One further element of IP addressing needs to be covered subnetting. Subnetting a network allows an administrator to carefully control network traffic. In large networks, it is important to control which machines can talk to each other, where network broadcasts are seen and to limit network collisions. Using a subnet mask, you can define which parts of an IP address are the network parts, and which are the node parts. To understand how this works, we need to delve into a little mathematics.
There are 5 different address classes that can be used:
As an example, 10.1.1.1 is a class A address, because the first octet (10) falls within 1 to 126 the class A range. Similarly 250.10.10.10 is a class E address, because the first octet (250) is between 240 and 254.
In simple, classful subnetting, the address class determines the subnet mask:
In the above subnet list, an octet set to 255 indicates a network address whilst an octet set to 0 indicates a node (computer) address. This defines which nodes are parts of the same network. As a practical example, consider the IP addresses and subnets in Figure 434.
The N signifies a network portion of the address, whilst an n signifies a node portion of the address. If the network portions of 2 subnets do not match, the IP addresses are in different networks and thus require a gateway to communicate with each other. In the above table, 192.168.100.120 and 192.168.100.150 are both in the same network, because the network portion of the IP addresses match. 192.168.101.130 is in a different network. If the reasoning for this is unclear, mentally draw a line to the right of the last N in all 3 columns. Everything to the left of this line must be identical for each IP address, otherwise they are in a different network. Think of this like an address the N refers to the street, city and country you live in, whilst the n specifies your unique house number.
It is an absolute rule of subnetting that no host may have the first or last IP address in the subnet assigned to them. The .0 and .255 addresses are reserved for network broadcasts.
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