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default subnetmask is defined according to class.
Ip is divided into different classes

class A , class B , class C and every class has its default subnetmask

like class A has 255.0.0.0
class B has 255.255.0.0
class C has 255.255.255.0

In this 255 shows all bits are 1 and it shows network bit
where as 0 bit shows host bit

In class A one octent has Network bit and 3 has host bit
In class B two network and two host octent
In class C three netwok and one host octent

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Posted by saurabh2dixit

Each class of address is associated with a default subnet mask, as shown in Table. An address using its default mask defines a single IP broadcast domain—all the hosts using that same network number and mask can receive each other's broadcasts and communicate via IP.

Class Default Mask
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0

One of the rules that Cisco devices follow is that a subnet mask must be a contiguous string of 1s followed by a contiguous string of 0s. There are no exceptions to this rule: A valid mask is always a string of 1s, followed by 0s to fill up the rest of the 32 bits.

Therefore, the only possible valid values in any given octet of a subnet mask are 0, 128, 192, 224, 240, 248, 252, 254, and 255. Any other value is invalid.

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Posted by steve10

For class A
255.0.0.0
For Class B
255.255.0.0
For Class C
255.255.255.0

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Posted by sagitraz

IP addresses are divided into classes
class A,B,C,D,E
class A has default subnet mask of
255.0.0.0
class B has 255.255.0.0
class c has 255.255.255.0
255 represents the network portion
0 represents hosts
these are the default subnet masks .when u are using class full ip addressing scheme .

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Posted by blackmamba