Finish the thoughts on CIDR

net/subnets.md

@@ -19,10 +19,11 @@ form, you get `00001010.00101010.00000111.00010011`. This is referred to as a
 bit string, which you'll notice contains exactly `32` bits as expected. Since
 an octet will always be `8` bits, the dot separators aren't needed in a bit
 string. This means that the bit string should really be written as
-`00001010001010100000011100010011`. This bit string is also known as a `32`-bit
-integer. In other words, it is a whole number that takes up exactly `32` bits of
-memory. This number can also be written in decimal form as `170526483`. We now
-have 3 ways to describe the same IP address:
+`00001010001010100000011100010011`. These bit strings can also be referred to as
+a `32`-bit integer. In other words, they are a whole number that takes up
+exactly `32` bits of memory. The example bit string number in this case could be
+written in decimal form as `170526483`. We now have 3 ways to describe the same
+IP address:
 
   - `10.42.7.19`
   - `00001010001010100000011100010011`
@@ -39,4 +40,12 @@ known as a subnet mask.
 A subnet mask is a 32-bit string, similar to an IPv4 address, with the added
 constraint that it must be an initial series of `1` bits immediately followed by
 a series of `0` bits. This means that as soon as the first `0` bit appears, all
-remaining bits must be `0`.
+remaining bits must be `0`. The following bit string is a valid subnet mask:
+`11111111111111111111111100000000`. If you split up the 32 bits into 4 groups of
+octets, and convert those groups to decimal form, you get `255`, `255`, `255`,
+and `0`. We would write this as `255.255.255.0`. Another way of writing a subnet
+mask is CIDR notation, which is of the form `/N`, where `N` represents the
+number of `1` bits at the beginning of the mask. This means that writing
+`10.42.7.19/24` indicates you have the IP address `10.42.7.19` with the subnet
+mask of `11111111111111111111111100000000` (or `255.255.255.0`). You'll notice
+the mask has 24 `1` bits, the value of `N`.