Understanding Networks

Objectives Compare client and network operating systems Learn about local area network technologies, including Ethernet, Token Ring, FDDI, and wireless Use the OSI model to understand networking Learn how network computers and servers are addressed

TCP/IP Protocols at Each Layer TCP/IP covers the first five layers of the OSI model, and is included in an operating system as a group of utilities called the TCP/IP stack. Figure 5-41 shows the four major groupings of the OSI model as applied to TCP/IP networks.

Protocols at the Application, Presentation, and Session Layers The first three layers of the OSI model are handled by the protocol specific to the application using it and are best treated as a single group rather than unique layers. Web browsers, , chat rooms, and FTP software are examples of the applications that use the Internet. The language or protocol each of these applications uses is listed at the Application, Presentation, and Session layers.

Protocols at the Transport Layer A TCP/IP network has two protocols that work at the Transport layer; one protocol guarantees delivery and the other does not. With TCP/IP, the protocol that guarantees delivery is TCP and the protocol that does not is UDP (User Datagram Protocol). TCP is used for client and server requests and responses.

Protocols at the Transport Layer (Continued) Because TCP establishes a connection, it is called a connection-oriented protocol. UDP is a protocol that sends data without caring about whether the data is received. It does not establish a connection first; thus, it is called a connectionless protocol.

Protocols at the Network Layer TCP and UDP communicate with the Network layer, which is sometimes called the Internet layer. Some of the other supporting protocols include ARP (Address Resolution Protocol), responsible for locating a host on a LAN; RARP (Reverse Address Resolution Protocol), responsible for discovering the Internet address of a host on a LAN; and ICMP (Internet Control Message Protocol), responsible for communicating problems with transmission to devices that need to know about these problems.

Protocols at the Data Link and Physical Layers PPP (Point-to-Point Protocol) is used over telephone lines, and allows a computer to connect to a network using a modem. PPP is the most popular protocol for managing network transmission from one modem to another.

MAC Addresses MAC addresses function at the lowest (Data Link) networking level. If a host does not know the MAC address of another host on a local area network, it uses the operating system to discover the MAC address.

IP Addresses All the protocols of the TCP/IP suite identify a device on the Internet or an intranet by its IP address. An IP address is 32 bits long, made up of 4 bytes separated by periods. Within an IP address, each of the four numbers separated by periods is called an octet. The first part of an IP address identifies the network, and the last part identifies the host.

Classes of IP Addresses IP addresses that can be used by companies and individuals are divided into three classes: Class A, Class B, and Class C, based on the number of possible IP addresses in each network within each class. The group of IP addresses assigned to an organization are unique to all other IP addresses on the Internet and are available for use on the Internet. The IP addresses available to the Internet are called public IP addresses.

Private IP Addresses Private IP addresses are IP addresses that are assigned by a network administrator for use on private intranets that are isolated from the Internet. The RFC 1918 recommends that the following IP addresses be used for private networks: – through – through – through

Dynamically Assigned IP Addresses Instead of IP addresses permanently being assigned to computers (called static IP addresses), an IP address is assigned for the current session only (called a dynamic IP address). Internet service providers (ISPs) are organizations through which individuals and businesses connect to the Internet.

Network Address Translation If the hosts on a network using private IP addresses need to access the Internet, a problem arises because the private IP addresses are not allowed on the Internet. The solution is to use NAT (Network Address Translation), which uses a single public IP address to access the Internet on behalf of all hosts on the network using other IP addresses.

Plans for New IP Addresses Because of an impending shortage of IP addresses, as well as some limitations in the current standards for IP, a new scheme of IP addresses called the IPv6 (IP version 6) standard is currently being developed and implemented. Current IP addresses using the current IPv4 (IP version 4) have 32 bits with eight bits in each of four octets.

Plans for New IP Addresses (Continued) With the new system, each address segment can have 32 bits, for a total of 128 bits for the entire address. A disadvantage of IPv6 is the fact that so much software used on the Internet would become outdated because current software is designed to hold 32-bit IP addresses and, with the new system, this number would no longer be sufficient.

Ports A port is a number used to address software or services running on a computer. A host computer might have several services running on it. Each server running on the host is assigned a port. The port is written at the end of the IP address, separated from the IP address with a colon—like this:

Tying It All Together: How Data Travels on the Network The Web browser wants to make a request to a Web server, and processes the request using an API call. The API process packages the data using HTTP format, which includes an HTTP header, and addresses it to an IP address and port 80, which is the default port for a Web server. HTTP delivers the package to TCP, giving the destination IP address and port.

Tying It All Together: How Data Travels on the Network (Continued) TCP guarantees delivery, and attempts to make a connection to the destination IP address and port using the three-way handshake. With the first attempt that TCP makes to communicate with the remote host, the first packet reaches the Network layer and IP uses ARP to resolve the IP address to a MAC address. When a packet reaches a hub on the Ethernet LAN, the hub replicates the packet and sends it to every device to which it is connected.

Tying It All Together: How Data Travels on the Network (Continued) When the IP layer receives a broadcast packet, it decides if it is appropriate to respond. After it knows the MAC address, it can send the first packet to make the connection. It prepares the data by attaching a TCP header in front of the data.

Summary The two approaches to managing network resources are peer-to-peer and client/server. Windows 2000 Server and Windows Server 2003 support multiple domain controllers. Windows XP has three operating systems: Windows XP Professional, Windows XP Home Edition, and Windows XP 64-Bit Edition. The most popular network technology used today is Ethernet; it can be configured either as a star or bus.

Summary (Continued) An IP address can be permanently assigned to a device (static IP addressing) or a different IP address can be assigned each time the device logs on to the network (dynamic IP addressing). When a host is configured to used TCP/IP, and IP address is bound to the MAC address. A proxy server uses NAT to substitute its own public IP address for the private IP address of a computer on the network that is requesting Internet access.