Subnet Mask 11111111 11111110 00000000 00000000 Starting Class C 110XXXXX XXXXXXX0 00000000 00000000 Ending Class C 110XXXXX XXXXXXX1 11111111 00000000
You can add in any valid combination of bits for the Xs. This results in the range of networks you require. For example, 205.126.0.0 works with the subnet mask of 255.254.0.0. This range includes 512 (you are using 9 bits, or 29 class C networks) networks. The actual network IDs range from 205.126.0.0 to 205.127.255.0. Notice again that all 0s and all 1s are valid (and required) in supernetting.
Summary
Simply put, subnetting is the hardest concept that you will have to deal with. The unfortunate part is that you may need to deal with subnetting fairly often in your organization. This chapter has looked at the why and then the how of subnetting. It also has given you the information you need to deal with it. The single largest stum- bling block is normally trying too hard. It is really very simple, as long as you don’t let the binary scare you. Building on the subnetting that you have seen here, the next chapter looks at IP routing, which is basically why you need to subnet.
Test Yourself
1. Why is subnetting required?
2. What does subnetting do from a binary perspective? 3. What is the least number of bits that you can subnet on?
4. How many different subnet masks are required for an organization with 17,938 hosts?
5. For each of the following number of networks, determine the number of bits needed in the subnet mask and how many networks there will be in total. 48 156 12 64 6 78 312 56 127 7
6. For each of the following number of hosts, determine the number of bits required and the maximum hosts supported.
50 250 125 300 800 2,000 60 95 4,000 1,500
7. Complete Subnet Mask and Hosts per Subnet columns in the following chart. Network ID Subnets Required Subnet Mask Hosts per Subnet
152.42.0.0 10 ___ ___ 120.0.0.0 250 ___ ___ 187.16.0.0 100 ___ ___ 210.125.36.0 2 ___ ___ 160.106.0.0 33 ___ ___
8. What numbers are valid in a subnet mask.
9. For each of the following, give the first three valid subnet IDs. 152.42.0.0 mask 255.255.240.0
120.0.0.0 mask 255.255.0.0 187.16.0.0 mask 255.255.254.0
210.125.36.0 mask 255.255.255.192 (only two) 160.106.0.0 mask 255.255.248.0
Complete the following table: Position Value 64 32 16 8 4 2 1 Subnet bits __ __ __ __ __ __ __ Subnets Available __ __ __ __ __ __ __ Subnet Mask __ __ __ __ __ __ __ Host bits __ __ __ __ __ __ __
11. For each of the following subnet IDs, give the range of valid host IDs. 152.42.64.0 mask 255.255.240.0
160.106.64.0 mask 255.255.192.0 198.78.16.64 mask 255.255.255.224 15.56.0.0 mask 255.255.0.0
131.107.64.0 mask 255.255.252.0
12. For each of the following host IDs, determine the range of hosts for the subnet. 175.42.36.52 mask 255.255.248.0
189.64.125.12 mask 255.255.192.0 164.53.47.8 mask 255.255.240.0 45.36.25.4 mask 255.255.0.0 160.106.78.52 mask 255.255 224.0 13. What is the purpose of supernetting?
14. Who do you have to work with to perform supernetting? Why?
15. For each of the following number of hosts, determine the number of class C addresses required for supernetting and the subnet mask.
12,245 160,782 852 6,254 85,765
Test Yourself Answers
1. Subnetting is required to enable organizations that have a large number of hosts to take the assigned network ID and break down into small pieces. This is generally done for performance reasons or to accommodate different physical locations.
2. Subnetting is basically the process of using more bits to identify the network than the standard mask uses. The extra networks generated this way become a division that can be managed internally.
3. Subnetting requires that the subnet ID not be all 0s or all 1s. This means you cannot use 1 bit to subnet. The least number of bits you can subnet on, there- fore, is 2.
4. One. When you plan the network, all the hosts on the network should use the same subnet mask—regardless of the number of hosts.
5. The answers are as follows:
48 6 bits 62 156 8 bits 255 12 4 bits 14 64 7 bits 126 6 3 bits 6 78 7 bits 126 312 9 bits 510 56 6 bits 62 127 8 bits 255 7 4 bits 14
6. The answers are as follows:
50 6 bits 62 250 8 bits 254 125 7 bits 126 300 9 bits 510 800 10 bits 1,022 2,000 11 bits 2,046
60 6 bits 62
95 7 bits 126
4,000 12 bits 4,094
1,500 11 bits 2,046
7. When completed, the table should look like this:
Network ID Subnets Required Subnet Mask Hosts per Subnet
152.42.0.0 10 255.255.240.0 4,094
120.0.0.0 250 255.255.0.0 65,534
187.16.0.0 100 255.255.254.0 510
210.125.36.0 2 255.255.255.192 62
160.106.0.0 33 255.255.248.0 2,046
8. The numbers valid in a subnet mask are 0, 192, 224, 240, 248, 252, 254, 255. 9. The valid subnet IDs are as follows:
For 152.42.0.0 mask 255.255.240.0 152.42.16.0, 152.42.32.0, 152.42.48.0 For 120.0.0.0 mask 255.255.0.0 120.1.0.0. 120.2.0.0, 120.3.0.0 For 187.16.0.0 mask 255.255.254.0 187.16.2.0, 187.16.4.0, 187.16.6.0
For 210.125.36.0 mask 255.255.255.192 (only two)
210.125.36.64, 210.125.36.128
For 160.106.0.0 mask 255.255.248.0
160.106.8.0, 160.106.16.0, 160.106.24.0 10. When completed, the table should look like this:
Position Value 64 32 16 8 4 2 1
Subnet bits 2 3 4 5 6 7 8
Subnets 22-2 23-2 24-2 25-2 26-2 27-2 28-2
Subnet Mask 128+64 192+32 224+16 240+8 248+4 252+2 254+1 =192 =224 =240 =248 =252 =254 =255
Host bits 6 5 4 3 2 1 0
11. The ranges are as follows:
For 152.42.64.0 mask 255.255.240.0 152.42.64.1 to 152.42.79.254 For 160.106.64.0 mask 255.255.192.0 160.106.64.1 to 160.106.65.254 For 198.78.16.64 mask 255.255.255.224 198.78.16.65 to 198.78.16.94 For 15.56.0.0 mask 255.255.0.0 15.56.0.1 to 15.56.255.254 For 131.107.64.0 mask 255.255.252.0 131.107.64.1 to 131.107.67.254 12. The ranges are as follows:
For 175.42.36.52 mask 255.255.248.0 175.42.32.1 to 175.42.39.254 For 189.64.125.12 mask 255.255.192.0 189.64.64.1 to 189.64.127.254 For 164.53.47.8 mask 255.255.240.0 164.53.32.1 to 164.53.47.254 For 45.36.25.4 mask 255.255.0.0 45.36.0.1 to 45.36.255.254 For 160.106.78.52 mask 255.255 224.0 160.106.64.1 to 160.106.95.254
13. Supernetting enables you to combine groups of class networks in a single network ID. This is required to get around the lack of available addresses. 14. You must work with your ISP to obtain a supernetted address. Your ISP needs to
15. The number of class C addresses and the subnet masks are as follows:
Hosts Number of Addresses Subnet Mask
12,245 64 255.255.192.0
160,782 1,024 255.252.0.0
852 4 255.255.252.0
6,254 32 255.255.224.0