Basic principles of subnetting
I'll let Wikipedia explain subnetting in more detail, but basically it's used to make routing more efficient and to logically seperate a network hierarchically.
I'll be adding more to this page as time goes on, but for now...
We'll start with a basic lesson on how to find the Subnet, Broadcast address & range of addresses in a subnet, given only an IP address and the Subnet Mask.
How to find the resident subnet, broadcast address & range of addresses in a subnet
1. Write down the mask and IP address in a table, in dotted-decimal format.
2. Find the interesting octet of the mask (the octet that is neither a 255, nor a 0). Draw a rectangle around that column of the table, for ease of practice.
3. Write down values for 3 octets of the subnet number, as follows:
a. For the octets to the left, copy the IP address' value.
b. For the octets to the right, write down 0s.
4. For the interesting octet's value:
a. Calculate the magic number. Magic number = 256-x, where x=the subnet mask's value in the interesting octet.
b. Calculate the integer multiples of the magic number, starting at 0, through 256.
c. Find the multiple that is closest to the IP address' value in the interesting octet, but not bigger than the IP address' value. Write this multiple down as the subnet number's value in the interesting octet.
5. To find the subnet broadcast address:
a. For octets to the left of the rectangle, copy the subnet number (or IP address') value.
b. For octets to the right of the rectangle, write down 255s.
c. In the interesting octet, add the subnet number's value to the magic number, and subract 1.
6. For the 1st IP address in the range of addresses, copy the subnet number, but add 1 to the 4th octet.
7. For the last IP address in the range of addresses, copy the subnet broadcast address, but subtract 1 from the 4th octet.
Example:
We are given the IP address 172.16.159.159 /22 ( /22 = 255.255.252.0 )
| IP Address | 172 | 16 | 159 | 159 |
| Subnet Mask | 255 | 255 | 252 | 0 |
| Subnet | 172 | 16 | 156 | 0 |
| First IP address | 172 | 16 | 156 | 1 |
| Last IP address | 172 | 16 | 159 | 254 |
| Broadcast IP address | 172 | 16 | 159 | 255 |
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