadband routers support viewing and sometimes changing MAC addresses. Some ISPs track their customers by MAC address. Changing a MAC address can be necessary in some cases to keep an Internet connection working.

Changing MAC addresses may also increase privacy in some situations, though MAC addresses do not reveal any geographic or ISP location information like IP addresses do.

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ARP:

ARP converts an IP address to its corresponding physical network address. It is a low-level protocol (at layer 2 in the OSI model) usually implemented in the device drivers of network operating systems. ARP is most commonly seen on Ethernet networks, but ARP has also been implemented for ATM, Token Ring, and other physical networks. The first RFC discussing ARP (for Ethernet) was RFC 826.

Ethernet network adapters are produced with a physical address (called the Media Access Control or MAC address) embedded in the hardware. Manufacturers take care to ensure these 6-byte addresses are unique, and Ethernet relies on these unique identifiers for frame delivery. When an IP packet arrives at a network gateway, the gateway needs to convert the destination IP address to the appropriate MAC address so that it can be delivered over Ethernet. Some IP-to-MAC address mappings are maintained in an ARP cache, but if the given IP address does not appear there, the gateway will send an ARP request that is broadcast on the local subnet. The host with the given IP address sends an ARP reply to the gateway, who in turn delivers the packet (and updates its cache).

Also Known As: Address Resolution Protocol

NAT:

NAT allows an IP-based network to manage its public (Internet) addresses separately from its private (intranet) addresses. It is a popular technology for Net connection sharing on DSL or cable LANs. With NAT, each private IP address can be translated to a different public address, or multiple private addresses can be aliased to a single public one. To accomplish this, NAT software snoops both incoming and outgoing packets on the network. It modifies the source or destination address in the IP header (and the affected checksums) to reflect the mapping between internal and external addressing for that network.

NAT functionality appears on routers and other gateway devices with low-level access to packets at the network boundary. Several variations on NAT have also been implemented to provide additional support for application-level protocols. NAT for the Internet is defined in RFC 1631.

Also Known As: Network Address Translation

[ Last edited by Remy_3D on 25-2-2004 at 05:51 PM ]

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Lesson 7

TCP/IP - Transmission Control Protocol / Internet Protocol

Transmission Control Protocol (TCP) and Internet Protocol (IP) are two distinct network protocols, technically speaking. TCP and IP are so commonly used together, however, that TCP/IP has become standard terminology to refer to either or both of the protocols.

IP corresponds to the Network layer (Layer 3) in the OSI model, whereas TCP corresponds to the Transport layer (Layer 4) in OSI.  In other words, the term TCP/IP refers to network communications where the TCP transport is used to deliver data across IP networks.

The average person on the Internet works in a predominately TCP/IP environment. Web browsers, for example, use TCP/IP to communicate with Web servers.

Also Known As: Transmission Control Protocol / Internet Protocol

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The User Datagram Protocol (UDP) supports network applications that need to transport data between computers. Applications that use UDP include client/server programs like video conferencing systems. Although UDP has been in use for many years -- and overshadowed by more glamorous alternatives -- it remains an interesting and viable technology.

UDP -- like its cousin the Transmission Control Protocol (TCP) -- sits directly on top of the base Internet Protocol (IP). Recalling the Open Systems Interconnection (OSI) model of networking, UDP (and TCP) are transport layer protocols as shown below.


UDP in the OSI Reference Model

In general, UDP implements a fairly "lightweight" layer above the Internet Protocol. UDP's main purpose is to abstract network traffic in the form of datagrams. A datagram comprises one single "unit" of binary data; the first eight (8) bytes of a datagram contain the header information and the remaining bytes contain the data itself.

UDP Headers

The UDP header consists of four (4) fields of two bytes each:

    * source port number
    * destination port number
    * datagram size
    * checksum

UDP port numbers allow different applications to maintain their own "channels" for data; both UDP and TCP use this mechanism to support multiple applications sending and receiving data concurrently. The sending application (that could be a client or a server) sends UDP datagrams through the source port, and the recipient of the packet accepts this datagram through the destination port. Some applications use static port numbers that are reserved for or registered to the application. Other applications use dynamic (unregistered) port numbers. Because the UDP port headers are two bytes long, valid port numbers range from 0 to 65535; by convention, values above 49151 represent dynamic ports.

The UDP datagram size is a simple count of the number of bytes contained in the header and data sections . Because the header length is a fixed size, this field essentially refers to the length of the variable-sized data portion (sometimes called the payload). The maximum size of a datagram varies depending on the operating environment. With a two-byte size field, the theoretical maximum size is 65535 bytes. However, some implementations of UDP restrict the datagram to a smaller number -- sometimes as low as 8192 bytes.

UDP checksums work as a safety feature. The checksum value represents an encoding of the datagram data that is calculated first by the sender and later by the receiver. Should an individual datagram be tampered with (due to a hacker) or get corrupted during transmission (due to line noise, for example), the calculations of the sender and receiver will not match, and the UDP protocol will detect this error. The algorithm is not fool-proof, but it is effective in many cases. In UDP, checksumming is optional -- turning it off squeezes a little extra performance from the system -- as opposed to TCP where checksums are mandatory.

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UDP vs. TCP: Speed vs. Reliability

The primary difference between UDP and TCP lies in their respective implementations of reliable messaging. TCP includes support for guaranteed delivery, meaning that the recipient automatically acknowledges the sender when a message is received, and the sender waits and retries in cases where the receiver does not respond in a timely way.

UDP, on the other hand, does not implement guaranteed message delivery. A UDP datagram can get "lost" on the way from sender to receiver, and the protocol itself does nothing to detect or report this condition. UDP is sometimes called an unreliable transport for this reason.

Another way in which UDP works unreliably is in the receipt of a burst of multiple datagrams. Unlike TCP, UDP provides no guarantees that the order of delivery is preserved. For example, a client application might send the following four datagrams to a server

        D1       
        D22
        D333       
        D4444

but UDP may present the datagrams to the server-side application in this order instead:

        D333       
        D1       
        D4444       
        D22


In practice, UDP datagrams arrive out-of-order relatively infrequently -- generally only under heavy traffic conditions.

The Case For UDP

On the surface, an "unreliable" network protocol may not seem very worthwhile or desirable. But in fact, UDP can be very useful in certain situations, and it enjoys one key advantage over TCP -- speed. The reliability features built into TCP can be expensive in terms of overhead at execution time. Also note that UDP does not preclude reliable message delivery, it merely defers those details to a higher level of the network stack.

The original specification for UDP is RFC 768, published in 1980. Despite its age, UDP continues to be used in mainstream applications. Video conferencing systems in particular have proven a good fit for UDP because they have been willing to sacrifice some reliability (i.e., picture quality) in return for performance (i.e., higher frame rates). This is the classic tradeoff between UDP and TCP, and it appears both transports still have their place in today's networking world.

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FTP allows one to transfer files between computers on the Internet. Technically, FTP is a simple network protocol based on IP, but many also use the term "FTP" to refer to this type of file sharing service.

The FTP service is based on a client/server architecture. An FTP client program initiates a connection to a remote computer running FTP server software. After the connection is established, the client can choose to send and/or receive copies of files, singly or in groups. To connect to an FTP server, a client generally requires a username and password as set by the administrator of the server. Many public FTP archives follow a special convention for that accepts a username of "anonymous."

FTP clients are included with most network operating systems, but most operating system clients (such as FTP.EXE on Windows) support a relatively unfriendly command-line interface. Many freeware and shareware third-party FTP clients have been developed that support graphic user interfaces (GUIs) and additional convenience features. In either command-line or graphic interfaces, FTP clients identify the server either by its IP address (such as 192.168.0.1) or by its host name (such as ftp.about.com).

The FTP protocol supports two modes of data transfer: plain text (ASCII), and binary. The mode an FTP client uses must generally be configured by the end user. The mode usually defaults to plain text. The most common error one makes in using FTP occurs when attempting to transfer a binary file (such as a program or music file) while in text mode. A copy of the file is made, but this copy will often be unusable. When working with FTP clients and files, learn to use the transfer mode properly.

Also Known As: File Transfer Protocol

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Protocol:

A network protocol defines a "language" of rules and conventions for communication between devices. A protocol includes formatting rules that specify how data is packaged into messages. It also may include conventions like message acknowledgement or data compression to support reliable and/or high-performance communication.

Many protocols exist in computer networking ranging from high level (like SOAP) to low level (like ARP). The Internet Protocol family includes IP and all higher-level network protocols built on top of it, such as TCP, UDP, HTTP, and FTP. Modern operating systems include services or daemons that implement support for specific protocols. Some protocols, like TCP/IP, have also been implemented in silicon hardware for optimized performance.

Also Known As: network protocol

HTTP:

HTTP is an application layer network protocol built on top of TCP. HTTP allows Web browsers and Web servers to communicate.

HTTP clients and servers communicate via request and response messages. The three main HTTP message types are GET, POST, and HEAD.

HTTP utilizes TCP port 80 by default, though other ports such as 8080 are also used.

The current version of HTTP in widespread use - HTTP version 1.1 - was developed to address some of the performance limitations of the original version - HTTP 1.0. HTTP 1.1 is documented in RFC 2068.

Also Known As: HyperText Transfer Protocol

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Additional Reading

Introduction to Sockets and Socket Programming

An essential network programming concept

A socket is one of the most fundamental technologies of computer networking. Sockets allow applications to communicate using standard mechanisms built into network hardware and operating systems. Although network software may seem to be a relatively new "Web" phenomenon, socket technology actually has been employed for roughly two decades.

Software applications that rely on the Internet and other computer networks continue to grow in popularity. Many of today's most popular software packages -- including Web browsers, ICQ, and Napster -- rely on sockets.

Point-to-Point Communication

In a nutshell, a socket represents a single connection between exactly two pieces of software. More than two pieces of software can communicate in client/server or distributed systems (for example, many Web browsers can simultaneously communicate with a single Web server) but multiple sockets are required to do this. Socket-based software usually runs on two separate computers on the network, but sockets can also be used to communicate locally (interprocess) on a single computer.

Sockets are bidirectional, meaning that either side of the connection is capable of both sending and receiving data. Sometimes the one application that initiates communication is termed the client and the other application the server, but this terminology leads to confusion in non-client/server systems and should generally be avoided.

Libraries

Programmers access sockets using code libraries packaged with the operating system. Several libraries that implement standard application programming interfaces (APIs) exist. The first mainstream package - the Berkeley Socket Library is still widely in use on UNIX

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Internet Protocol Summary - Facts about IP

Name: Internet Protocol - abbreviated "IP"

Description: IP is used by many higher level network protocols, principally TCP and UDP. Many Internet software applications including Web browsers, FTP clients, and email programs, rely on Internet Protocol.

OSI Model: Network layer (Layer 3)

Datagram Format: A base header 20 bytes (5 "longwords") in length, with the option for expanded header options, followed by data.

Header:

Word 1 -

    * Version - 4 bits
    * Header Length (in longwords) - 4 bits
    * Type of Service / Differentiated Services Code Point (DSCP) - 8 bits
    * Datagram Length (in bytes) - 16 bits

Word 2 -

    * ID Number - 16 bits
    * Fragmentation Flags - 3 bits
    * Fragmentation Offset - 13 bits

Word 3 -

    * Time to Live - 8 bits
    * Transport Protocol - 8 bits
    * Header Checksum - 16 bits

Word 4 -

    * Source IP Address - 32 bits

bWord 5 -

    * Destination IP Address - 32 bits

Variable length fields -

    * Options
    * Padding

Payload: IP datagram payloads can be of variable length. The minimum size of an IP datagram is 28 bytes, using the minimum 20 bytes of header information, followed by the minimum of 8 bytes of data. The maximum size of an IP datagram payload is 65,535 bytes minus the header size.

Footer: Internet Protocol does not use its own datagram footer.

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