Calculate Sub net mask

Here is a subnet mask table that tells how many IP addresses are in a subnet based on the mask used:

ID	# Bits	Number Hosts	Usable Hosts	Netmask		Note
6	/6  	67108864	67108862	 252.0.0.0  	
7	/7 	33554432	33554430	254.0.0.0 	
8	/8 	16777216	16777214	255.0.0.0 	Class A Network 
9	/9 	8388608		8388606		255.128.0.0 	
10	/10 	4194304		4194302		255.192.0.0 	
11	/11 	2097152		2097150		255.224.0.0 	
12	/12 	1048576		1048574		255.240.0.0 	
13	/13 	524288		524286		255.248.0.0 	
14	/14 	262144		262142		255.252.0.0 	
15	/15 	131072		131070		255.254.0.0 	
16	/16 	65536		65534		255.255.0.0 	Class B Network 
17	/17 	32768		32766		255.255.128.0 	
18	/18 	16384		16382		255.255.192.0 	
19	/19 	8192		8190		255.255.224.0 	
20	/20 	4096		4094		255.255.240.0 	
21	/21 	2048		2046		255.255.248.0 	
22	/22 	1024		1022		255.255.252.0 	
23	/23 	512		510		255.255.254.0 	
24	/24 	256		254		255.255.255.0 	Class C Network 
25	/25 	128		126		255.255.255.128 	
26	/26 	64		62		255.255.255.192 	
28	/28	16		14		255.255.255.240	
27	/27	32		30		255.255.255.224	
29	/29 	8		6		255.255.255.248 	
30	/30 	4		2		255.255.255.252

The subnet mask when applied to the IP address basically extracts the subnet portion of the IP address. This is done when you apply the AND operation against the IP address. What you do is convert the IP address into a binary number. Then you convert the subnet mask into a binary number. AND the numbers together, and the result is the subnet address.

Example:

IP: 1101 1000 . 0000 0011 . 1000 0000 . 0000 1100 (216.003.128.012)
Mask: 1111 1111 . 1111 1111 . 1111 1111 . 0000 0000 (255.255.255.000)
---------------------------------------------
1101 1000 . 0000 0011 . 1000 0000 . 0000 0000 (216.003.128.000)