Telecom and Network Security, IPv4 Address Classes
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At about 38min in to the CISSP Telecom and Network Security Episode, Adam and Don describe how the first octet of the IPv4 Addresses was used to determine the class of an IP Address (A, B, or C). It looks like they incorrectly depict that Class A begins with 01??????, Class B with 001?????, and Class C with 0001????.
Just to be clear. The first octet for a Class A address begins with 0??????? (1-126), Class B begins with 10????? (128-191). and Class C with 11?????? (192-223). -
In the show notes for Network+ episode 2.3 they go into more detail about it:
Classes
A = N.H.H.H - 1st Number = 1-126 - Starts with 0 16,777,214 Usable IPs
? = Loopback - 1st Number = 127 - Starts with 01111111
B = N.N.H.H - 1st Number = 128-191 - Starts with 10 65,534 Usable IPs
C = N.N.N.H - 1st Number = 192-223 - Starts with 110 254 Usable IPs
D = ?.?.?.? - 1st Number = 224-239 - Starts with 1110
E = ?.?.?.? - 1st Number = 240-254 - Starts with 11110
In the early days of networking, there probably was a reason for this. It's too perfect to be an accident. Remember that the computers don't see the decimal numbers and they don't see the dots, they just see a stream of binary digits. What we see as 192.168.1.100, the machine sees as: 11000000101010000000000101100100
So 25 years ago some very expensive yet underpowered system could look at this address flying by and when it sees the "110" at the front, it knows how to treat this, even without a subnet mask.