1. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Subnetting

2. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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 School Subnetting

3. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Network mask and subnetwork mask

5. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Example 1 What is the subnetwork address if the destination address is 200.45.34.56 and the subnet mask is 255.255.240.0?

9. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Solution 11001000 00101101 00100010 00111000 11111111 11111111 11110000 00000000 000000000000 11001000 00101101 00100000 00000000 The subnetwork address is 200.45.32.0.

10. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Example 2 What is the subnetwork address if the destination address is 19.30.80.5 and the mask is 255.255.192.0? Solution See Figure

12. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Example 2

13. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-7 Comparison of a default mask and a subnet mask

14. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 The number of subnets must be a power of 2.

15. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Example 3 A company is granted the site address 201.70.64.0 (class C). The company needs six subnets. Design the subnets. Solution The number of 1s in the default mask is 24 (class C).

16. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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 (2 3 ). 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 (32  27). The mask is

17. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Solution (Continued) 11111111 11111111 11111111 11100000 or 255.255.255.224 The number of subnets is 8. The number of addresses in each subnet is 2 5 (5 is the number of 0s) or 32.

18. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Example 4 A company is granted the site address 181.56.0.0 (class B). The company needs 1000 subnets. Design the subnets. Solution The number of 1s in the default mask is 16 (class B).

19. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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 (2 10 ). We need 10 more 1s in the subnet mask. The total number of 1s in the subnet mask is 26 (16  10). The total number of 0s is 6 (32  26).

20. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Solution (Continued) The mask is 11111111 11111111 11111111 11000000 or 255.255.255.192. The number of subnets is 1024. The number of addresses in each subnet is 2 6 (6 is the number of 0s) or 64. See Figure 5.9

21. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Default mask and subnet mask

22. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Comparison of subnet, default, and supernet masks

23. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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.

24. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Note In classless addressing, the prefix defines the network and the suffix defines the host.

25. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Slash notation

26. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Example The following addresses are defined using slash notations. a. In the address 12.23.24.78/8, the network mask is 255.0.0.0. The mask has eight 1s and twenty-four 0s. The prefix length is 8; the suffix length is 24. b. In the address 130.11.232.156/16, the network mask is 255.255.0.0. The mask has sixteen 1s and sixteen 0s.The prefix length is 16; the suffix length is 16. c. In the address 167.199.170.82/27, the network mask is 255.255.255.224. The mask has twenty-seven 1s and five 0s. The prefix length is 27; the suffix length is 5.