Subnet calculator

http://www.subnet-calculator.com/

Subnet Calculator
Network Class A  B  C	First Octet Range
IP Address	Hex IP Address
Subnet Mask   255.255.255.0  255.255.255.128  255.255.255.192  255.255.255.224  255.255.255.240  255.255.255.248  255.255.255.252	Wildcard Mask
Subnet Bits   0  1  2  3  4  5  6	Mask Bits   24  25  26  27  28  29  30
Maximum Subnets   1  2  4  8  16  32  64	Hosts per Subnet   254  126  62  30  14  6  2
Host Address Range
Subnet ID	Broadcast Address
Subnet Bitmap 110nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh

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Notes about the Subnet Calculator

1. The subnet calculator implements a classful / classed IP addressing scheme where the following rules are adhered to:
   
   • Class A addresses have their first octet in the range 1 to 126 (binary address begins with 0).
     
   • Class B addresses have their first octet in the range 128 to 191 (binary address begins with 10).
     
   • Class C addresses have their first octet in the range 192 to 223 (binary address begins with 110).
     
2. The subnet calculator allows the use of a single subnet bit - for example, a class C address with a subnet mask of 255.255.255.128 is permitted.
   
3. The subnet calculator allows a subnet ID to have its final octet equal to the final octet of its subnet mask - for example, a class C network address of 192.168.0.192 with a subnet mask of 255.255.255.192 is permitted.
   

The above is generally accepted as being 'normal', however, certification students should keep in mind that, in some certification programs, the final two points are regarded as inacceptible.
For classless subnetting, you can use the CIDR calculator.