Mod -1

INTRODUCTION TO

COMPUTER NETWORKS

OSI - TCP/IP

OSI _{TCP / IP}

Application (Layer7)

Application Presentation (Layer6)

Session (Layer 5)

Transport (Layer 4) Transport

Network (Layer 3) Internet

Data Link (Layer 2)

Application Layer Transport Layer Network Layer Data Link Layer Physical Layer

HTTP Request

HTTP TCP HTTP TCP IP HTTP TCP IP Ethernet

Sender _Receiver

Request Request Request Application Layer Transport Layer Network Layer Data Link Layer Physical Layer

HTTP Request

Network Devices

Devices and the layers at which

they operate

Layer Name of Layer Device

3 Network Routers, layer 3

switches

2 Data Link Switches,

bridges, NIC’s

NIC’s

(

Network Interface Cards

)

Repeaters

• Signal attenuation or signal loss – signal degrades over distance

• Repeaters clean, amplify, and resend signals that are weakened by long cable length.

Hubs

• OSI layer 1 hardware

• Hubs regenerate and retime network signals

• Hubs propagate signals through the network

• They cannot filter network traffic

• They cannot determine best path

• They are used as network concentration points

• They are really multi-port repeaters

Bridges

• A layer 2 device designed to create two or more LAN segments, each of which is a

separate collision domain.

• The purpose is to filter traffic on a LAN, to

keep local traffic local, yet allow connectivity to other segments of the network.

• Filter traffic by looking at the MAC address

Bridges

• If the frame is addressed to a MAC address on the local side of the bridge, it is not forwarded to the other segment

• MAC addresses on the other segment are forwarded

Switched networks

• Shared ethernet networks perform best when kept to 30-40 percent full capacity

• This is a result of CSMA/CD

• A LAN switch is a high-speed multiport bridge which segments each port into its own

Switches

Store and Forward Switches

• Do error checking on each frame after the entire frame has arrived into the switch

• If the error checking algorithm determines there is no error, the switch looks in its MAC address table for the port to which to forward the destination device

• Highly reliable because doesn’t forward bad frames

Cut Through Switch

• Faster than store and forward because doesn’t perform error checking on frames

• Reads address information for each frame as the frames enter the switch

• After looking up the port of the destination device, frame is forwarded

• Forwards bad frames

Fragment free cut through switch

• Combines speed of cut through switch with error checking functionality

• Forwards all frames initially, but determines that if a particular port is receiving too many bad frames, it reconfigures the port to store and forward mode

Unmanaged/Intelligent switches

• Unmanaged – provides LAN’s with all the benefits of switching

• Fine in small networks

• Intelligent switches tracks and reports LAN performance statistics

• Have a database ASIC (application specific integrated circuit) on board to collect and

Layer 3 switch

• By definition a switch filters or forwards frames based on MAC addresses. This makes a switch a layer 2 device.

• Now we have layer 3 switches which have routing

capability. If a data frame can’t be switched it is routed.

• Each port is a separate LAN port, but the forwarding

engine actually calculates and stores routes based on IP addresses, not MAC addresses

VLAN Switches

• Virtual local area network

• Each port on a switch defines a collision domain

• The entire switch forms a single broadcast domain

• VLANs can define multiple broadcast domains

• Network traffic that is directed to all computers on the network can be segmented to transmit only on a specific VLAN.

What is an IP Address?

• An IP address is a unique global address for a network interface

• An IP address:

- is a 32 bit long identifier

- encodes a network number (network prefix)

Dotted Decimal Notation

• IP addresses are written in a so-called dotted

decimal notation

• Each byte is identified by a decimal number in the range [0..255]:

• Example:

10001111

10000000 10001001 10010000

1st _Byte

= 128

2nd _Byte

= 143

3rd _Byte

= 137

4th _Byte

= 144

• The network prefix identifies a network and the host number identifies a specific host (actually, interface on the network).

• How do we know how long the network prefix is?

– The network prefix is implicitly defined (see class-based

addressing)

– The network prefix is indicated by a netmask.

Network prefix and Host number

CIDR - Classless Interdomain Routing

• Goals:

– Restructure IP address assignments to increase efficiency

– Hierarchical routing aggregation to minimize route table entries

Key Concept: The length of the network id (prefix) in the IP addresses is kept arbitrary

CIDR Example

• CIDR notation of a network address: 192.0.2.0/18

• "18" says that the first 18 bits are the network part of the address (and 14 bits are available for specific host addresses)

• The network part is called the prefix

• Assume that a site requires a network address with 1000 addresses

Variable Length Subnet Masking (VLSM)

VLSM, allows you to apply different subnet masks to the same class address space Classful protocols, such as RIPv1 and IGRP, do not

support VLSM.

What class is our IP address block?

How many physical subnets are on the network today? (A “physical subnet” generally refers to a broadcast domain on a LAN; a set of hosts on a physical network bounded by routers.)

Do we anticipate adding any more physical networks in the near future, and if so, how many?

How many hosts do we have in the largest of our subnets today?

How many hosts do we anticipate having in the largest subnet in the near future?