There are two kinds of addresses that are widely used on the Internet. One is a person's e-mail address, and the other is the address of a Web site, which is known as a URL. Following is an explanation of Internet e-mail addresses only. For more on URLs, see URL and Internet domain name.
E-Mail Address Format
The format for addressing a message to an Internet user is USER NAME @ DOMAIN NAME. For example, the address of the author of this database is dinesh@gmail.com. There are no spaces between any of the words. DINESH is the user name and GMAIL.COM is the domain name. The .COM stands for the commercial top level domain category (see Internet domain name). For the actual structure of an Internet address, see IP address.
IP Address
(Internet Protocol address) The address of a device attached to an IP network (TCP/IP network). Every client, server and network device is assigned an IP address, and every IP packet traversing an IP network contains a source IP address and a destination IP address.
Every IP address that is exposed to the public Internet is unique. In contrast, IP addresses within a local network use the same private addresses; thus, a user's computer in company A can have the same address as a user in company B and thousands of other companies. However, private IP addresses are not reachable from the outside world.
Logical Vs. Physical
An IP address is a logical address that is assigned by software residing in a server or router. In order to locate a device in the network, the logical IP address is converted to a physical address by a function within the TCP/IP protocol software. The physical address is actually built into the hardware.
Static and Dynamic IP
Network infrastructure devices such as servers, routers and firewalls are typically assigned permanent "static" IP addresses. The client machines can also be assigned static IPs by a network administrator, but most often are automatically assigned temporary "dynamic" IP addresses via software that uses the "dynamic host configuration protocol". Cable and DSL modems typically use dynamic IP with a new IP address assigned to the modem each time it is rebooted.
The Dotted Decimal Address: x.x.x.x
IP addresses are written in "dotted decimal" notation, which is four sets of numbers separated by decimal points; for example, 204.171.64.2. Instead of the domain name of a Web site, the actual IP address can be entered into the browser. However, the Domain Name System (DNS) exists so users can enter computerlanguage.com instead of an IP address, and the domain (the URL) computerlanguage.com is converted to the numeric IP address.
Class A, B and C
Based on the split of the 32 bits, an IP address is either Class A, B or C, the most common of which is Class C. More than two million Class C addresses are assigned, quite often in large blocks to network access providers for use by their customers. The fewest are Class A networks, which are reserved for government agencies and huge companies.
Although people identify the class by the first number in the IP address (see table below), a computer identifies class by the first three bits of the IP address (A=0; B=10; C=110). This class system has also been greatly expanded, eliminating the huge disparity in the number of hosts that each class can accommodate.
NETWORKS VERSUS HOSTS IN IPV4 IP ADDRESSES
Maximum Maximum Number of
Class Number Hosts Bits used in
Number of per Network/Host
Class Range Networks Network ID ID
A 1-126 127 16,777,214 7/24
B 128-191 16,383 65,534 14/16
C 192-223 2,097,151 254 21/8
127 reserved for loopback test
An IP address is first divided between networks and hosts. The host bits are further divided between subnets and hosts. See subnet mask.
Domain Name
A name that identifies one or more IP addresses. For example, the domain name microsoft.com represents about a dozen IP addresses. Domain names are used in URLs to identify particular Web pages. For example, in the URL
http://www.pcwebopedia.com/index.html, the domain name is pcwebopedia.com.
Every domain name has a suffix that indicates which top level domain (TLD) it belongs to. There are only a limited number of such domains. For example:
· gov - Government agencies
· edu - Educational institutions
· org - Organizations (nonprofit)
· mil - Military
· com - commercial business
· net - Network organizations
· ca - Canada
· th - Thailand
Because the Internet is based on IP addresses, not on domain names, every Web server requires a Domain Name System (DNS) server to translate domain names into IP addresses.
Internet Protocol version 6 (IPv6)
It is the next-generation Internet Protocol version designated as the successor to IPv4, the first implementation used in the Internet that is still in dominant use currently. It is an Internet Layer protocol for packet-switched internetworks. The main driving force for the redesign of Internet Protocol is the foreseeable IPv4 address exhaustion. IPv6 was defined in December 1998 by the Internet Engineering Task Force (IETF) with the publication of an Internet standard specification.
Features and differences from IPv4
In most regards, IPv6 is a conservative extension of IPv4. Most transport- and application-layer protocols need little or no change to operate over IPv6; exceptions are application protocols that embed network-layer addresses, such as FTP or NTPv3 (Network Time Protocol).
Larger address space
The most important feature of IPv6 is a much larger address space than that of IPv4: addresses in IPv6 are 128 bits long, compared to 32-bit addresses in IPv4.
The very large IPv6 address space supports a total of 2128 (about 3.4×1038) addresses—or approximately 5×1028 (roughly 295) addresses for each of the roughly 6.5 billion (6.5×109) people alive in 2006.
Stateless address auto configuration
IPv6 hosts can configure themselves automatically when connected to a routed IPv6 network using ICMPv6 router discovery messages. When first connected to a network, a host sends a link-local multicast router solicitation request for its configuration parameters; if configured suitably, routers respond to such a request with a router advertisement packet that contains network-layer configuration parameters.
If IPv6 stateless address auto configuration is unsuitable for an application, a network may use stateful configuration with the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) or hosts may be configured statically.
Multicast
Multicast, the ability to send a single packet to multiple destinations, is part of the base specification in IPv6. This is unlike IPv4, where it is optional (although usually implemented).
IPv6 does not implement broadcast, which is the ability to send a packet to all hosts on the attached link. The same effect can be achieved by sending a packet to the link-local all hosts multicast group. It therefore lacks the notion of a broadcast address—the highest address in a subnet (the broadcast address for that subnet in IPv4) is considered a normal address in IPv6.
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