IPv4 Subnetting - BASICS
Intro
IPv4 subnetting is the process of taking a 32-bit address space and splitting it into smaller networks (subnets) providing:
-More efficient use of allocated space
-Easier management
-Segmentation
-Security
An IPv4 address is split up into two parts: The network portion and the host portion. The subnet mask is what 'identifies' the network portion. In particular, the number of consecutive '1's from left to right.
Classful / Fixed Length Subnet Masking (FLSM)
Classful addressing is how the Internet Assigned Numbers Authority (IANA) historically allocated public IPv4 address ranges. The term 'FLSM' is how individual businesses allocate the IPv4 space they are given.
The three separate unicast classes are defined by the leading digits in the subnet mask (circled in red). Class B and Class C networks stretch the boundary to the right, strictly along the decimal notation. In these examples, as the network field increases the host field decreases.
Below is a typical example of a class C network:
192.168.1.0 255.255.255.0
You may notice similar address ranges on your home computer or internet router. They are usually used as default network ranges.
Note,
127.0.0.0/8 is reserved for loopback addressing usually applied to the 'localhost' of the computer you are on. If you are setting up a web server and want to point software to your local machine, this is the address you would use, for example, 127.0.0.1/8.
Try it,
Open up a command prompt window and type the following:
ping localhost -4
You should get a reply stating the machine name of the computer you are on. The '-4' option forces the command to use IPv4 as the default action would be to reply via IPv6.
Other reserved public address ranges are:
0.0.0.0/8
192.0.0.0/24
223.255.255.0/24
Classless Inter-Domain Routing (CIDR) / Variable Length Subnet Masking (VLSM)
Classful/FLSM allocation can be very wasteful, especially if a business only requires, for example, 300 routable hosts and is assigned the range 126.0.0.0/8 (16,777,212 hosts)...... The next step is to break down these networks even further.
As with the previous example, CIDR is how more efficient public IPv4 addresses are allocated by IANA. VLSM is how an individual business breaks the allocated range down even further.
The easiest example to illustrate this is splitting up a class C unicast /24 network as shown on the left. A /24 network will provide 252 usable hosts (.0 being the network address and .254 being the gateway).
This /24 network can be split into more 'smaller' networks by moving the network portion of the subnet mask towards the right (increasing the total amount of 1's in the subnet mask. As you can see in the 'Usable Hosts' column if there is only a requirement for a network of 10 hosts, a /28 would be the most suitable to apply resulting in a wastage of only 4 x IP's.
When breaking down a /24 into smaller equal networks you can see just how many of each fit into that /24 space. For example, a /24 can be broken down into 2 x /25's, or 4 x /26's or 64 x /30's.