127 Class A? 2

[y2k1981]

hi all, I'm studying for my CCNA right now. can somebody explain to me if 127.* is considered Class A or not? On a Cisco simulated prep-test last nigth I said that A was 1-127 but it said it was 1-126. Correct me if I'm wrong, but the only rules for the bits in a class A address are that the first bit should be 0 - which means you can go all the way from
00000001 - 01111111 -- ie 1 - 127. I know that 127 is reserved but isn't it still Class A?

thanks in advance

 

[Cluebird]

Technically the 127.x.x.x is in the class A range. However, because it is a special address used for loopback, the address has been removed from legitimate class A usage. Tests want you to understand this so the class A range is 1-126. Technically, the 0 address is also in the class A range, but it is also excluded since all zeros are not allowed.

 

[bierhunter]

Network classes are:
Class-A = 0xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
Class-B = 10xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
Class-C = 110xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

It's the bit position that determines the class.

127.x.x.x is internal to the machine, but it is still class-A.

 

[y2k1981]

thank you both for your responses. I think you made a very valid point there cluebird about the tests wanting people to understand that 127 is reserved so I guess that's why the answer on my test was 1-126. some of the other options were 0-126 & 0-127 but I automaticaly discounted them because I knew neither the network or host portion could be all zero's or all one's.

anyway, I'm just ranting on here. thanks again for your responses.

 

[chieftan999]

The easiest way to remember this is as follows:-

Class A - First bit of first byte MUST be switched off. Turn all the other bits off and then on and you get your range as follows:-

00000000 = 0 (Obviously a 1 is required)
01111111 = 127 (127 is used for loopback stack testing so cannot be used)

Range A = 1 - 126

Class B - First bit is ALWAYS ON and the second bit MUST be OFF, as follows:-

10000000 = 128
10111111 = 191

Class C - First 2 bits ALWAYS ON and third bit off. Turn all the other bits off and then on to find the range.

11000000 - 192
11011111 - 223

 

[NetEng631]

All ZERO's and all ONEs are not allowed in the old classful IP routing.