THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical OSI Model Chapter 1 Review By: Allan Johnson
Table of Contents Go There! Go There! Go There! Go There! Go There! Go There! Review the OSI Model LAN Devices & Technologies IP Addressing CIDR Notation Routing Transport Layer
THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical Review The Model Open Systems Interconnected Reference Model Table of Contents
Why A Layered Model? Application Presentation Session Transport Network Data-Link Physical Reduces complexity Standardizes interfaces Facilitates modular engineering Ensures interoperable technology Accelerates evolution Simplifies teaching & learning
What is the OSI Model? Each of these different layers has its own set of functions and only communicates with the layers directly above and below and with its opposite layer on other computers.
Application Layer Application Presentation Session Transport Network Data-Link Physical Provides network services (processes) to applications. For example, a computer on a LAN can save files to a server using a network redirector supplied by NOSs like Novell. Network redirectors allow applications like Word and Excel to see the network.
Presentation Layer Application Presentation Session Transport Network Data-Link Physical Provides data representation and code formatting. Code formatting includes compression and encryption Basically, the presentation layer is responsible for representing data so that the source and destination can communicate at the application layer.
Session Layer Application Presentation Session Transport Network Data-Link Physical Provides inter-host communication by establishing, maintaining, and terminating sessions. Session uses dialog control and dialog separation to manage the session Some Session protocols: NFS (Network File System) SQL (Structured Query Language) RCP (Remote Call Procedure) ASP (AppleTalk Session Protocol) SCP (Session Control Protocol) X-window
Transport Layer Application Presentation Session Transport Network Data-Link Physical Provides reliability, flow control, and error correction through the use of TCP. TCP segments the data, adding a header with control information for sequencing and acknowledging packets received. The segment header also includes source and destination ports for upper-layer applications TCP is connection-oriented and uses windowing. UDP is connectionless. UDP does not acknowledge the receipt of packets.
Network Layer Application Presentation Session Transport Network Data-Link Physical Responsible for logically addressing the packet and path determination. Addressing is done through routed protocols such as IP, IPX, AppleTalk, and DECnet. Path Selection is done by using routing protocols such as RIP, IGRP, EIGRP, OSPF, and BGP. Routers operate at the Network Layer
Data-Link Layer Application Presentation Session Transport Network Data-Link Physical Provides access to the media Handles error notification, network topology issues, and physically addressing the frame. Media Access Control through either... Deterministic token passing Non-deterministic broadcast topology (collision domains) Important concept: CSMA/CD
Physical Layer Application Presentation Session Transport Network Data-Link Physical Provides electrical, mechanical, procedural and functional means for activating and maintaining links between systems. Includes the medium through which bits flow. Media can be... CAT 5 cable Coaxial cable Fiber Optics cable The atmosphere
Peer-to-Peer Communications Peers communicate using the PDU of their layer. For example, the network layers of the source and destination are peers and use packets to communicate with each other. Application Presentation Session Transport Network Data-Link Physical Data Data Data Segments Packets Frames Bits Application Presentation Session Transport Network Data-Link Physical
Encapsulation Example Application Presentation Session Transport Network Data-Link Physical You type an email message. SMTP takes the data and passes it to the Presentation Layer. Presentation codes the data as ASCII. Session establishes a connection with the destination for the purpose of transporting the data.
Encapsulation Example Application Presentation Session Transport Network Data-Link Physical Transport segments the data using TCP and hands it to the Network Layer for addressing Network addresses the packet using IP. Data-Link then encaps. the packet in a frame and addresses it for local delivery (MACs) The Physical layer sends the bits down the wire.
THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical LAN Devices & Technologies The Data-Link & Physical Layers Table of Contents
Devices What layer device? What does it do? Connects LAN segments; Filters traffic based on MAC addresses; and Separates collision domains based upon MAC addresses.
Devices What layer device? What does it do? Since it is a multiport bridge, it can also Connect LAN segments; Filter traffic based on MAC addresses; and Separate collision domains However, switches also offer full-duplex, dedicated bandwidth to segments or desktops.
Devices What layer device? What does it do? Concentrates LAN connections from multiple devices into one location Repeats the signal (a hub is a multi-port repeater)
Devices What layer device? What does it do? Interconnects networks and provides broadcast control Determines the path using a routing protocol or static route Re-encapsulates the packet in the appropriate frame format and switches it out the interface Uses logical addressing (i.e. IP addresses) to determine the path
Media Types
LAN Technologies Three Most Common Used Today in Networking
Ethernet/802.3 Cable Specifications: 10Base2 Called Thinnet; uses coax Max. distance = 185 meters (almost 200) 10Base5 Called Thicknet; uses coax Max. distance = 500 meters 10BaseT Uses Twisted-pair Max. distance = 100 meters 10 means 10 Mbps
Ethernet/802.3 Ethernet is broadcast topology. What does that mean? Every devices on the Ethernet segment sees every frame. Frames are addressed with source and destination addresses. When a source does not know the destination or wants to communicate with every device, it encapsulates the frame with a broadcast MAC address: FFFF.FFFF.FFFF What is the main network traffic problem caused by Ethernet broadcast topologies?
Ethernet/802.3 Ethernet topologies are also shared media. That means media access is controlled on a first come, first serve basis. This results in collisions between the data of two simultaneously transmitting devices. Collisions are resolved using what method?
Ethernet/802.3 CSMA/CD (Carrier Sense Multiple Access with Collision Detection) Describe how CSMA/CD works: A node needing to transmit listens for activity on the media. If there is none, it transmits. The node continues to listen. A collision is detected by a spike in voltage (a bit can only be a 0 or a 1--it cannot be a 2) The node generates a jam signal to tell all devices to stop transmitting for a random amount of time (back-off algorithm). When media is clear of any transmissions, the node can attempt to retransmit.
Address Resolution Protocol In broadcast topologies, we need a way to resolve unknown destination MAC addresses. ARP is protocol where the sending device sends out a broadcast ARP request which says, What s you MAC address? If the destination exists on the same LAN segment as the source, then the destination replies with its MAC address. However, if the destination and source are separated by a router, the router will not forward the broadcast (an important function of routers). Instead the router replies with its own MAC address.
THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical IP Addressing Subnetting Review Table of Contents
Logical Addressing At the network layer, we use logical, hierarchical addressing. With Internet Protocol (IP), this address is a 32-bit addressing scheme divided into four octets. Do you remember the classes 1st octet s value? Class A: 1-126 Class B: 128-191 Class C: 192-223 Class D: 224-239 (multicasting) Class E: 240-255 (experimental)
Network vs. Host Class A: 2 7 = 126 networks; 2 24 > 16 million hosts N H H H Class B : 2 14 = 16,384 networks; 2 16 > 65,534 hosts N N H H Class C : 2 21 > 2 million networks; 2 8 = 254 hosts N N N H
Why Subnet? Remember: we are usually dealing with a broadcast topology. Can you imagine what the network traffic overhead would be like on a network with 254 hosts trying to discover each others MAC addresses? Subnetting allows us to segment LANs into logical broadcast domains called subnets, thereby improving network performance.
Four Subnetting Steps To correctly subnet a given network address into subnet addresses, ask yourself the following questions: 1. How many bits do I need to borrow? 2. What s the subnet mask? 3. What s the magic number or multiplier? 4. What are the first three subnetwork addresses? Let s look at each of these questions in detail
1. How many bits to borrow? First, you need to know how many bits you have to work with. Second, you must know either how many subnets you need or how many hosts per subnet you need. Finally, you need to figure out the number of bits to borrow.
1. How many bits to borrow? How many bits do I have to work with? Depends on the class of your network address. Class C: 8 host bits Class B: 16 host bits Class A: 24 host bits Remember: you must borrow at least 2 bits for subnets and leave at least 2 bits for host addresses. 2 bits borrowed allows 2 2-2 = 2 subnets
1. How many bits to borrow? How many subnets or hosts do I need? A simple formula: Total Bits = Bits Borrowed + Bits Left TB = BB + BL I need x subnets: 2 BB 2 x I need x hosts: 2 BL 2 x Remember: we need to subtract two to provide for the subnetwork and broadcast addresses.
1. How many bits to borrow? Class C Example: 210.93.45.0 Design goals specify at least 5 subnets so how many bits do we borrow? How many bits in the host portion do we have to work with (TB)? What s the BB in our TB = BB + BL formula? (8 = BB + BL) 2 to the what power will give us at least 5 subnets? 2 3-2 = 6 subnets
1. How many bits to borrow? How many bits are left for hosts? TB = BB + BL 8 = 3 + BL BL = 5 So how many hosts can we assign to each subnet? 2 5-2 = 30 hosts
1. How many bits to borrow? Class B Example: 185.75.0.0 Design goals specify no more than 126 hosts per subnet, so how many bits do we need to leave (BL)? How many bits in the host portion do we have to work with (TB)? What s the BL in our TB = BB + BL formula? (16 = BB + BL) 2 to the what power will insure no more than 126 hosts per subnet and give us the most subnets? 2 7-2 = 126 hosts
1. How many bits to borrow? How many bits are left for subnets? TB = BB + BL 16 = BB + 7 BL = 9 So how many subnets can we have? 2 9-2 = 510 subnets
2. What s the subnet mask? We determine the subnet mask by adding up the decimal value of the bits we borrowed. In the previous Class C example, we borrowed 3 bits. Below is the host octet showing the bits we borrowed and their decimal values. 1 1 1 128 64 32 16 8 4 2 1 We add up the decimal value of these bits and get 224. That s the last non-zero octet of our subnet mask. So our subnet mask is 255.255.255.224
3. What s the magic number? To find the magic number or the multiplier we will use to determine the subnetwork addresses, we subtract the last non-zero octet from 256. In our Class C example, our subnet mask was 255.255.255.224. 224 is our last non-zero octet. Our magic number is 256-224 = 32
Last Non-Zero Octet Memorize this table. You should be able to: Quickly calculate the last non-zero octet when given the number of bits borrowed. Determine the number of bits borrowed given the last non-zero octet. Determine the amount of bits left over for hosts and the number of host addresses available. Bits Borrowed Non-Zero Octet Hosts 2 192 62 3 224 30 4 240 14 5 248 6 6 252 2
4. What are the subnets? We now take our magic number and use it as a multiplier. Our Class C address was 210.93.45.0. We borrowed bits in the fourth octet, so that s where our multiplier occurs 1st subnet: 210.93.45.32 2nd subnet: 210.93.45.64 3rd subnet: 210.93.45.96 We keep adding 32 in the fourth octet to get all six available subnet addresses.
Host & Broadcast Addresses Now you can see why we subtract 2 when determining the number of host address. Let s look at our 1st subnet: 210.93.45.32 What is the total range of addresses up to our next subnet, 210.93.45.64? 210.93.45.32 to 210.93.45.63 or 32 addresses.32 cannot be assigned to a host. Why?.63 cannot be assigned to a host. Why? So our host addresses are.33 -.62 or 30 host addresses--just like we figured out earlier.
THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical CIDR Notation A Different Way to Represent a Subnet Mask Table of Contents
CIDR Notation Classless Interdomain Routing is a method of representing an IP address and its subnet mask with a prefix. For example: 192.168.50.0/27 What do you think the 27 tells you? 27 is the number of 1 bits in the subnet mask. Therefore, 255.255.255.224 Also, you know 192 is a Class C, so we borrowed 3 bits!! Finally, you know the magic number is 256-224 = 32, so the first useable subnet address is 197.168.50.32!! Let s see the power of CIDR notation.
202.151.37.0/26 Subnet mask? 255.255.255.192 Bits borrowed? Class C so 2 bits borrowed Magic Number? 256-192 = 64 First useable subnet address? 202.151.37.64 Third useable subnet address? 64 + 64 + 64 = 192, so 202.151.37.192
198.53.67.0/30 Subnet mask? 255.255.255.252 Bits borrowed? Class C so 6 bits borrowed Magic Number? 256-252 = 4 Third useable subnet address? 4 + 4 + 4 = 12, so 198.53.67.12 Second subnet s broadcast address? 4 + 4 + 4-1 = 11, so 198.53.67.11
200.39.89.0/28 What kind of address is 200.39.89.0? Class C, so 4 bits borrowed Last non-zero octet is 240 Magic number is 256-240 = 16 32 is a multiple of 16 so 200.39.89.32 is a subnet address--the second subnet address!! What s the broadcast address of 200.39.89.32? 32 + 16-1 = 47, so 200.39.89.47
194.53.45.0/29 What kind of address is 194.53.45.26? Class C, so 5 bits borrowed Last non-zero octet is 248 Magic number is 256-248 = 8 Subnets are.8,.16,.24,.32, ect. So 194.53.45.26 belongs to the third subnet address (194.53.45.24) and is a host address. What broadcast address would this host use to communicate with other devices on the same subnet? It belongs to.24 and the next is.32, so 1 less is.31 (194.53.45.31)
No Worksheet Needed! After some practice, you should never need a subnetting worksheet again. The only information you need is the IP address and the CIDR notation. For example, the address 221.39.50/26 You can quickly determine that the first subnet address is 221.39.50.64. How? Class C, 2 bits borrowed 256-192 = 64, so 221.39.50.64 For the rest of the addresses, just do multiples of 64 (.64,.128,.192).
The Key!! MEMORIZE THIS TABLE!!! Bits Borrowed Non-Zero Octet Hosts 2 192 62 3 224 30 4 240 14 5 248 6 6 252 2
Practice On Your Own Below are some practice problems. Take out a sheet of paper and calculate... Bits borrowed Last non-zero octet Second subnet address and broadcast address 1. 192.168.15.0/26 2. 220.75.32.0/30 3. 200.39.79.0/29 4. 195.50.120.0/27 5. 202.139.67.0/28 6. Challenge: 132.59.0.0/19 Answers 7. Challenge: 64.0.0.0/16
THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical Routing Basics Path Determination & Packet Switching Table of Contents
A Router s Functions A router is responsible for determining the packet s path and switching the packet out the correct port. A router does this in five steps: 1. De-encapsulates the packet 2. Performs the ANDing operation 3. Looks for entry in routing table 4. Re-encapsulates packet into a frame 5. Switches the packet out the correct interface
Routed v. Routing Protocols What is a routed protocol? Routed protocols are protocols that enable data to be transmitted across a collection of networks or internetworks using a hierarchical addressing scheme. Examples include IP, IPX and AppleTalk. A routable protocol provides both a network and node number to each device on the network. Routers AND the address to discover the network portion of the address. An example of a protocol that is not routable is NetBEUI because it does not have a network/node structure.
Routed v. Routing Protocols What is a routing protocol? A routing protocol is a protocol that determines the path a routed protocol will follow to its destination. Routers use routing protocols to create a map of the network. These maps allow path determination and packet switching. Maps become part of the router s routing table. Examples of routing protocols include: RIP, IGRP, EIGRP, & OSPF
Multi-protocol Routing Routers are capable of running multiple routing protocols (RIP, IGRP, OSPF, etc.) as well as running multiple routed protocols (IP, IPX, AppleTalk). For a router to be able use different routing and routing protocols, you must enable the protocols using the appropriate commands.
Dynamic v. Static Routing Dynamic routing refers to the process of allowing the router to determine the path to the destination. Routing protocols enable dynamic routing where multiple paths to the same destination exist.
Dynamic v. Static Routing Static routing means that the network administrator directly assigns the path router are to take to the destination. Static routing is most often used with stub networks where only one path exists to the destination.
Default Routes A default route is usually to a border or gateway router that all routers on a network can send packets to if they do not know the route for a particular network.
Routing Protocol Classes Routing protocols can be divided into three classes: Distance vector: determines the route based on the direction (vector) and distance to the destination Link-state: opens the shortest path first to the destination by recreating an exact topology of the network in its routing table Hybrid: combines aspects of both
Convergence Convergence means that all routers share the same information about the network. In other words, each router knows its neighbor routers routing table Every time there is a topology change, routing protocols update the routers until the network is said to have converged again. The time of convergence varies depending upon the routing protocol being used.
Distance-vector Routing Each router receives a routing table periodically from its directly connected neighboring routers. For example, in the graphic, Router B receives information from Router A. Router B adds a distance-vector number (such as a number of hops), and then passes this new routing table to its other neighbor, Router C.
Link-state Routing Link-state protocols maintain complex databases that summarize routes to the entire network. Each time a new route is added or a route goes down, each router receives a message and then recalculates a spanning tree algorithm and updates its topology database.
Comparing the Two DISTANCE-VECTOR LINK-STATE Views network topology from neighbor s perspective Adds distance vectors from router to router Frequent, periodic updates: slow convergence Passes copies of routing tables to neighbors Gets common view of entire network topology Calculates the shortest path to other routers Event triggered updates: fast convergence Passes link-state routing updates to all routers in the system.
Hybrid Routing Cisco s proprietary routing protocol, EIGRP, is considered a hybrid. EIGRP uses distance-vector metrics. However, it uses event-triggered topology changes instead of periodic passing of routing tables.
THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical Transport Layer A Quick Review Table of Contents
Transport Layer Functions Synchronization of the connection Three-way handshake Flow Control Slow down, you re overloading my memory buffer!! Reliability & Error Recovery Windowing: How much data can I send before getting an acknowledgement? Retransmission of lost or unacknowledged segments
Transport s Two Protocols TCP Transmission Control Protocol Connection-oriented Acknowledgment & Retransmission of segments Windowing Applications: Email File Transfer E-Commerce UDP User Datagram Protocol Connectionless No Acknowledgements Applications: Routing Protocols Streaming Audio Gaming Video Conferencing