1. Subnetting

2. Subnetting - Why?
Problem: Organizations have multiple networks which are independently managed
Solution 1: Allocate one or more addresses for each network
Difficult to manage
From the outside of the organization, each network must be addressable.
Solution 2: Add another level of hierarchy to the IP addressing structure
University Network
Medical
School
Library
Engineering
Subnetting

3. Basic Idea of Subnetting
Split the host number portion of an IP address into a subnet number and a (smaller) host number.
Result is a 3-layer hierarchy
Subnets can be freely assigned within the organization
Internally, subnets are treated as separate networks
Subnet structure is not visible outside the organization

4. Network mask and subnetwork mask

5. Advantages of Subnetting
With subnetting, IP addresses use a 3-layer hierarchy:
Network
Subnet
Host
Improves efficiency of IP addresses by not consuming an entire address space for each physical network.
Reduces router complexity. Since external routers do not know about subnetting, the complexity of routing tables at external routers is reduced.
Note: Length of the subnet mask need not be identical at all subnetworks.

6. 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.

7. 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.

8. Example 1
What is the subnetwork address if the destination address is and the subnet mask is ?

9. The subnetwork address is 200.45.32.0.
Solution
The subnetwork address is

10. 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.

11. Example 2
What is the subnetwork address if the destination address is and the mask is ?
Solution
See Figure

12. Example 2

13. Comparison of a default mask and
Figure 5-7
Comparison of a default mask and
a subnet mask

14. The number of subnets must be a power of 2.

15. The number of 1s in the default mask is 24 (class C).
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).

16. The total number of 0s is 5 (32 - 27). The mask is
Solution (Continued)
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 is

17. 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.

19. The number of 1s in the default mask is 16 (class B).
Example 4
A company is granted the site address (class B). The company needs 1000 subnets. Design the subnets.
Solution
The number of 1s in the default mask is 16 (class B).

20. The total number of 1s in the subnet mask is 26 (16 + 10).
Solution (Continued)
The company needs 1000 subnets. This number is not a power of 2. The next number that is a power of 2 is 1024 (210). We need 10 more 1s in the subnet mask.
The total number of 1s in the subnet mask is 26 ( ).
The total number of 0s is 6 ( ).

21. The number of subnets is 1024.
Solution (Continued)
The mask is or
The number of subnets is 1024.
The number of addresses in each subnet is 26 (6 is the number of 0s) or 64.
See Figure 5.9

23. Default mask and subnet mask

24. Comparison of subnet, default, and supernet masks

25. CLASSLESS ADDRESSING
Subnetting and supernetting in classful addressing did not really solve the address depletion problem. With the growth of the Internet, it was clear that a larger address space was needed as a long-term solution. Although the long-range solution has already been devised and is called IPv6, a short-term solution was also devised to use the same address space but to change the distribution of addresses to provide a fair share to each organization. The short-term solution still uses IPv4 addresses, but it is called classless addressing.

26. In classless addressing, the prefix defines the network and the suffix
Note
In classless addressing, the prefix defines the network and the suffix
defines the host.

27. Slash notation

28. Example The following addresses are defined using slash notations.
a. In the address /8, the network mask is The mask has eight 1s and twenty-four 0s. The prefix length is 8; the suffix length is 24.
b. In the address /16, the network mask is The mask has sixteen 1s and sixteen 0s.The prefix length is 16; the suffix length is 16.
c. In the address /27, the network mask is The mask has twenty-seven 1s and five 0s. The prefix length is 27; the suffix length is 5.