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.
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.
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. |
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.
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 |
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 |
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 |
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
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.
Table 18 summarizes the WAN technologies.
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.