Subnetting and Variable Length Subnet Masks

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• Subnetting and Variable Length Subnet Masks

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Topics

• Planning Designing
• Design a simple LAN
• Design an IP addressing scheme to meet design
  requirements
• Design a simple internetwork
• Technology
• Evaluate TCP/IP communication process and its
  associated protocols
• Troubleshooting
• Perform LAN and VLAN troubleshooting
• Troubleshoot routing protocols
• Troubleshoot IP addressing and host configuration
• Troubleshoot a device as part of a working network

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Benefits of Subnetting

• Reduced network traffic
• as a result of segmenting the network
• reducing broadcast traffic
• Optimized network performance
• Simplified management
• Facilitated spanning of large geographical
  distances
• by connecting smaller networks together

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Subnet Masks

• A 32-bit value that separates the network ID of
  the IP address from the host ID
• Identifies that part of the host address used for
  the subnet

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Subnetting Basics

• An IP address can be separated into network and
  host portions by its class or subnet mask
• Bits are borrowed from the host portion of the
  IP address to create the subnetworks

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Creating Subnets

• Determine the number of network IDs needed,
  typically
• 1 ID per subnet
• 1 ID per WAN connection
• Determine how many hosts IDs per subnet are
  needed, typically
• 1 ID per host
• 1 ID per router
• Create
• a subnet mask for the entire network
• a subnet ID for each physical subnet
• A range of host IDs for each subnet

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Class Subnet Masks
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Classless Inter-Domain Routing (CIDR)

• Does not use class boundaries for distinguishing
  network and hosts portions of IP address
• Allocates an amount of IP address space to a
  company, home, or customer, based upon customer
  requirements
• Uses the slash notation (/) to indicate how many
  bits are used for the network portion of the IP
  address also indicates subnet mask value

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CIDR Values
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Subnetting Class C Addresses

• Class C addresses use 24 bits for identifying
  network IDs
• Bits are borrowed from the host portion to
  subnet, yielding the following subnet masks
  Binary Decimal CIDR 10000000 128
  /25
• 11000000 192 /26
• 11100000 224 /27
• 11110000 240 /28
• 11111000 248 /29
• 11111100 252 /30
• /31 and /32 provide fewer than 2 host IDs

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Binary Subnetting

• Determine number of bits used for subnet
• Example 1000 0000 (1 bit used for subnet)
• Example 1100 0000 (2 bits for subnet)
• Determine all possible subnet values for that
  number of bits
• 00, 01, 10, and 11 are subnets for1100 0000
• Determine number of hosts per subnet
• for 1100 0000, six bits define 26 2
  hosts/subnet
• Determine broadcast address and host ID range
• Broadcast address has all 1s in host portion

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Subnets 00 and 01
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Subnets 10, 11
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Subnetting Class C Addresses Fast Method
Answer Five Simple Questions
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How Many Subnets?

• 2x number of subnets
• x is the number of masked bits (1s)
• For example, if 2 bits are used to identify the
  subnet (1100 00002 19210), there are 22 or 4
  subnets

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How Many Hosts Per Subnet?

• 2y - 2 number of hosts per subnet
• y is the number of unmasked bits (0s)
• For example, if 2 bits are used for the network
  ID, then 6 bits are left for host IDs
• Thus the number of hosts per subnet is 26-2 or 62.

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What Are The Valid Subnets?

• 256 - subnet mask block size and subnet base
  value (Critical values know these and everything
  else is simple)
• For example 256 - 19264. 64 is the first
  subnet.
• The next subnet would be the base value plus the
  block size or 64 64 128, the second subnet

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Subnet Broadcast Addresses

• The broadcast address is determined by setting
  all host bits to 1
• This is the number immediately preceding the next
  subnet

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Valid Host IDs

• Valid hosts are the values between the subnets,
  omitting all 0s and all 1s IDs

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Variable Length Subnet Masks (VLSM)

• Some segments do not need as many IDs as do other
  segments
• VLSM conserves address space by allotting the
  minimum number of IDs to each segment
• Like the name implies this is accomplished by
  using masks of different lengths

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Routing Protocols that Support VLSM

• RIPv2, EIGRP, and OSPF accommodate VLSM by
  including a field for subnet information
• RIPv1 and IGRP do not
• Protocols that support VLSM are said to be
  classless
• Protocols that do not support VLSM are called
  classful

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Same Mask Subnetting
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Applying VLSM
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Implementing VLSM

• Relate number of hosts needed to block sizes
• Create VLSM table using VLSM Worksheet to avoid
  overlap

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VLSM Worksheet
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VSLM, Example 1
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VLSM Worksheet for Example 1
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VLSM, Example 2
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VLSM Worksheet for Example 2
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What are the IP addresses for the Fast Ethernet
and Serial Interfaces?
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Answer
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Summarization or Route Aggregation

• Advertises many networks as one
• Reduces size of routing tables
• Determine block size and summary mask

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Troubleshooting

• The following examples provide practice at
  troubleshooting subnet addressing problems
• General trouble shooting steps
• ping localhost
• ping local IP address
• ping default gateway
• ping remote server
• Consider DNS (ping host names and IP addresses)

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Why Cant the Sales LAN Contact Server A?
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Why Cant Hosts in the Sales LAN Contact Server B?
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What is the Host Address?
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Which Addresses Can Be Assigned to the Hosts?
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Which IP Addresses Can Be Assigned?
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Which Class C Mask Should Be Used?