IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway)
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In the classless system, the classes of the original IP addressing scheme are tossed out the window. The division between the network ID and host ID can occur at an arbitrary point, not just on octet boundaries like in the classful scheme.
The dividing point is indicated by putting the number of bits used for the network ID, called the prefix length, after the address (recall that the network ID bits are also sometimes called the network prefix, so the network ID size is the prefix length). For example, if 126.96.36.199 is part of a network where the first 27 bits are used for the network ID, that network would be specified as 188.8.131.52/27. The /27 is conceptually the same as the 255.255.255.224 subnet mask, since it has 27 one bits followed by 5 zeroes.
Did I just confuse the heck out of you? Sorryand don't worry. I'm simply introducing the concepts of classful, subnetted and classless addressing and showing you how they impact the way the IP address is interpreted. This means of necessity that I have greatly summarized important concepts here. All three methods are explained in their own sections in full detail.
As you can see, in the original classful scheme the division between network ID and host ID is implied. However, if either subnetting or classless addressing is used, then the subnet mask or slash number are required to fully qualify the address. These numbers are considered adjuncts to the IP address and usually mentioned in the same breath as the address itself, because without them, it is not possible to know where the network ID ends and the host ID begins.
One other number that is often specified along with the IP address for a device is the default gateway identifier. In simplest terms, this is the IP address of the router that provides default routing functions for a particular device. When a device on an IP network wants to send a datagram to a device it can't see on its local IP network, it sends it to the default gateway which takes care of routing functions. Without this, each IP device would also have to have knowledge of routing functions and routes, which would be inefficient. See the sections on routing concepts and TCP/IP routing protocols for more information.
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