Classful Addressing

Class Start End Default Subnet Mask CIDR
Class A /8
Class B /16
Class C /24
Class D (multicast)
Class E (reserved)

In classful addressing each address class has a set subnet mask, meaning that the network and host portions of the IP address are determined by the class and cannot be changed. The above table show the address ranges and subnet masks in classful addressing for IPv4.

Subnet Mask

Like an IP address the subnet mask is made up of four binary octets. The subnet mask specifies the boundary between the network portion and the host portion of an IP address. The first portion of the subnet mask (in binary) is all ones and represents the network portion of an IP address. The second part of the subnet mask (in binary) is all zeros and represents the host portion of an IP address.

Class Subnet Mask
A 255 0 0 0
11111111 00000000 00000000 00000000
B 255 255 0 0
11111111 11111111 00000000 00000000
C 255 255 255 0
11111111 11111111 11111111 00000000

The Delta Table

The Delta Table is a tool that is used to make it easier to subnet a class C ip address. We borrow bits from the host portion of the ip address and use them for the network. In the table, look in the max networks field for the smallest number thats greater than the number of networks that you need. The number directly above that in the bits row is the numbers of host bits that you need to borrow to get the networks you need. The number directly below that is the Delta, or the magic number that can be used to determine the networks. Below that is the subnet mask.

Click to View an Example

Bits 1 2 3 4 5 6 7 8
MAX/networks 0 2 6 14 30 62 126 254
DELTA 128 64 32 16 8 4 2 1
Subnet Mask 128 192 224 240 248 252 254 255

The network and host buttons below open interactive dialogs based on whether you need a specific number of networks or hosts. You enter the number of networks or hosts that you need and it will tell you how many bits you need. The "Quiz Me" button will tell you how many hosts or networks you need and give you a field to enter the number of bits you need. It will check and tell whether you are right, if you are not it will give you the correct number of bits. They are geared towards the subnetting of a class C address. Use the above Delta Table to assist you.



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