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Routing Fundamentals and Subnets. Objectives. Routed protocol IP routing protocols The mechanics of subnetting. Routed Protocol. Routable and Routed Protocols . A routed protocol allows the router to forward data between nodes on different networks.

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Routing Fundamentals and Subnets

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Routing fundamentals and subnets l.jpg

Routing Fundamentals and Subnets

Objectives l.jpg


  • Routed protocol

  • IP routing protocols

  • The mechanics of subnetting

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Routed Protocol

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Routable and Routed Protocols

  • A routed protocol allows the router to forward data between nodes on different networks.

  • In order for a protocol to be routable, it must provide the ability to assign a network number and a host number to each individual device.

  • These protocols also require a network mask in order to differentiate the two numbers.

  • The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit.

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IP as a Routed Protocol

  • IP is a connectionless, unreliable, best-effort delivery protocol.

  • As information flows down the layers of the OSI model; the data is processed at each layer.

  • IP accepts whatever data is passed down to it from the upper layers.

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Packet Propagation and Switching Within a Router

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Packet Propagation and Switching Within a Router

  • As a frame is received at a router interface.

  • The MAC address is checked to see if the frame is directly addressed to the router interface, or a broadcast.

  • The frame header and trailer are removed and the packet is passed up to Layer 3.

  • The destination IP address is compared to the routing table to find a match.

  • The packet is switched to the outgoing interface and given the proper frame header.

  • The frame is then transmitted.

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Internet Protocol (IP): Connectionless

  • The Internet is a gigantic, connectionless network in which all packet deliveries are handled by IP.

  • TCP adds Layer 4, connection-oriented reliability services to IP.

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Telephone Calls: Connection-oriented

A connection is established between the sender and the recipient before any data is transferred.

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Anatomy of an IP Packet

  • While the IP source and destination addresses are important, the other header fields have made IP very flexible.

  • The header fields are the information that is provided to the upper layer protocols defining the data in the packet.

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IP Routing Protocols

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Routing Overview

  • A router is a network layer device that uses one or more routing metrics to determine the optimal path.

  • Routing metrics are values used in determining the advantage of one route over another.

  • Routing protocols use various combinations of metrics for determining the best path for data.

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Routing Versus Switching

  • This distinction is routing and switching use different information in the process of moving data from source to destination.

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Routing Versus Switching

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Routed Versus Routing

  • A routed protocol:

    • Includes any network protocol suite that provides enough information in its network layer address to allow a router to forward it to the next device and ultimately to its destination.

    • Defines the format and use of the fields within a packet.

  • A routing protocol:

    • Provides processes for sharing route information.

    • Allows routers to communicate with other routers to update and maintain the routing tables.

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Path Determination

  • Path determination enables a router to compare the destination address to the available routes in its routing table, and to select the best path.

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Routing Tables

  • Routers keep track of the following:

    • Protocol type

    • Destination/next-hop associations

    • Routing metric

    • Outbound interfaces

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Routing Algorithms and Metrics

  • Routing protocols have one or more of the following design goals:

    • Optimization

    • Simplicity and low overhead

    • Robustness and stability

    • Flexibility

    • Rapid convergence

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  • IGPs route data within an autonomous system.


  • EGPs route data between autonomous systems

    • Border Gateway Protocol (BGP)

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LinkState and Distance Vector

  • Examples of distance-vector protocols:

    • Routing Information Protocol (RIP)

    • Interior Gateway Routing Protocol (IGRP)

    • Enhanced IGRP (EIGRP)

  • Examples of link-state protocols:

    • Open Shortest Path First (OSPF)

    • Intermediate System-to-Intermediate System (IS-IS)

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Routing Protocols

  • RIP

  • RIP v2

  • IGRP


  • OSPF

  • IS-IS

  • BGP

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Mechanics of Subnetting

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Classes of Network IP Addresses

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Introduction to Subnetting

  • Host bits must are reassigned (or “borrowed”) as network bits.

  • The starting point is always the leftmost host bit.

3 bits borrowed allows 23-2 or 6 subnets

5 bits borrowed allows 25-2 or 30 subnets

12 bits borrowed allows 212-2 or 4094 subnets

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Reasons for Subnetting

  • Provides addressing flexibility for the network administrator.

    • Each LAN must have its own network or subnetwork address.

  • Provides broadcast containment and low-level security on the LAN.

  • Provides some security since access to other subnets is only available through the services of a router.

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Establishing the Subnet Mask Address

  • Determines which part of an IP address is the network field and which part is the host field.

  • Follow these steps to determine the subnet mask:

    • 1. Express the subnetwork IP address in binary form.

    • 2. Replace the network and subnet portion of the address with all 1s.

    • 3. Replace the host portion of the address with all 0s.

    • 4. Convert the binary expression back to dotted-decimal notation.

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Establishing the Subnet Mask Address

  • To determine the number of bits to be used, the network designer needs to calculate how many hosts the largest subnetwork requires and the number of subnetworks needed.

  • The “slash format” is a shorter way of representing the subnet mask:

    • /25 represents the 25 one bits in the subnet mask

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Establishing the Subnet Mask Address

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Subnetting Class A and B Networks

  • The available bits for assignment to the subnet field in a Class A address is 22 bits while a Class B address has 14 bits.

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Calculating the Subnetwork With ANDing

  • ANDing is a binary process by which the router calculates the subnetwork ID for an incoming packet.

    • 1 AND 1 = 1; 1 AND 0 = 0; 0 AND 0 = 0

  • The router then uses that information to forward the packet across the correct interface.

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