Process Application Layer, CCNA 640-801 Exam Guide

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Cisco Hierarchical Model:
For more information about this, please read our separate tutorial titled "The Cisco Hierarchical Model".

OSI Model:
The OSI model is a layered model and a conceptual standard used for defining standards to promote multi-vendor integration as well as maintain constant interfaces and isolate changes of implementation to a single layer. It is NOT application or protocol specific. In order to pass any Cisco exam, you need to know the OSI model inside and out.

The OSI Model consists of 7 layers as follows:

Layer	Description	Device	Protocol
Application	Provides network access for applications, flow control and error recovery. Provides communications services to applications by identifying and establishing the availability of other computers as well as to determine if sufficient resources exist for communication purposes.	Gateway	NCP, SMB, SMTP, FTP, SNMP, Telnet, Appletalk
Presentation	Performs protocol conversion, encryption and data compression	Gateway and redirectors	NCP, AFP, TDI
Session	Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers. Handles connection establishment, data transfer and connection release	Gateway	NetBios
Transport	Repackages messages into smaller formats, provides error free delivery and error handling functions	Gateway	NetBEUI, TCP, SPX, and NWLink
Network	Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management.	Router and brouter	IP, IPX, NWLink, NetBEUI
**Data Link	Packages raw bits into frames making it transmitable across a network link and includes a cyclical redundancy check(CRC). It consists of the LLC sublayer and the MAC sublayer. The MAC sublayer is important to remember, as it is responsible for appending the MAC address of the next hop to the frame header. On the contrary, LLC sublayer uses Destination Service Access Points and Source Service Access Points to create links for the MAC sublayers.	Switch, bridge and brouter	None
Physical	Physical layer works with the physical media for transmitting and receiving data bits via certain encoding schemes. It also includes specifications for certain mechanical connection features, such as the adaptor connector.	Multiplexer and repeater	None

Here is an easy way to memorize the order of the layers:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers. It is a little corny, but it works.

Class	Range	Explanation
A	1-126	IP addresses can be class A, B or C. Class A addresses are for networks with a large number of hosts. The first octet is the netid and the 3 remaining octets are the hostid. Class B addresses are used in medium to large networks with the first 2 octets making up the netid and the remaining 2 are the hostid. A class C is for smaller networks with the first 3 octets making up the netid and the last octet comprising the hostid. The later two classes aren’t used for networks.
B	128-191
C	192-223
D	224-239 (Multicasting)
E	240-255 (Experimental)

A subnet mask blocks out a portion of an IP address and is used to differentiate between the hostid and netid. The default subnet masks are as follows:

Class	Default Subnet	# of Subnets	# of Hosts Per Subnet
Class A	255.0.0.0	126	16,777,214
Class B	255.255.0.0	16,384	65,534
Class C	255.255.255.0	2,097,152	254

In these cases, the part of the IP address blocked out by 255 is the Net ID.

3COM’s IP addressing tutorial is just superior. It covers basic IP addressing options as well as subnetting and VLSM/CIDR.

IPX/SPX:
IPX will also be an important issue to consider in network management given the fact there many companies still use Netware servers. There are two parts to every IPX Network address - the Network ID and the Host ID. The first 8 hex digits represent the network ID, while the remaining hex digits represent the host ID, which is most likely the same as the MAC address, meaning we do not need to manually assign node addresses. Note that valid hexadecimal digits range from 0 through 9, and hexadecimal letters range from A through F. FFFFFFFF in hexadecimal notation = 4292967295 in decimal.

Sequenced Packet Exchange(SPX) belongs to the Transport layer, and is connection-oriented. It creates virtual circuits between hosts, and that each host is given a connection ID in the SPX header for identifying the connection. Service Advertisement Protocol(SAP) is used by NetWare servers to advertise network services via broadcast at an interval of every 60 minutes by default.

Process Application Layer

Defines protocols for node-to-node application communication and also controls user interface specifications. Consists of a set of services that provide ubiquitous access to all types of networks. Applications utilize the services to communicate with other devices and remote applications

Protocols and Applications

Port	Protocol	Description
23	Telnet	Terminal Emulation (Telephone network)
21	FTP	Allows file transfers between computers (File Transfer Protocol)
69	TFTP	Have to know what you want and where it is on the server, no directory browsing, no user authentication (Trivial File Transfer Protocol)
2049	NFS	Allows remote file systems to be mounted as local (Network File System)
25	SMTP	Used to send mail between mail servers (Simple Mail Transfer Protocol)
515	LPD	Used for print sharing of network printers with TCP/IP (Line Printer Daemon)
161	SNMP	Collect and manipulates network information (Simple Network Management Protocol)
53	DNS	Resolves FQDN to IP addresses (Domain Name Service)
67	BootP	Used by diskless workstations to receive boot file and other information via TFTP
	DHCP	Assigns IP addresses to hosts from a pool. Can send IP address, Subnet mask, Domain Name, Default Gateway, DNS IP, WINS info. (Dynamic Host Configuration Protocol)

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