Class B Custom Subnet Mask Calculation Example

Now, let's do the same example with our Class B network (166.113.0.0) with 5 bits for the subnet ID (with a bit less narration this time; see Figure 77):

1. Determine Default Subnet Mask: For Class B, the subnet mask is 255.255.0.0. In binary, this is:

11111111 11111111 00000000 00000000

2. Change Left-Most Zeroes To Ones For Subnet Bits: We have decided to use 5 bits for the subnet ID, so, we change the 5 left-most 0 bits from a 0 to 1, shown highlighted below, to give us our binary custom subnet mask:

11111111 11111111 11111000 00000000

3. Convert Subnet Mask To Dotted Decimal Notation: We take each of the octets in the subnet mask and convert it to decimal, to give us a custom subnet mask of 255.255.248.0
   
   
4. Express Subnet Mask In “Slash Notation”: We can express the subnet mask 255.255.248.0 as “/21”, since it is 21 ones followed by 11 zeroes. In other words, its prefix length is 21.

Now, what could be easier than that? Well, you could simply refer to the subnetting summary tables. Find the table for the appropriate class, and then find the row that you selected in the previous step that matches the number of subnet ID bits you want to use. You can see the matching subnet mask right there.

(Hey, it's good to know how to do it yourself! You may not always have tables to refer to!)