2. Subnetting, The Struggle for Understanding John Skyers HBC Regional Academy London SE17 1JE United Kingdom

3. Subnetting is the logical creation of new IP networks, from one IP network address.

4. In order to subnet a network, you need to have a good understanding of IP addresses

5. So lets review IP addresses and roll straight through into subnetting

6. IP addresses are: IP addresses are: Assigned by American Registry for Internet Numbers (ARIN). Assigned by American Registry for Internet Numbers (ARIN). Four 8 bit binary numbers connected by periods. Each number division is called an octet. These numbers are displayed in decimal form eg. 210.10.4.20 Four 8 bit binary numbers connected by periods. Each number division is called an octet. These numbers are displayed in decimal form eg. 210.10.4.20 Divided into classes dependent on the number within the first octet. Divided into classes dependent on the number within the first octet.

7. The class of address? Class A 0-127 (0 Default; 127 Loop back test) Class B 128-191 Class C 192-223 Class D 224-239 (Multicasting) Class E 240-254 (Experimental; Research)

8. Each class of address has a different code. The code displays the network and host octets of each address and its corresponding subnet mask Class A N.H.H.H Class A N.H.H.H Class B N.N.H.H Class B N.N.H.H Class C N.N.N.H Class C N.N.N.H

9. “Subnet Mask?”

10. A subnet mask allows computers to distinguish between the network, sub-network and host bits of a given IP address

11. Default Subnet Masks are used when subnetting is not occurring. The defaults are: 255.0.0.0 for Class A addresses 255.0.0.0 for Class A addresses 255.255.0.0 for Class B addresses and 255.255.0.0 for Class B addresses and 255.255.255.0 for Class C addresses. 255.255.255.0 for Class C addresses.

12. Now lets subnet!

13. Given the Class C network address 210.50.4.0, create 6 subnets. Given the Class C network address 210.50.4.0, create 6 subnets.

14. Number of bits to borrow First step, find out how many bits you need to borrow for subnet. (Note: You can only borrow from host octets; you must borrow at least 2 bits and you must leave at least 2 bits) First step, find out how many bits you need to borrow for subnet. (Note: You can only borrow from host octets; you must borrow at least 2 bits and you must leave at least 2 bits)

15. Number of bits to borrow By using a constant (2), 2³ = 8, we discover that 3 bits need to be borrowed (Answer must be <=6 when you minus 2, I will explain later). By using a constant (2), 2³ = 8, we discover that 3 bits need to be borrowed (Answer must be <=6 when you minus 2, I will explain later).

16. Subnet Number Now, by adding the 3 (number of bits borrowed) high order binary numbers (128 +64 +32) we get the number 224. This number is known as the subnet number. Now, by adding the 3 (number of bits borrowed) high order binary numbers (128 +64 +32) we get the number 224. This number is known as the subnet number.

17. Custom subnet mask The Default Subnet Mask is only used when subnetting is not occurring. The Default Subnet Mask is only used when subnetting is not occurring. The moment you start subnetting you have to create a Custom Subnet Mask. The moment you start subnetting you have to create a Custom Subnet Mask. This is simply a matter of dropping the subnet number into the first host octet of the Default Subnet Mask. This is simply a matter of dropping the subnet number into the first host octet of the Default Subnet Mask. This creates a Custom Subnet Mask for our present example: 255.255.255.224 This creates a Custom Subnet Mask for our present example: 255.255.255.224

18. Subnet range of addresses If we minus the subnet number (224) from another constant, 256 (magic number), we get the number 32 (the first useable subnet address). If we minus the subnet number (224) from another constant, 256 (magic number), we get the number 32 (the first useable subnet address). We then add up in multiples of 32 until we have 6 IP address ranges, which look like…. We then add up in multiples of 32 until we have 6 IP address ranges, which look like….

19. THIS! Subnet (1) 32 – 63 Subnet (1) 32 – 63 Subnet (2) 64 – 95 Subnet (2) 64 – 95 Subnet (3) 96 - 127 Subnet (3) 96 - 127 Subnet (4) 128 – 159 Subnet (4) 128 – 159 Subnet (5) 160 – 191 Subnet (5) 160 – 191 Subnet (6) 192 - 223 Subnet (6) 192 - 223

20. The first number in range is the Subnet Address Subnet (1) 32 – 63 Subnet (1) 32 – 63 Subnet (2) 64 – 95 Subnet (2) 64 – 95 Subnet (3) 96 - 127 Subnet (3) 96 - 127 Subnet (4) 128 – 159 Subnet (4) 128 – 159 Subnet (5) 160 – 191 Subnet (5) 160 – 191 Subnet (6) 192 - 223 Subnet (6) 192 - 223

21. While the last number in range is the subnet’s Broadcast Address Subnet (1) 32 – 63 Subnet (1) 32 – 63 Subnet (2) 64 – 95 Subnet (2) 64 – 95 Subnet (3) 96 - 127 Subnet (3) 96 - 127 Subnet (4) 128 – 159 Subnet (4) 128 – 159 Subnet (5) 160 – 191 Subnet (5) 160 – 191 Subnet (6) 192 - 223 Subnet (6) 192 - 223

22. The subnet addresses and broadcast addresses cannot be used for hosts or subnets therefore for every range, two addresses cannot be used by devices and by subnets (this was why we minus 2 in our previous subnet calculation) Subnet (1) 32 (Useable Host Addresses) 63 Subnet (1) 32 (Useable Host Addresses) 63 Subnet (2) 64 ((Useable Host Addresses) 95 Subnet (2) 64 ((Useable Host Addresses) 95 Subnet (3) 96 (Useable Host Addresses) 127 Subnet (3) 96 (Useable Host Addresses) 127 Subnet (4) 128 (Useable Host Addresses) 159 Subnet (4) 128 (Useable Host Addresses) 159 Subnet (5) 160 (Useable Host Addresses) 191 Subnet (5) 160 (Useable Host Addresses) 191 Subnet (6) 192 (Useable Host Addresses) 223 Subnet (6) 192 (Useable Host Addresses) 223

23. Formula’s you need to know!

24. Subnets Total subnets = 2^(Bits used for subnet) Total subnets = 2^(Bits used for subnet) Total useable subnets = 2^(Bits used for subnet)-2 Total useable subnets = 2^(Bits used for subnet)-2

25. Hosts Total hosts = 2^(Bits used for hosts) Total hosts = 2^(Bits used for hosts) Total useable hosts = 2^(Bits used for hosts) - 2 Total useable hosts = 2^(Bits used for hosts) - 2

26. Now let me let you in on a secret. The key to mastering IP addressing and Subnetting is……

27. PRACTICE! PRACTICE! PRACTICE! Good luck on your CNAP at