Subnetting Basicsl

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Chapter 6

• Subnetting Basicsl

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Summary of the Three Classes of Networks
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Subnetting

• Benefits
• Reduce Network Traffic
• Routers breakup Broadcast Domains
• Optimal Performance
• Simplified Management
• Subnets
• Allow you to break large networks into smaller
  ones
• Must take bits from the hostid part of the
  address
• Always take from the left most hostid bits

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Subnetting

• Process of subdividing a single class of network
  into multiple, smaller networks

FIGURE 11-2 IP address before and after subnets
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Subnetting

• Extended network prefix
• The combination of an addresss network and
  subnet information
• (netid,subnetid,hostid)
• Subnet mask
• Special 32-bit number
• Only used within Internally
• Used to determine (netid, subnetid portion of
  an address

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Subnetting
IP Address and Subnet Mask are logically ANDed to
determine the network/subnet portion of an IP
address
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Subnetting
FIGURE 11-4 Subnetted network connected to the
Internet
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Subnetting
Given IP Address 128.12.34.71
Subnet Mask 255.255.240.0How many
subnets? What is the Subnet Number? How
many hosts? What is the Host Number?
11111111.11111111.11110000.00000000
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32
4094
2.71
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First Things First

• Determine the number of networks needed
• One for each subnetwork
• One for each WAN segment
• Determine the number of hosts required
• One for each TCP/IP host
• One for each Router interface
• Create the following
• A subnet mask for the entire network
• A subnet id for each network segment
• A range of host ids for each subnet segment

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Subnet Masks
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Subnetting Class C Addresses

• Class C address 8 host bits
• Possible Subnet Mask Bits
• 10000000 128
• 11000000 192
• 11100000 224
• 11110000 240
• 11111000 248
• 11111100 252
• 11111110 254
• Rules
• Cannot have only 1 bit for subnetting or hostid
• Subnets 128 254 are illegal

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The Binary Method using 2 bits
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The Alternate Method

• 1. How many subnets does the subnet mask produce?
• 2x xnumber of bits for subnet
• 2. How many valid hosts per subnet? 2y
• ynumber of bits for host ids
• 3. What are the valid subnets?
• 256 subnet first subnetwork (block)
• add the block to subnetwork
• stop when subnet mask is reached
• 4. What is the broadcast address of each subnet?
• Next subnetwork 1
• 5. What are the valid hosts in each subnet?
• First Host Subnetwork 1
• Last Host Broadcast - 1

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Things to Know

• Calculating subnet mask, host addresses,
  broadcast address
• Understand the concept of VLSM
• Four diagnostic steps

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