1. Chapter 5 Addressing
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Lecture

2. Subnetting
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Lecture

3. SUBNETTING
When we talked about CLASSFUL addressing – we realized the problem of wasted host addresses and depleting available network addresses.
In subnetting, a network is divided into several smaller networks called subnetworks or subnets – each subnet will have it’s own address
Typically, there are 2 steps in reaching a destination: first we must reach the network (netid) and then we reach the destination (hostid)
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4. A network with two levels of hierarchy (not subnetted)
The 2 level approach is not enough some times – you can only have 1 physical network – in example, all host are at the same level – no grouping
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5. A network with three levels of hierarchy (subnetted)
(0-63)
(64-127)
With subnetting, hosts can be grouped
( )
( )
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6. Addresses in a network with and without subnetting
With subnetting, there are 3 levels (versus 2 levels). Partition the hostid space into subnetid and hostid.
(1st) network, (2nd) subnetwork and (3rd) host
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7. Similar to Hierarchy concept in a telephone number
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8. Default mask and subnet mask
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9. Finding the Subnet Address
Given an IP address, we can find the subnet address the same way we found the network address in the previous chapter. We apply the mask to the address. We can do this in two ways: straight or short-cut.
Straight Method
In the straight method, we use binary notation for both the address and the mask and then apply the AND operation to find the subnet address.
Short-Cut Method
** If the byte in the mask is 255, copy the byte in the address.
** If the byte in the mask is 0, replace the byte in the address with 0.
** If the byte in the mask is neither 255 nor 0, we write the mask and the address in binary and apply the AND operation.
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10. Subnet Mask Form
In the early days, non-contiguous 1’s masks were used (0’s and 1’s could alternate)
Today, as a best practice, contiguous 1’s masks are used
In either case, the black box can perform the “masking” process
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Lecture

11. Example 1
What is the subnetwork address if the destination address is and the subnet mask is ?
Solution
The subnetwork address is
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12. Recall - 5-bit Address Space Illustration
1-bit Netid case (no subnets)
16 addresses/block
Number of blocks: 2
Address range per block: 0 to 15
Netids: 0, 1
Network Addresses : 00000, 10000
Broadcast Addresses: 01111, 11111
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13. 5-bit Address Space Illustration
subnet
1-bit Subnet case
Number of blocks/networks: 2
Number subnets per block: 2
8 addresses/subnet
Address range per subnet: 0 to 7
Subnet ids: 0, 1
Network Addresses : 00000, 01000, 10000, 11000
Broadcast Addresses: 00111, 01111, 10111, 11111
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14. 5-bit Address Space Illustration
subnet
2-bit Subnet case
Number of blocks/networks: 2
Number subnets per block: 4
4 addresses/subnet
Address range per subnet: 0 to 3
Subnet ids: 00, 01, 10, 11
Network Addresses : 00000, 00100, 01000, 01100
10000, 10100, 11000, 11100
Broadcast Addresses: 00011, 00111, 01011, 01111
10011, 10111, 11011, 11111
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15. Example 2
What is the subnetwork address if the destination address is and the mask is ?
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16. Comparison of a default mask and
a subnet mask
A portion of the hostid space is divided between some contiguous 1’s and 0’s
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17. The number of subnets must be a power of 2.
Determine the number of subnets added by looking at the number of 1s added to the default mask and performing 2 raised to that number
For example, 23 = 8 subnets
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18. Example 3
A company is granted the site address (class C). The company needs six subnets. Design the subnets.
Solution
The number of 1s in the default mask is 24 (class C).
The company needs six subnets. This number 6 is not a power of 2. The next number that is a power of 2 is 8 (23). We need 3 more 1s in the subnet mask. The total number of 1s in the subnet mask is 27 (24 + 3).
The total number of 0s is 5 ( ). The mask would be
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19. The number of subnets is 8.
Solution (Continued) or
The number of subnets is 8.
The number of addresses in each subnet is 25 (5 is the number of 0s) or 32.
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20. Example 3
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