Understanding the network.pdf - Back to Home

Figure 2.3. IP address classes compared, in terms of
NOTE
bit significance of the first octet.
The IP subnet mask is the most important and misunderstood part of the IP address.
This is mostly due to the confusion between classful and classless routing. According
to RFC950, all hosts are required to support subnet mask addressing (some do it
better than others). The subnet mask, like the IPv4 address, is 32-bits long, in
binary format, and expressed as 4 octets. The masks are defined from left to right
using ones or zeros.
NOTE
There is a Arabic/Hexadecimal//Binary conversion chart in Appendix A, "Binary
Conversion Table."
With classful addressing, the natural mask is determined by the values of the first
few bits contained in the first octet of the address. By examining the IP address in
binary form, you'll see that the first three bits of a Class A network are 000, the first
three in a Class B network are 100, and, in a Class C network, are 111.
The first eight bits in an address class natural network mask are