2.0—Protocols and Standards

A MAC address is a 6-byte address that allows a NIC to be uniquely identified on the network. The first three bytes (00:D0:59) identify the manufacturer of the card; The last three bytes (09:07:51) are the Universal LAN MAC address, which makes the interface unique.

OSI Model

As data is passed up or down through the OSI model structure, headers are added (going down) or removed (going up) at each layer—a process called encapsulation (added) or decapsulation (removed).

Table 12 provides a summary of the OSI model layers, and Table 13 shows how each device maps to the OSI model.

Table 12. Summary of the OSI Model

OSI Layer	Major Functions
Application	Provides access to the network for applications and certain end-user functions. Displays incoming information and prepares outgoing information for network access.
Presentation	Converts data from the application layer into a format that can be sent over the network. Converts data from the session layer into a format that can be understood by the application layer. Handles encryption and decryption of data. Provides compression and decompression functionality.
Session	Synchronizes the data exchange between applications on separate devices. Handles error detection and notification to the peer layer on the other device.
Transport	Establishes, maintains, and breaks connections between two devices. Determines the ordering and priorities of data. Performs error checking and verification and handles retransmissions, if necessary.
Network	Provides mechanisms for the routing of data between devices across single or multiple network segments. Handles the discovery of destination systems and addressing.
Data-link	Has two distinct sublayers: LLC and MAC. Performs error detection and handling for the transmitted signals. Defines the method by which the medium is accessed. Defines hardware addressing through the MAC sublayer.
Physical	Defines the physical structure of the network. Defines voltage/signal rates and the physical connection methods. Defines the physical topology.

Table 13. Mapping Device Drivers to the OSI Model

Device	OSI Layer at Which the Device Operates
Hub	Physical (Layer 1)
Switch	Data-link (Layer 2)
Bridge	Data-link (Layer 2)
Router	Network (Layer 3)
NIC	Data-link (Layer 2)

Application protocols map to the application, presentation, and session layers of the OSI model. Application protocols include AFT, FTP, TFTP, NCP, and SNMP.

Transport protocols map to the transport layer of the OSI model and are responsible for the transporting of data across the network. Transport protocols include ATP, NetBEUI, SPX, TCP, and UDP.

The NetBEUI protocol uses names as addresses.

Network protocols are responsible for providing the addressing and routing information. Network protocols include IP, IPX, and DDP.

RIP is responsible for the routing of packets on an IPX/SPX network.

Table 14 provides information on each protocol. Table 15 summarizes TCP/IP, including each protocol in the TCP/IP suite, Table 16 shows you the TCP/IP port assignments, and Table 17 describes the TCP/IP services.

Table 14. Protocol Summary

Protocol	Network Operating System	Routable?	Configuration	Primary Use
TCP/IP	Used by default with Unix, Linux, NetWare, and Windows systems; supported by Macintosh and just about every other computing platform available	Yes	Comparatively difficult to configure; has a number of different configuration requirements	Used on many networks of all shapes and sizes; is the protocol of the Internet
IPX/SPX	Used to be the default protocol for NetWare, but now TCP/IP is preferred; can also be used with Linux; Windows supports NWLink, a version of the IPX/SPX protocol suite that was created by Microsoft for cross-platform compatibility	Yes	Very easy to configure because most information is autoconfigured	Primarily used on legacy NetWare networks
AppleTalk	Used by Macintosh, with some support on other platforms	Yes	Minimal configuration difficulty; requires a node address (automatically assigned when systems boot) and a network address	Used on legacy Macintosh networks
NetBEUI	Used by Windows	No	Easy network configuration, requiring only the computer's NetBIOS name	Primarily used on small networks where routing is not required

Table 15. Summary of the TCP/IP Protocol Suite

Protocol	Full Name	Description
IP	Internet Protocol	Connectionless protocol used for moving data around a network
TCP	Transmission Control Protocol	Connection-oriented protocol that offers flow control, sequencing, and retransmission of dropped packets
UDP	User Datagram Protocol	Connectionless alternative to TCP that is used for applications that do not require the functions offered by TCP
FTP	File Transfer Protocol	Protocol for uploading and downloading files to and from a remote host; also accommodates basic file management tasks
TFTP	Trivial File Transfer Protocol	File transfer protocol that does not have the security or error-checking capabilities of FTP; uses UDP as a transport protocol and is therefore connectionless
SMTP	Simple Mail Transfer Protocol	Mechanism for transporting email across networks
HTTP	Hypertext Transfer Protocol	Protocol for retrieving files from a Web server
HTTPS	Hypertext Transfer Protocol Secure	Secure protocol for retrieving files from a Web server
POP/IMAP	Post Office Protocol/Internet Message Access Protocol	Used for retrieving email from a server on which the mail is stored
Telnet	Telnet	Allows sessions to be opened on a remote host
ICMP	Internet Control Message Protocol	Used for error reporting, flow control, and route testing
ARP	Address Resolution Protocol	Resolves IP addresses to MAC addresses, to enable communication between devices
NTP	Network Time Protocol	Used to communicate time synchronization information between devices

Table 16. Summary of TCP/IP Port Assignments

Protocol	Port Assignment
FTP	21
SSH	22
Telnet	23
SMTP	25
DNS	53
TFTP	69
HTTP	80
POP3	110
NNTP	119
NTP	123
IMAP4	143
SNMP	161
HTTPS	443

Table 17. Summary of TCP/IP Services

Service	Purpose/Function
DHCP/BOOTP	Automatically assigns IP addressing information
DNS	Resolves hostnames to IP addresses
NAT/ICS	Translates private network addresses into public network addresses
WINS	Resolves NetBIOS names to IP addresses
SNMP	Provides network management facilities on TCP/IP-based networks

TCP/IP

In a network that does not use DHCP, you need to watch for duplicate IP addresses that prevent a user from logging onto the network.

Following is a description of the classes of IP addresses:

• A Class A address uses only the first octet to represent the network portion, a Class B address uses two octets, and a Class C address uses three octets. Class A addresses span from 1 to 126, with a default subnet mask of 255.0.0.0.

• Class B addresses span from 128 to 191, with a default subnet mask of 255.255.0.0.

• Class C addresses span from 192 to 223, with a default subnet mask of 255.255.255.0.

The 127 network ID is reserved for the local loopback.

An example of a valid IPv6 address is

42DE:7E55:63F2:21AA:CBD4:D773:CC21:554F

Public Versus Private Networks

A public network is a network to which anyone can connect, such as the Internet. Internet Assigned Numbers Authority (IANA) is responsible for assigning IP addresses to public networks.

A private network is any network to which access is restricted. Reserved IP addresses are 10.0.0.0, 172.16.0.0, and 192.168.0.0.

WAN Technologies

Table 18 summarizes the WAN technologies.

Table 18. WAN Technologies

WAN Technology	Speed	Supported Media	Switching Method Used	Key Characteristics
ISDN	BRI: 64Kbps to 128Kbps PRI: 64Kbps to 1.5Mbps	Copper/fiber-optic	Can be used for circuit-switching or packet-switching connections	ISDN can be used to transmit all types of traffic, including voice, video, and data. BRI uses 2B+D channels, PRI uses 23B+D channels. B channels are 64Kbps. ISDN uses the public network and requires dial-in access.
T-carrier (T1, T3)	T1: 1.544Mbps T3: 44.736Mbps	Copper/fiber-optic	Circuit switching	T-carrier is used to create point-to-point network connections for private networks.
FDDI	100Mbps	Fiber-optic	N/A	FDDI uses a dual-ring configuration for fault tolerance. FDDI uses a token-passing media-access method. FDDI uses beaconing for error detection.
ATM	1.544Mbps to 622Mbps	Copper/fiber-optic	Cell switching	ATM uses fixed cells that are 53 bytes long.
X.25	56Kbps	Copper/fiber-optic	Packet switching	X.25 is limited to 56Kbps. X.25 provides a packet-switching network over standard phone lines.
Frame Relay	56Kbps to 1.544Mbps	Copper/fiber-optic	PVCs and SVCs	Frame Relay is a packet-oriented protocol, and it uses variable-length packets.
SONET	51.8Mbps to 2.4Gbps	Fiber-optic	N/A	SONET defines synchronous data transfer over optical cable. The European equivalent of SONET is SDH.

Remote Access and Security Protocols

When a connection is made to the RAS server, the client is authenticated and the system that is dialing in becomes a part of the network.

RAS supports remote connectivity from all the major client operating systems.

Although the system is called RAS, the underlying technologies that enable the RAS process are dial-up protocols such as PPP and SLIP:

• SLIP also does not provide error checking or packet addressing, so it can be used only in serial communications.

• PPP provides a number of security enhancements compared to SLIP. The most important of these is the encryption of usernames and passwords during the authentication process.

Windows 2000 natively supports SLIP and PPP.

ICA protocol allows client systems to access and run applications on a server, using the resources of the server, with only the user interface, keystrokes, and mouse movement being transferred between the client and server computers.

IPSec is designed to encrypt data during communication between two computers. IPSec operates at the network layer of the OSI model and provides security for protocols that operate at higher layers of the OSI model.

L2F allows tunneling to be utilized as a connection method over insecure networks.

L2TP is a combination of PPTP and Cisco's L2F technology and uses tunneling to deliver data. L2TP operates at the data-link layer, making it protocol independent.

SSL is a security protocol that is used on the Internet. Secure Web site URLs begin with https:// instead of http://. HTTPS connections require a browser to establish a secure connection. Secure SSL connections for Web pages are made through port 443 by default.

Kerberos provides a method to verify the identity of a computer system over an insecure network connection.

The security tokens used in Kerberos are known as tickets.