IP Address in Decimal Notation
Reading IP addresses would be very difficult if we actually used its binary notation or hexadecimal notation. To simplify the reading, a standard was developed known as the doted decimal notation, which divides a 32-bit IP address into a four 8-bit.
Look at the following figure.
Figure 8.3 demonstrates a Class B network, doted in decimal notation. Now it simplifies the reading doesn't it?
So how do we recognize what type of network are we dealing with? The following table may answer the question.
|Subnet Classes||Dotted Decimal|
|A /8 Prefixes||0||1.xxx.xxx.xxx through 126.xxx.xxx.xxx|
|B /16 Prefixes||10||128.0.xxx.xxx through 191.255.xxx.xxx|
|C /24 Prefixes||110||192.0.0.xxx through 223.255.255.xxx|
Note. 127.0.0.1 is out of the table, because this address is the reserved loop back.
As you have already noticed, the classes A, B and C are easy to understand, and easy to allocate, but the allocation of addresses is truly inefficient because there are no classes to support medium sized networks.
If you notice a Class C, /24 which supports 254 hosts is too small and a class B /16 which support 65,534 hosts is too big. And assigning either one to an organization really makes a great impact when increasing the size of the Internet's routing table.
Anyways, if you are working in a private LAN behind a router or proxy server, you are probably using a Class C or Class B. Many system administrators are unaware of why they are using these types of networks; others don't even know that classes exist. All they know is that they have to place a subnet mask.