Routing Fundamentals and Subnets

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Routing Fundamentals and Subnets

• Objectives
• Routed Protocols
• IP Routing Protocols
• The Mechanism of Subnetting
• By
• Adwoa Afful (Mrs)

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IP Address
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IP Address Grouping
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Routed and Routing Protocols

• Consider that a packet needs to be sent from node
  A to node F. How would it decide which path to
  take?

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Routing Protocol vs Routed Protocol

• A routed protocol
• defines the end to end addressing and the packet
  format of a packet that is forwarded between
  nodes on different networks.
• Internet Protocol (IP) is a routed protocol
• A routing protocol
• exchanges topology information with adjacent
  routers to update and maintain their routing
  tables.
• selects the best path through a network
• RIP is a routing protocol

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Routed Protocol

• A protocol is a set of rules
• A routed protocol is a set of rules that
  determines how computers at the source and
  destination communicate with each other across
  networks
• packet format
• end to end addressing
• In order for a protocol to be routable, it must
  provide the ability to assign both a network
  number and a host number for each individual
  device.

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Internet Protocol IP

• 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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IP Packet Header
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Network Layer Devices in Data Flow
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Network Layer Devices in Data Flow

• 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 (datagram) is placed in a new frame
  with the MAC address of the next hop interface.
• The frame is then transmitted.

If a match is found or there is a default route,
the packet will be sent to the interface
specified in the matched routing table statement
otherwise packet is discarded
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Packets Travel Across Links in a Frame

• Packets NEVER travel through the network they
  are carried within frames
• A new frame MUST be created to carry the packet
  over each individual link
• Routers provide the IP address of the next hop
  interface (router or host)
• The ARP table provides the MAC address of this IP
  address for the frame destination

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Router Protocol Stripping
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Connectionless Network Services
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Telephone Calls Connection-oriented
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Connectionless vs. Connection-Oriented

• In a connection oriented system is established
  between the sender and the recipient before any
  data is transferred.
• example Telephone
• In a connectionless system, the destination is
  not contacted before a packet is sent.
• example Postal system
• TCP is connection oriented
• IP is connectionless

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Connectionless Network Services

• The Internet is a huge network where packets are
  routed according to their IP addresses.
• IP is unreliable and best-effort as IP does not
  verify that the data reached its destination and
  therefore does not resend missing packets.
• Reliability and resending of packets is handled
  by the upper layer protocols.
• IP may be used in conjunction with TCP to add a
  Layer 4, connection-oriented service that checks
  for missing segments and resends them to provide
  reliability.

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The IPv4 Packet Header
Time-to-live (TTL) Count Decreases with every
hop This prevents packets from looping endlessly.
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• Routing

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The Network Layer
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Routing

• Routing is an OSI Layer 3 function.
• Routers connect networks (or subnetworks)
• Routing is the process of finding the most
  efficient path from one device to another
  (router)
• Routers must maintain routing tables and make
  sure other routers know of changes in the network
  topology. This function is performed using a
  routing protocol to communicate network
  information with other routers

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Routing Through a Network

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

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Routing Metrics
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Data Encapsulation
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Layer 3 Routing and Layer 2 Switching
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Routers Reduce the Size of Broadcast Domains

• Routers block LAN broadcasts, so a broadcast
  storm only affects the broadcast domain from
  which it originated
• Switched networks do not block broadcasts

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Routing and Switching in a Network
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Layer 2 Switching and Layer 3 Routing
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The Routing Process
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ARP Tables and Routing Tables
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Router and Switch Features Comparison
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• The difference between a routed and routing
  protocol revisited

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Routed Protocol
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Routing Protocol
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Routed Vs Routing protocols

• A Routed Protocol
• A 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.
• The Internet Protocol (IP) and Novell's
  Internetwork Packet Exchange (IPX), DECnet,
  AppleTalk, Banyan VINES, and Xerox Network
  Systems (XNS)
• A Routing Protocol
• Provides processes for sharing route
  information. Exchange topology info. To
  determining the best routing paths and
  transporting packets through an internetwork
• Also allows routers to communicate with other
  routers to update and maintain the routing
  tables.
• Routing Information Protocol (RIP), Interior
  Gateway Routing Protocol (IGRP), Open Shortest
  Path First (OSPF), Border Gateway Protocol (BGP),
  and Enhanced IGRP (EIGRP).

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• Back to Routing

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Path Determination
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Path Determination
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Routing Tables

• Routing tables contain the best routes to all
  known networks.
• These routes can be either
• Static routes, which are entered manually by the
  system administrator
• Or dynamic routes, which are constructed from
  information passed between adjacent routers.
• A routing table entry contains
• Each Destination
• The next hop IP address to reach that
  destination
• The metric for the route via that next hop
• Outbound router interface for the next hop

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Routing Tables
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Routing Algorithms and Metrics
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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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Routing Algorithms and Metrics
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Interior and Exterior Gateway Protocols
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Interior and Exterior Gateway Protocols

• IGPs route data within an autonomous system.
• RIP, RIPv2, IGRP, EIGRP, OSPF, IS-IS
• EGPs route data between autonomous systems
• Border Gateway Protocol (BGP)

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Interior Gateway Routing Protocols

• Link State and Distance Vector Routing Protocols
• 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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• Mechanics of Subnetting

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Subnetting

• 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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IP Address Bit Patterns
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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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Subnetting Chart (Bit Position and Value)
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Subnetting Chart (Subnet Mask Identifier)
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Subnetting
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Subnetting Chart
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Subnetting Example

• This is an example of subnetting the 192.168.10.0
  class C network into 8 subnets with 32 host
  addresses per subnet

• Note that the first and last subnets are not used
  (the first can be)
• Also the first and last host address in each
  subnet are not used

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Example Host IP Address from Subnet 2

• The subnet mask is ANDed with the packet address
  to determine the subnet address - as shown in the
  next slides

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The Logical ANDing Process
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The Logical ANDing Process
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Class A and B Hosts
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Calculating the Subnet ID
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Subnet Mask Defines the Number of Subnets
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Find the Subnet Mask
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Summary