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Old 05-17-2005, 11:10 PM   #1
Registered: Jan 2005
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Class A,B,C licenses

I thought I knew this for a while but I went to school and they are teaching me a different way to figure this out. I googled this and saw there were two different explainations on figuring this out. Post what you think this actually is and lets see if we all get confused as I am getting.

The two different ways to figure this out is...

Class A,B,C lisenses
Class A - 1-127 range of IPs.
Class B - 128-191 range of IPs
Class C - 192-223 range of IPS

so a IP of is a Class A and is a Class C

The other way I am finding is that it is all based on the Subnet mask

meaning if you had an IP of and a mask of then it is a Class A because of the subnet mask...
So by this method the 201 IP is still a class A because of the subnet..
example: and a mask of is a Class A IP

Old 05-18-2005, 12:57 AM   #2
Registered: Nov 2004
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Technically, the netmasks are the relevant portions of the system. The ranges you mentioned at the beginning are the ranges as ASSIGNED to owners: they can subnet all they want.

The "Class" is essentially the number of "network" octets.

A.B.C.D/ - Class A
A.B.C.D/ - Class B
A.B.C.D/ - Class C

Now look at CIDR notation: it defies the 'Class' grouping: You can have a /20 network: 20 bits for network address. This is halfway between Class B(/16) and Class C(/24)... aka, A.B.C.D/

I've even seen, on my campus network, .192 and .64 netmasks.
Old 05-18-2005, 08:05 AM   #3
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With all the different routing protocols (like rip, ospf, eigrp, bgp etc..), the term classful ( and classless ( are more meaningful than the traditional term class. A classful routing protocol (like rip) does not look at the netmask, but rather the high order bits of the first octet to determine the class/netmask. i.e. network portion (n) vs host portion (h)
0nnnnnnn . hhhhhhhh . hhhhhhhh . hhhhhhhh
10nnnnnn . nnnnnnnn . hhhhhhhh . hhhhhhhh
110nnnnn . nnnnnnnn . nnnnnnnn . hhhhhhhh
Note how the bit pattern value of the high order bits corresponds to the class a, b, c ranges.

On the other side of the coin, a classless routing protocol would look at the netmask and not the high order bits.
Old 05-18-2005, 09:53 AM   #4
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Originally posted by scowles
0nnnnnnn . hhhhhhhh . hhhhhhhh . hhhhhhhh
10nnnnnn . nnnnnnnn . hhhhhhhh . hhhhhhhh
110nnnnn . nnnnnnnn . nnnnnnnn . hhhhhhhh
Originally posted by Matir
The "Class" is essentially the number of "network" octets.

A.B.C.D/ - Class A
A.B.C.D/ - Class B
A.B.C.D/ - Class C
Originally posted by Mishra100
Class A,B,C licenses
Class A - 1-127 range of IPs.
Class B - 128-191 range of IPs
Class C - 192-223 range of IPS
You are all right! If that confuses you more, let me try to clarify.

In a classful implimentation of IP, the range of the first octet determines the "class":
Class A is 1-127 denoted in binary as 0nnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh with a netmask of aka /8

Class B is 128-191 denoted in binary as 10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh with a netmask of aka /16

Class C is 192-223 denoted in binary as 110nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh with a netmask of aka /24
Note that the range of numbers translates to binary such that they start with either 0, 10, or 110 ie 12 decimal is 00000110 binary which starts with 0 while 150 decimal is 10010110 binary which starts with 10.

Again, in a classful implimentation of IP each of these ranges has a predefined netmask so if the IP address starts with 64 then it has a netmask of

What determines the class is the first octet (or more specifically the first 3 binary bits of that first octet) and as a result of what class it is it will have a given netmask.

In what is called classless IP you can further divide an IP block from one class into multiple networks, this is called subnetting. It's also possible to own multiple concurrent blocks in a given class and combine them, this is called supernetting.

If you subnet the addresses from a higher class by 8 or 16 more bits, then you will end up with a subnet mask that looks like it's from another class. Sometimes when this is done the netmask is just referred to by the class letter. As an example, if you take the (or block of class A addresses and divide them up by taking 16 more host bits you end up with (or and this is sometimes just called a class C mask. This is still a class A address block, it has just been subnetted and has a new netmask which is easier to refer to as a class C netmask than saying two-fifty-five dot two-fifty-five dot two-fifty-five dot zero.

The original routing protocols like RIP version 1 do not understand classless IP so if you use an address that starts with 64, it will assume the netmask is This makes RIP1 a lighter routing protocol because it doesn't even store a netmask for the networks. Newer routing protocols use CIDR (Classless Inter-Domain Routing) and they will use a provided netmask to distinguish subnetted blocks of IP addresses on different networks.

Basically, the first number defines what "class" it is but each class has a pre-defined netmask although the netmask doesn't determine what class it belongs to.


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