What is Advanced Subnetting?
Cisco defines advanced subnetting as when the network part of the subnet mask is neither 24, 16, nor 8 bits. (In other words: Subnet masks that contain numbers other than .255. and .0.)

Why use advanced subnetting?
Because the /24 (255.255.255.0), /16 (255.255.0.0), and /8 (255.0.0.0) subnetting-methods are limited. If you have only had 3 PCs, and you want to bring them on the net, the best subnet mask would be 255.255.255.0 (or /24). A /24 subnet provides room for 253 hosts within a subnet, and if you only have 3 PCs, 250 possible IP Addresses would go to waste.

That's an expression I made up myself, but it matches perfectly what I am going to describe: For each subnet, 2 IPs are wasted: Network Address, and Broadcast Address. In addition to these two, I prefer to eliminate one more IP Address, since most subnets also need a gateway to communicate with the outside world.

# Advanced Subnet Calculations

There are many things you can calculate regarding advanced subnetting. Some of these things include:
• Given an IP Address and a subnet mask.. What is the network address?
• Given an IP Address and a subnet mask.. What is the broadcast address?
• Given an IP Address and a subnet mask.. What available IPs exists within that subnet?
There are more than these, but these are the most common, so I will explain the algorithm for these here. I assume that you know how to convert decimal numbers into binary numbers, so I will not explain that here.

To speed up your calculations, here are some translations from decimal to binary. Apart from 255 and 0, which are fairly simple to convert to binary, these numbers are the ones that can appear in a subnet masks. The octet that contains one of these numbers are to be considered as "The Interesting Octet":

```254:  11111110
252:  11111100
248:  11111000
240:  11110000
224:  11100000
196:  11000000
128:  10000000
```

Given an IP Address and a subnet mask.. What is the network address?
The procedure isn't as hard as it looks. It might take time, yes, but when you get the hang of it, it'll only take a few seconds. Here we'll use 255.255.224.0 as subnet mask, and 172.60.50.2 as IP Address.

Step 1:
Convert the interesting octet of the subnet mask into binary. The interesting octet is octet number three. As the above table shows, 224dec is 11100000bin.

Step 2:
Do the same with the IP Address. As stated earlier, I assume that you already know how to convert from decimal to binary. To speed things up, I’ll do it for you. As you probably remember, the interesting octet is octet number three ( .50. ). 50dec is 00110010bin.

Step 3:
Write the two binary numbers above each other and start to calculate:
0 + 0 = 0
0 + 1 = 0
1 + 1 = 1

here it is:
```subnet mask: 11100000
---------------------
Result:      00100000
```

Step 4:
Convert the result back to decimal (00100000 = 32), and replace the interesting octet of the IP Address with the decimal number. Enter 0 in any octets right of the Interesting Octet. Now you've got the Network Address (If you've done it right, it should be 172.60.32.0). Congratulations, you made it!