In this webcast we'll talk about address space, the purpose of the network mask and the reasons we manipulate network masks. Presented by: 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.

5. Logical AND
AND gate

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

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