Because subnets can have different sizes, we need to have a method for hosts to know what they should consider as their subnet. You do not want the host to try and find the host in the local subnet when the host it is trying to talk to is on the other side of the planet. Remember that an IP address is just a 32-bit number. The IP address 193.110.157.77 can be written in bits as 11000001 01101110 10011101 01001101. What do we know about these bits for the subnet that contains all the addresses in 193.110.157.*? Well, we notice that some of the bits, in our case the first 24, are always the same. The last 8 bits change, depending on the number we want that "*" to be, as anything from 0 to 255. This is exactly what the subnet mask (also called the netmask) tells us. It is also a series of 32 bits, but now the bits do not represent a number, but the property of a bit in the IP address.

Let us visualize this in a table, because it sounds a lot more complex than it really is. Let us write down our IP address, but also the first and last address possible in our subnet. The parts in bold in the table below never change, and are part of the subnet, and thus receive a 1 in the netmask.

As expected, the only difference between IP addresses in the 193.110.157.* range are the last 8 bits: the first 24 bits (3 bytes) are always the same. We can also see another property of the netmask. It will always start with 1s and at one (and only one) point, it will switch to zeros. This is because our subnets will always be a continuous set of increasing numbers, e.g. from 0 to 255.

So if we want to describe our IP address and its subnet, we could use the decimal syntax 193.110.157.77/255.255.255.0. This gives us all the information we need. Our host's IP address is 193.110.157.77, and all the IP addresses that fall within 193.110.157.* can be reached directly.

But since sysadmins are inherently lazy, they do not want to write all these netmask numbers every time they need an address. Instead, a shorthand notation is used. For instance, for '255.255.255.0' we count the number of 1s in the netmask, and write that. So, the most common notation for our machine here would be 193.110.157.77/24. If we want to describe the entire subnet instead of a single host in a subnet, we would use the lowest address in that subnet. Our subnet would be written as 193.110.157.0/24. This is called the CIDR notation, the Classless Internet Domain Routing notation.

Let us now see how this works with a second range, often used elsewhere in this book, and go from CIDR to a full network description. The CIDR notation is 205.150.200.223/28. First, we rewrite the full netmask. 28 bits is 8 bits + 8 bits + 8 bits + 4 bits. So we have a netmask of 11111111 11111111 11111111 11110000, which in decimal is 255.255.255.240.

So, the bits in the netmask that are zero are those that can be changed for this subnet. Again, to get the lowest address, we write zeros in the IP address for which the netmask has a zero, which means that bit can be changed. To get the highest address, we write ones in the IP address for which the netmask has a zero.

The following table might help make this clear:

The lowest address is also called the network address, the highest address is also called the broadcast address, as this is used to send messages that need to reach all the hosts in the subnet. It is important to realize that a netmask does not mean anything on its own; it must be used in conjunction with an actual IP address.