Network Design Basics

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Network Design Basics

• MM Clements

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Introduction

• Subnetting Review
• LAN review
• Applying network numbering
• Adding more LANs
• Adding another router
• Network numbering again
• Routing introduction

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Subnetting

• The IPv4 address is 32 bits long
• Broken into dotted decimal for us humans
• e.g. 135.35.235.14
• We need to use binary to work out subnetting
• Vital to understand decimal to binary conversion
  and vice versa

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Network and Host

• IP address has 2 parts network and host
• This cannot be seen from the IP address alone
• We need a way to show the boundary
• This is done with the subnet mask which is a 32
  bit number
• S/M uses 1s to show the network portion and 0s to
  show the host portion

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Subnet Mask

• Router performs logical AND with IP address and
  the subnet mask
• Reveals the network this is all that is
  required when routing across internetworks
• Host part only required at destination LAN
• Can be represented using slash notation
• Simply count the 1s in the subnet mask

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Use host 195.60.61.24 as an example
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LAN Review

• LAN is a group of hosts in a broadcast domain
• Broadcast crosses layer 2 devices
• Does not pass layer 3 e.g. routers
• Router interface defines extent of a LAN
• Need Ethernet interface to create an Ethernet LAN
• Ethernet interface is normally given the lowest
  usable IP address in the IP range

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Defining IP Range

• How many host addresses required in LAN?
• Consider future expansion too at 10 25
• Number of host IPs available 2n -2 where n
  number of host bits
• E.g. 200 hosts requires 8 bits
• 28 -2 254.. Plenty of addresses even
  including expansion

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Applying to router interface

• In Packet Tracer 5.x, choose a router and make
  sure it has sufficient Ethernet interfaces
• If not, add WIC or NMs that do have Ethernet
• Choose lowest available IP address from the /24
  range of addresses that you have chosen
• Allocate this to the Ethernet interface
• Make sure that the correct S/M is entered

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Adding more LANs

• More LANs can be added to the same router
• Add sufficient Ethernet interfaces
• Number the interfaces as previously
• Packet Tracer will give an error if addresses
  overlap
• PCs on LAN will need to be numbered
• Also need to know default gateway IP (router)
• This could be manual or via DHCP

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LANs on same router

• LANs on same router can communicate
• Router can see both interfaces
• No need for routing protocol here
• Use ping to check communications
• show ip route command displays routers routing
  table
• Any network that appears in the routing table can
  be communicated with by router

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Adding another router

• Make sure routers have at least one serial
  interface add these in Physical Device View
• Connect the serial interfaces between both
  routers and decide which interface will have the
  clock
• Usually interface closest to network core
• This is now the DCE interface
• Decide on a clock rate 64000 is common but slow
• Apply the clockrate to the DCE interface only
• Non-clocked interface is known as DTE

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Numbering serial interfaces

• Serial link will only ever have 2 hosts
• It would be a waste to add a /24 address range
• Solution is to use /30 address range
• /30 subnet has only 2 host addresses
• Apply these to the serial interfaces
• Make sure the interface is turned on
• show ip interface brief command will give a
  summary of the interface states

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Troubleshooting
Screenshot above shows that Serial 0/1 interface
is not functioning. It has IP address but its
Status is administratively down and Protocol
is down Check that the interface is actually
turned on CLI uses no shutdown command to
activate an interface
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Add LAN to second router

• Apply numbering to new Ethernet LAN interface on
  the 2nd router
• Add some host PCs they need IP addressing
• Manual configuration or DHCP is OK
• LAN on first router cannot connect to second
  routers LAN
• Examine routing table to discover why

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Routing table
Router can only see connected networks C
connected, look at Codes
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Router communications

• Router will need to know how to get to the other
  LANs
• This information is learned by a routing protocol
• Packet tracer implements RIP, OSPF and EIGRP
• Implement a routing protocol on all routers in
  the network
• Compare the routing table now
• How did it learn about the other LANs?
• Check the codes to see which routing protocol was
  used

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Routing Table
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Routing Protocols

• Routing can be either static or dynamic
• Routing protocols allow for dynamic routing
• Changes to network are automatically catered for
• 2 types of dynamic routing protocols exist
• Link State and Distance Vector
• More next week

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Summary

• Address Ethernet interfaces on router
• Lowest IP address on LAN for interface
• Add WICs or NMs as needed
• Use /30 between routers on serial interfaces
• Choose DCE and DTE on serial links
• A routing protocol is needed to send data between
  2 routers
• Practical work reinforces these concepts