IP CIDR Addressing Example
(Page 4 of 4)
Third Level of Division
We now take each of the four /18 networks above and further subdivide it. We want to make each of these contain a number of blocks of different sizes corresponding to our potential customers. One way to do this would be as follows:
01000111 01011111 00000000 00000000
We substitute into these five bits 00000, 00001, 00010 and so on, giving us 32 different /23 networks in this block, each containing 9 bits for the host ID, for 510 hosts. The first will be sub-sub-subnetwork #1-0-0, 184.108.40.206/23; the second sub-sub-subnetwork #1-0-1, 220.127.116.11/23; the last will be sub-sub-subnetwork #1-0-31: 18.104.22.168/23.
01000111 01011111 01000000 00000000
This gives us 64 different /24 networks. The first will be sub-sub-subnetwork #1-1-0, 22.214.171.124/24, the second sub-sub-subnetwork #1-1-1, 126.96.36.199/24, and so on.
01000111 01011111 10000000 00000000
Seven bits allow 128 of these /25 networks within our /18 block. The first will be 188.8.131.52/25, the second 184.108.40.206/25, the third 220.127.116.11/25, and so on.
01000111 01011111 11000000 00000000
This gives us 256 different /26 networks within our /18 block. The first will be 18.104.22.168/26, the second 22.214.171.124/26, and so on.
Above all else, CIDR is about flexibilitythis is only one of many different ways to slice up this pie (sheet of brownies, whatever!) The ISP might decide that creating four different sizes of customer networks in advance was not the right way to go. They might instead just take the tack of dividing the pie in half, dividing it in half again, and so on, as many times as needed to create pie slices of the right size. Alternately, if most of their customers need around 50, 100, 200 or 500 hosts, the example above might be the easiest to administer.
It would still be possible for the ISP to further divide any of the smaller blocks further if they needed. They could split a /26 sub-sub-subnetwork into four /28 sub-sub-sub-subnetworks for very small customers, for example. Also, an individual customer of this ISP could do the same thing, dividing their own block to suit the internal structure of their network.
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