Going Back to Class

When the Internet Protocol was first introduced, the designers had no idea just how widely it would become adopted.  At the time, it competed head to end with Novell’s IPX protocol, along with Appletalk and a number of others, before IP finally achieved dominance.  In the beginning uses, groups of addresses were placed in classes which represented the way the addresses treated parts of the address, namely network and node (or host).  I love Wendell Odom’s explanation of these concepts, which he uses in his CCNA ICND1 Official Exam Certification Guide by Cisco Press (his website can be found at:http://www.certskills.com/) .  The network portion of the address is similar to a postal zip code, a broader representation of locations in a geographical area, after which street addresses and cities narrow down to a specific location.  A letter carrier on his/her route doesn’t care about zip codes several states away (in this analogy, addresses in other networks), but only the ones local to them.  With this in mind, let’s look at how address classes break out in terms of network length and node length.

Class A addresses have a network length of 8 bits, which is the first byte/octet of that address space.  Let’s use the example of, which would use 4 as the network portion and 233.10.40 as the node portion.  The natural or default mask is, or /8 using the examples described in the last blog.  Look at the representation of the possible ranges of Class A addresses in binary: 00000000 – 01111111. Two things should jump out at you, first, that only the leading digit (0) is consistent, which should always clue you in that a leading zero represents a Class A address.  Second, if you do the conversion from binary back to decimal, you will find the range to be 0 through 127 in the first octet.  Strictly speaking, the  range is reserved for internal loopback usage and zero ist permitted, so the actual usable range is –

To save space and brain cells, I will just summarize the Class B and C address characteristics, but keep in mind that the binary math works similarly to what is discussed above:

Address Class                        Leading Bit(s)                       Valid Network Numbers                       Network Bits                       Host Bits

A                                               0                                     to                                8                                             24

B                                               10                                   to                            16                                           16

C                                               11                                    to                           24                                           8

Notice that if you add the host bits and network bits you end up with 32 bits (4 bytes), which is the total address space in an IP Address.  Being able to recognize the Class of address in both binary and decimal will be very helpful not only on the exam, but later on when we discuss subnetting.

More to come…

– Joe

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