Windows Explorer and My Computer
12.16 An Overview of TCP/IP
When discussing communication, especially the Internet, you will hear the term TCP/IP (Transmission Control Protocol/Internet Protocol). TCP/IP is truly the protocol of the Internet. Data is transferred over the Internet through the protocols called TCP/IP.
Using the Internet is not unlike making a telephone call from Los Angeles to your mother in Phoenix. You know that you do not have a direct phone line connection to your mother’s home. You dial and the phone company decides the best way to route your call. If Los Angeles is very busy, the phone company may send your call to Phoenix through Denver because Denver is not as busy and can process the data you are sending faster. It is not important to you how the phone company manages communication as long as you can talk to your mother.
On the Internet, data usually travels through several networks before it gets to its destination. Each network has a router, a device that connects networks. Data is sent in packets, units of information. A router transfers a packet to another network only when the packet is addressed to a station outside of its own network. The router can make intelligent decisions as to which network provides the best route for the data.
The rules for creating, addressing, and sending packets are specified by the TCP/IP protocols. TCP and IP have different jobs and are actually two different protocols. TCP is what divides the data into packets and then numbers each packet so it can be reassembled correctly at the receiving end. IP is responsible for specifying the addresses of the sending and receiving computers and sending the packets on their way. An IP address tells routers where to route the data. Data is divided into packets for two major reasons. The first is to ensure that sending a large file will not take up all of a network’s time, and the second is to ensure that the data will be transferred correctly. Each packet is verified as having been received correctly. If a packet is corrupt, only the corrupted packet has to be resent, not the entire file.
A large company, college, or university will maintain a permanent open connec-tion to the Internet (a T1 or T3 line), but this is not the case for a small office or a stand-alone PC. As mentioned previously, a single user often accesses the Internet through a dial-up, cable, or DSL connection instead. This procedure provides a temporary connection known as a PPP (Point-to-Point Protocol) connection. An-other older protocol that accomplishes the same task is SLIP (Serial Line Internet Protocol). This connection provides full access to the Internet as long as you are online. However, if you are using a cable modem or a DSL connection, you do not need to dial up; you are always connected to your provider (the cable company or phone company). The connection is more like a LAN. You are always connected to the server, which in turn is connected to the gateway to the Internet.
Each computer connected to the Internet must have the TCP/IP protocols installed, as well as a unique IP address. The IP address identifies the computer on the Internet.
If you are connected to the Internet through a permanent connection, the IP address remains a static (constant) address. If you have a dial-up account, a cable modem account, or other type of connection, you typically get a dynamic (temporary) IP
12.16 AN OVERVIEW OF TCP/IP
address. It is a leased address and will change depending on how long the hosting server runs its leases for.
The Internet Corporation for Assigned Names and Numbers (ICANN, http://
www.icann.org/general/abouticann.htm) is the non-profit corporation that was formed to assume responsibility for the IP address space allocation, protocol param-eter assignment, domain name system management, and root server system manage-ment functions previously performed under U.S. Governmanage-ment contract by IANA (Internet Assigned Numbers Authority, http://www.ican.org) and other entities. It is a nonprofit organization established for the purpose of administration and registration of IP numbers for the geographical areas previously managed by Network Solutions, Inc. When an organization applies for IP addresses, ICANN assigns a range of addresses appropriate to the number of hosts on the asking organization’s network.
An IP address is made up of four numbers separated by periods. An IP address is 32 bits long, making each of the four numbers 8 bits long. These 8-bit numbers are called octets. The largest possible octet is 11111111. In decimal notation, that is equal to 255. So the largest possible IP address is 255.255.255.255. This format is called dotted decimal notation, also referred to as “dotted quad.”
Bit number 0 31
10010001 . 00001010 . 00100010 . 00000011
145 10 34 3
145.10.34.3 Figure 12.14A Dotted Quad Address
As originally designed, IP address space was divided into three different address classes: Class A, Class B, and Class C. A Class A network receives a number that is used in the first octet of the address. Class A network numbers range from 0 to 127. If an organization was assigned 95 as its network address, the hosts in the network would have IP addresses like 95.0.0.1, 95.0.0.2, 95.0.0.3, and so forth. There are no Class A network addresses remaining. Class A networks are now referred to as /8 (pronounced “slash eight”) or sometimes just 8 since they have an 8-bit network prefix.
A Class B network has its network address assigned as the first two octets. The first octet can range between 128 and 191. The second octet can range between 0 and 255. If an organization was assigned 145.21, the hosts in the network would have IP addresses like 145.21.0.1, 145.21.0.2, 145.21.0.3, and so on. Class B networks are now referred to as /16 since they have a 16-bit network prefix. There are also no Class B network addresses remaining.
Today, Class C network addresses are still available. These are assigned the first three octets as their network address. The first octet can range from 192 to 254. If an organization was assigned 199.91.14, the hosts in the network would have IP ad-dresses like 199.91.14.1, 199.91.14.2, 199.91.14.3, and so on. Class C networks are now referred to as /24 since they have a 24-bit network prefix.
There are two additional classes: Class D, which is used to support multicasting, and Class E, which is reserved for experimental use. With the explosive expansion of
697
the Internet, IP addresses are going to be depleted. The appropriate parties are working on a solution to this problem by developing a new standard, called IP Next Generation (IPv6). In the meantime, the current system remains in place.
Even with the current system, if you had an organization with a large number of computers, you would still run out of IP addresses fairly quickly. A solution is to not assign a permanent (static) IP address to a computer, but rather to assign an IP address to be used for the current work session only when the computer goes online (a dynamic IP address). In this system, when you log off, your IP address is returned to the list of available addresses, and, since not everyone is online at the same time, not as many IP addresses are needed. The server that manages dynamic IP addresses is called a dynamic host configuration protocol (DHCP) server. Some ISPs (Internet service providers) use this method to assign IP addresses to their dial-up clients.
Others assign the address to the modem you dial into.
It would be difficult for most people to remember a numeric IP address. People remember names better than numbers. Phone numbers such as 1-800-FLOWERS or 1-800-URENTIT became popular for this very reason. Although you may not name your personal computer, computers in organizations are named so one computer can be distinguished from another. Organizations may choose names such as pc1, pc2, mac1, mac2 or do it by department such as sales. Often, a computer’s name will reflect its major role in the company. Thus, a computer devoted to handling electronic mail is often named mail, whereas a computer devoted to running the company’s World Wide Web service is often called www. Both are easy-to-remember host names. These are in-house business names, not IP addresses for the Internet. If the computer is on the Internet, it has an IP address. An IP address can change, but typically it is not an organization’s name. To give Internet addresses easy-to-remember names like this, the Internet is divided into domains. A domain is a general category that a computer on the Internet belongs to. A domain name is an easy-to-understand name given to an Internet host, as opposed to the numerical IP address. A user or organization applies for a domain name through the Internet Network Information Center (InterNIC) to ensure that each name is unique. InterNIC is now not the only organization respon-sible for assigning domain names. Some examples of domain names are
saddleback.cc.ca.us, solano.cc.ca.us, fbeedle.com, unl.edu, loyola.edu, ces.sdsu.edu, uci.edu, bookbiz.com, dell.com, and microsoft.com.
A fully qualified domain name (FQDN) is the host name plus the domain name. As an example, a host name could be mail and the domain name could be fbeedle.com.
The FQDN would be mail.fbeedle.com. Another host name could be www with a domain name of microsoft.com; thus the FQDN would be www.microsoft.com. A fully qualified domain name must be resolved into the numeric IP address in order to communicate across the Internet.
The Domain Name System (DNS) provides this name resolution. It ensures that every site on the Internet has a unique address. Part of its job is to divide the Internet into a series of networks called domains. Each site attached to the Internet belongs to a domain. Large domains are divided into smaller domains, with each domain responsible for maintaining unique addresses in the next lower-level domain or subdomain. DNS maintains a distributed database. When a new domain name is assigned, the domain name and its IP address are placed into a database on a top-level domain name server (domain root server), which is a special computer that
12.16 AN OVERVIEW OF TCP/IP
Internet name of the
keeps information about addresses in its domain. When a remote computer tries to access a domain name and does not know the IP address, it queries its DNS server. If that DNS server does not have the IP address in its database, it contacts a root DNS server for the authoritative server responsible for that domain. Then, the DNS server goes directly to the authoritative server to get the IP address and other needed infor-mation, updates its database, and informs the remote computer of the domain name’s IP address.
When you use a browser to access a site on the Internet, you key in the URL (uniform resource locator). The browser program contacts the remote server for a copy of the requested page. The server on the remote system returns the page, tells the browser how to display the information, and gives a URL for each item you can click on the page. Figure 12.15 describes the parts of a URL.
http://www.microsoft.com/support
Figure 12.15The Parts of a URL
The URL in the above figure is for the page that gives you support for Microsoft products.
This somewhat technical discussion is not intended to confuse you, but to give you some idea of Internet jargon. Terms like IP address, URL, and domain name are commonly used in conjunction with the Internet. Having some understanding and familiarity with the terms will help you navigate the Internet.