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As others have pointed out, RFC1918 defines 3 private IP ranges. In 1996, there was still legacy equipment around that didn't support CIDR, so one range was created for each class. Class B addresses start at 128.0.0.0, and class C addresses start at 192.0.0.0; 168 was chosen just because it was unallocated.

But this raises another question — why was a class C range required? Since the only difference between classes A, B and C is the network size, why not just use 10.0.0.0/8? According to RFC1918:

If a suitable subnetting scheme can be designed and is supported by the equipment concerned, it is advisable to use the 24-bit block (class A network) of private address space and make an addressing plan with a good growth path. If subnetting is a problem, the 16-bit block (class C networks), or the 20-bit block (class B networks) of private address space can be used.

I'm not sure exactly what kind of "problems" with subnetting the authors were thinking of. Perhaps some pre-CIDR hardware didn't support Class A networks due to memory constraints (though you would think it's the number of hosts, not the number of potential hosts, that matters).

Also, class C networks are /24s, even though 192.168.x.x is a /16 — so in classful networking 192.168.x.x actually contains 256 subnets. This may have been useful for large organisations that wanted to run private subnets on pre-CIDR hardware.

As others have pointed out, RFC1918 defines 3 private IP ranges. In 1996, there was still legacy equipment around that didn't support CIDR, so one range was created for each class. Class B addresses start at 128.0.0.0, and class C addresses start at 192.0.0.0; 168 was chosen just because it was unallocated.

But this raises another question — why was a class C range required? Since the only difference between classes A, B and C is the network size, why not just use 10.0.0.0/8? According to RFC1918:

If a suitable subnetting scheme can be designed and is supported by the equipment concerned, it is advisable to use the 24-bit block (class A network) of private address space and make an addressing plan with a good growth path. If subnetting is a problem, the 16-bit block (class C networks), or the 20-bit block (class B networks) of private address space can be used.

I'm not sure exactly what kind of "problems" with subnetting the authors were thinking of. Perhaps some pre-CIDR hardware didn't support Class A networks due to memory constraints (though you would think it's the number of hosts, not the number of potential hosts, that matters).

As others have pointed out, RFC1918 defines 3 private IP ranges. In 1996, there was still legacy equipment around that didn't support CIDR, so one range was created for each class. Class B addresses start at 128.0.0.0, and class C addresses start at 192.0.0.0; 168 was chosen just because it was unallocated.

But this raises another question — why was a class C range required? Since the only difference between classes A, B and C is the network size, why not just use 10.0.0.0/8? According to RFC1918:

If a suitable subnetting scheme can be designed and is supported by the equipment concerned, it is advisable to use the 24-bit block (class A network) of private address space and make an addressing plan with a good growth path. If subnetting is a problem, the 16-bit block (class C networks), or the 20-bit block (class B networks) of private address space can be used.

I'm not sure exactly what kind of "problems" with subnetting the authors were thinking of. Perhaps some pre-CIDR hardware didn't support Class A networks due to memory constraints (though you would think it's the number of hosts, not the number of potential hosts, that matters).

Also, class C networks are /24s, even though 192.168.x.x is a /16 — so in classful networking 192.168.x.x actually contains 256 subnets. This may have been useful for large organisations that wanted to run private subnets on pre-CIDR hardware.

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source | link

As others have pointed out, RFC1918 defines 3 private IP ranges. In 1996, there was still legacy equipment around that didn't support CIDR, so one range was created for each class. Class B addresses start at 128.0.0.0, and class C addresses start at 192.0.0.0; 168 was chosen just because it was unallocated.

But this raises another question — why was a class C range required? Since the only difference between classes A, B and C is the network size, why not just use 10.0.0.0/8? According to RFC1918:

If a suitable subnetting scheme can be designed and is supported by the equipment concerned, it is advisable to use the 24-bit block (class A network) of private address space and make an addressing plan with a good growth path. If subnetting is a problem, the 16-bit block (class C networks), or the 20-bit block (class B networks) of private address space can be used.

I'm not sure exactly what kind of "problems" with subnetting the authors were thinking of. Perhaps some pre-CIDR hardware didn't support Class A networks due to memory constraints (though you would think it's the number of hosts, not the number of potential hosts, that matters).