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Ch. 7 Subnetting and Other
    Masking Acrobatics
         Bruce Hartpence
 The Packet Guide to Core Network
             Protocols
Introduction
• Let’s start with the idea of a network
  – The 10,000 foot view is that it is a bunch of
    computers connected together
  – But the technical view is a little more precise
• Computers on the same network
  – Share the same IP address range
  – Have the same default gateway (router)
  – Have the same broadcast address
What is a Mask?
• 192.168.100.0 255.255.255.0
• Used to determine the network for a host
• Also used by routers to determine forwarding
  information
• The normal or natural network mask
  (netmask) is defined by address class type
• Not much flexibility
How do we use the mask?



•Classes vary in size and the number of hosts.
    •Each class has it’s own address range and mask.
•Early networks could be given an entire class.
    •For example, RIT has a Class B network address.
•But, this is inefficient and with more and more folks desiring
connectivity, was not sustainable.
Logical AND

 AND gate
Example: Class C network host
           200.150.100.95
• Steps 1 & 2: Convert the address and mask to
  binary
  11001000.10010110.01100100.01011111
  11111111.11111111.11111111.00000000
• Perform the ANDing operation from RT to LT
  11001000.10010110.01100100.00000000
• Convert back to base 10 numbers
  200.150.100.0
• Easy right?
IP packets
• Network Masks
  are not included
  in the IP packet
• All of the
  processing is
  done on devices
• But, you never
  know the mask
  for the other
  end
Revisiting the mask structure
• From the class structure
  – for a class A address: 255.0.0.0
  – for a class B address: 255.255.0.0
  – for a class C address: 255.255.255.0
• When we convert to binary
  – There 1’s indicate the network portion
  – The 0’s indicate the host portion
From an earlier chapter




• So for our example, the network address is 200.150.100.0 (all
  0’s) and the broadcast address is 200.150.100.255 (all 1’s)
• This also means that after ANDing, the host addresses from
  200.150.100.1 to 200.150.100.254 are on the same network.
What is a subnet?
• “Logically visible sub-      • Created by
  sections” RFC 917              manipulating the
• Work exactly like the          network mask
  classful network and         • This splits the network
  they still have:               into smaller networks
   – Network (subnetwork)      • Bits are “stolen” from
     address                     the host portion of the
   – Broadcast address           network
   – All hosts on the subnet
     typically use the same
                               • A third portion is added
     gateway                     to the IP address
OK, we can subnet but why?
• As a network grows in terms of the number of
  hosts, the traffic grows as well
  – Broadcast (ARP, Windows, DHCP, etc.)
• Users or departments may have varying
  security concerns
• The desire to provide different quality of
  service options
• Other overhead: SNMP, routing
• Rule of thumb: 100/30
Subnet behavior
• Local traffic is limited to the subnet
  – ARP traffic (and everything else) is now limited
  – Limited broadcast address 255.255.255.255
  – Broadcast frame address ff-ff-ff-ff-ff-ff
• MAC addresses belong to the subnet, not the
  classful network
• A router (or routing function) is required to
  get traffic between subnets
So how do you decide?
• First, how many subnets do you need?
  –   Organization of the groups
  –   Addresses required
  –   Growth
  –   Current equipment
• Once this has been determined, “steal the bits”
  – Number of bits stolen is determined by the number of
    required subnets
  – In the host portion, work to the right, converting 0s to
    1’s as you go
Common subnet patterns
• As bits are stolen, the number of subnets
  increases and the number of hosts/subnet
  decreases
Simple example
• A company desires 4 internal subnets within their
  200.150.100.0 network
• This requires 2 bits to be stolen because there are
  four possibilities in 2 bits
   – 00, 01, 10, 11
   – These are seen in the subnet field
   – Stolen bits are revealed in the mask
• New mask
   – Instead of 255.255.255.0 we have 255.255.255.192
Wait, how did this work?
• The classful network characteristics
  – Network 200.150.100.0 mask 255.255.255.0
  – Low useable address 200.150.100.1
  – High useable address 200.150.100.254
     • Often for the router
  – Broadcast address 200.150.100.255
• ANDing puts everyone on the same network
• Mask binary
  11111111.11111111.11111111.00000000
       Network portion       Host portion
Con’t
• Stealing 2 bits changes the mask
  – 255.255.255.192
  – 11111111.11111111.11111111.11000000
• The red bits represent the subnet field
• But what about the binary patterns?
  – The classful network address
  – 200.150.100.0
  – 11001000.10010110.01100100.00000000
  – The red bits are affected by the change to the
    mask
New subnet values
• Mask for all: 255.255.255.192
• Network ranges
  – Subnet ID           Subnet broadcast
  – 200.150.100.0 –     200.150.100.63
  – 200.150.100.64 –    200.150.100.127
  – 200.150.100.128 –   200.150.100.191
  – 200.150.100.192 –   200.150.100.255
Let’s look at an example address
• Node IP address: 200.150.100.137
  • Binary: 11001000.10010110.01100100.10001011
• Subnet mask: 255.255.255.192
  • Binary: 11111111.11111111.11111111.11000000
• AND
  • 11001000.10010110.01100100.10001011
  • 11111111.11111111.11111111.11000000
  • 11001000.10010110.01100100.10000000
• Base 10: 200.150.100.128
   • This is the new subnet for this node
Binary value details
• Node 200.150.100.137
   • 11001000.10010110.01100100.10001011
• Subnet mask: 255.255.255.192
  • 11111111.11111111.11111111.11000000
• Put all 0’s in for the host portion, you get the subnet
  ID
  • 11001000.10010110.01100100.10000000
• Put all 1’s in the host portion, you get the broadcast
  address
  • 11001000.10010110.01100100.10111111
Another look at the binary patterns
• Subnet address and binary
•   200.150.100.0:     11001000.10010110.01100100.00000000
•   200.150.100.64:    11001000.10010110.01100100.01000000
•   200.150.100.128:   11001000.10010110.01100100.10000000
•   200.150.100.192:   11001000.10010110.01100100.11000000
• So, where are the routers?
    –   200.150.100.62
    –   200.150.100.126
    –   200.150.100.190
    –   200.150.100.254
Another problem: 2 techniques
• Problem: Class B network 172.30.0.0, 8 subnets
  desired
  – Another way to ask this is “How many hosts do you
    need in each subnet?”
• Method 1 – Algebra
  –   How many hosts total? 65536
  –   How many subnets? 8
  –   Hosts/subnet? 65536/8 = 8192
  –   Bits for subnets? 3
  –   Mask? 255.255.224.0
  –   Always start at the classful network address.
Subnets
• The only tough question: What does 8191 look like in
  dotted quad notation?
• 172.30.0.0 - 172.30.31.255
• What is the next possible value?
    – 172.30.32.0 – this is the next subnet? Follow the pattern.
•   172.30.32.0 – 172.30.63.255
•   172.30.64.0 – 172.30.95.255
•   172.30.96.0 – 172.30.127.255
•   172.30.128.0 – 172.30.159.255
•   172.30.160.0 – 172.30.191.255
•   172.30.192.0 – 172.30.223.255
•   172.30.224.0 – 172.30.225.255
Method 2 – to the binary!
• Having selected the mask, start at the classful address
  and work through the binary patterns (172.30.0.0)
   –   172.30.00000000.00000000
   –   172.30.00100000.00000000
   –   172.30.01000000.00000000
   –   172.30.01100000.00000000
   –   172.30.10000000.00000000
   –   172.30.10100000.00000000
   –   172.30.11000000.00000000
   –   172.30.11100000.00000000
• Remember that these are the subnet addresses
Upper end of the range
• Broadcast addresses insert 1’s in the host portion
  –   172.30.00011111.11111111 (172.30.31.255)
  –   172.30.00111111.11111111
  –   172.30.01011111.11111111
  –   172.30.01111111.11111111
  –   172.30.10011111.11111111
  –   172.30.10111111.11111111
  –   172.30.11011111.11111111
  –   172.30.11111111.11111111
• Routers would typically be high/low in the range
Review and other ideas
• You should be able to go back to the simpler
  example and work both techniques
• Related ideas
  – Supernetting (combining networks)
  – Classless inter-domain routing (CIDR)
  – Variable length subnet masks (VLSM)
  – Aggregation
• All of these represent different ways to
  manipulate address space via the subnet mask
Thanks!
• Questions?
• Bruce.hartpence@rit.edu
• Keep an eye out for podcasts on itunes
Visit http://oreilly.com
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Oct. 14, 2011 webcast ch7 subnets bruce hartpence

  • 1. Ch. 7 Subnetting and Other Masking Acrobatics Bruce Hartpence The Packet Guide to Core Network Protocols
  • 2. Introduction • Let’s start with the idea of a network – The 10,000 foot view is that it is a bunch of computers connected together – But the technical view is a little more precise • Computers on the same network – Share the same IP address range – Have the same default gateway (router) – Have the same broadcast address
  • 3. What is a Mask? • 192.168.100.0 255.255.255.0 • Used to determine the network for a host • Also used by routers to determine forwarding information • The normal or natural network mask (netmask) is defined by address class type • Not much flexibility
  • 4. How do we use the mask? •Classes vary in size and the number of hosts. •Each class has it’s own address range and mask. •Early networks could be given an entire class. •For example, RIT has a Class B network address. •But, this is inefficient and with more and more folks desiring connectivity, was not sustainable.
  • 6. Example: Class C network host 200.150.100.95 • Steps 1 & 2: Convert the address and mask to binary 11001000.10010110.01100100.01011111 11111111.11111111.11111111.00000000 • Perform the ANDing operation from RT to LT 11001000.10010110.01100100.00000000 • Convert back to base 10 numbers 200.150.100.0 • Easy right?
  • 7. IP packets • Network Masks are not included in the IP packet • All of the processing is done on devices • But, you never know the mask for the other end
  • 8. Revisiting the mask structure • From the class structure – for a class A address: 255.0.0.0 – for a class B address: 255.255.0.0 – for a class C address: 255.255.255.0 • When we convert to binary – There 1’s indicate the network portion – The 0’s indicate the host portion
  • 9. From an earlier chapter • So for our example, the network address is 200.150.100.0 (all 0’s) and the broadcast address is 200.150.100.255 (all 1’s) • This also means that after ANDing, the host addresses from 200.150.100.1 to 200.150.100.254 are on the same network.
  • 10. What is a subnet? • “Logically visible sub- • Created by sections” RFC 917 manipulating the • Work exactly like the network mask classful network and • This splits the network they still have: into smaller networks – Network (subnetwork) • Bits are “stolen” from address the host portion of the – Broadcast address network – All hosts on the subnet typically use the same • A third portion is added gateway to the IP address
  • 11. OK, we can subnet but why? • As a network grows in terms of the number of hosts, the traffic grows as well – Broadcast (ARP, Windows, DHCP, etc.) • Users or departments may have varying security concerns • The desire to provide different quality of service options • Other overhead: SNMP, routing • Rule of thumb: 100/30
  • 12. Subnet behavior • Local traffic is limited to the subnet – ARP traffic (and everything else) is now limited – Limited broadcast address 255.255.255.255 – Broadcast frame address ff-ff-ff-ff-ff-ff • MAC addresses belong to the subnet, not the classful network • A router (or routing function) is required to get traffic between subnets
  • 13. So how do you decide? • First, how many subnets do you need? – Organization of the groups – Addresses required – Growth – Current equipment • Once this has been determined, “steal the bits” – Number of bits stolen is determined by the number of required subnets – In the host portion, work to the right, converting 0s to 1’s as you go
  • 14. Common subnet patterns • As bits are stolen, the number of subnets increases and the number of hosts/subnet decreases
  • 15. Simple example • A company desires 4 internal subnets within their 200.150.100.0 network • This requires 2 bits to be stolen because there are four possibilities in 2 bits – 00, 01, 10, 11 – These are seen in the subnet field – Stolen bits are revealed in the mask • New mask – Instead of 255.255.255.0 we have 255.255.255.192
  • 16. Wait, how did this work? • The classful network characteristics – Network 200.150.100.0 mask 255.255.255.0 – Low useable address 200.150.100.1 – High useable address 200.150.100.254 • Often for the router – Broadcast address 200.150.100.255 • ANDing puts everyone on the same network • Mask binary 11111111.11111111.11111111.00000000 Network portion Host portion
  • 17. Con’t • Stealing 2 bits changes the mask – 255.255.255.192 – 11111111.11111111.11111111.11000000 • The red bits represent the subnet field • But what about the binary patterns? – The classful network address – 200.150.100.0 – 11001000.10010110.01100100.00000000 – The red bits are affected by the change to the mask
  • 18. New subnet values • Mask for all: 255.255.255.192 • Network ranges – Subnet ID Subnet broadcast – 200.150.100.0 – 200.150.100.63 – 200.150.100.64 – 200.150.100.127 – 200.150.100.128 – 200.150.100.191 – 200.150.100.192 – 200.150.100.255
  • 19. Let’s look at an example address • Node IP address: 200.150.100.137 • Binary: 11001000.10010110.01100100.10001011 • Subnet mask: 255.255.255.192 • Binary: 11111111.11111111.11111111.11000000 • AND • 11001000.10010110.01100100.10001011 • 11111111.11111111.11111111.11000000 • 11001000.10010110.01100100.10000000 • Base 10: 200.150.100.128 • This is the new subnet for this node
  • 20. Binary value details • Node 200.150.100.137 • 11001000.10010110.01100100.10001011 • Subnet mask: 255.255.255.192 • 11111111.11111111.11111111.11000000 • Put all 0’s in for the host portion, you get the subnet ID • 11001000.10010110.01100100.10000000 • Put all 1’s in the host portion, you get the broadcast address • 11001000.10010110.01100100.10111111
  • 21. Another look at the binary patterns • Subnet address and binary • 200.150.100.0: 11001000.10010110.01100100.00000000 • 200.150.100.64: 11001000.10010110.01100100.01000000 • 200.150.100.128: 11001000.10010110.01100100.10000000 • 200.150.100.192: 11001000.10010110.01100100.11000000 • So, where are the routers? – 200.150.100.62 – 200.150.100.126 – 200.150.100.190 – 200.150.100.254
  • 22. Another problem: 2 techniques • Problem: Class B network 172.30.0.0, 8 subnets desired – Another way to ask this is “How many hosts do you need in each subnet?” • Method 1 – Algebra – How many hosts total? 65536 – How many subnets? 8 – Hosts/subnet? 65536/8 = 8192 – Bits for subnets? 3 – Mask? 255.255.224.0 – Always start at the classful network address.
  • 23. Subnets • The only tough question: What does 8191 look like in dotted quad notation? • 172.30.0.0 - 172.30.31.255 • What is the next possible value? – 172.30.32.0 – this is the next subnet? Follow the pattern. • 172.30.32.0 – 172.30.63.255 • 172.30.64.0 – 172.30.95.255 • 172.30.96.0 – 172.30.127.255 • 172.30.128.0 – 172.30.159.255 • 172.30.160.0 – 172.30.191.255 • 172.30.192.0 – 172.30.223.255 • 172.30.224.0 – 172.30.225.255
  • 24. Method 2 – to the binary! • Having selected the mask, start at the classful address and work through the binary patterns (172.30.0.0) – 172.30.00000000.00000000 – 172.30.00100000.00000000 – 172.30.01000000.00000000 – 172.30.01100000.00000000 – 172.30.10000000.00000000 – 172.30.10100000.00000000 – 172.30.11000000.00000000 – 172.30.11100000.00000000 • Remember that these are the subnet addresses
  • 25. Upper end of the range • Broadcast addresses insert 1’s in the host portion – 172.30.00011111.11111111 (172.30.31.255) – 172.30.00111111.11111111 – 172.30.01011111.11111111 – 172.30.01111111.11111111 – 172.30.10011111.11111111 – 172.30.10111111.11111111 – 172.30.11011111.11111111 – 172.30.11111111.11111111 • Routers would typically be high/low in the range
  • 26. Review and other ideas • You should be able to go back to the simpler example and work both techniques • Related ideas – Supernetting (combining networks) – Classless inter-domain routing (CIDR) – Variable length subnet masks (VLSM) – Aggregation • All of these represent different ways to manipulate address space via the subnet mask
  • 27. Thanks! • Questions? • Bruce.hartpence@rit.edu • Keep an eye out for podcasts on itunes
  • 28. Visit http://oreilly.com to purchase your copy of Core Network Protocols and enter code 4CAST to save 40% off print book & 50% off ebook with special code 4CAST Visit http://oreilly.com webcasts to view upcoming webcasts and online events.