IP Address - SubnetMask - Default Gateway

I have previuously read about IP address..i would like to share with you this interesting concept.
IP Address has 2 parts
Network ID
Host ID

N/W ID will be the same for all computers on a network
Host Ids were either decided by N/W Admin or DHCP
Subnet Mask: helps us to identify the N/w ID portion and Host ID portion of a IP address
Default Gateway: id the Ip address of the router.

IP Address - Classes
Class A
subnet mask:255.0.0.0
Format:     X.Y.Y.Y
X is Network ID portion and Y is Host ID Portion
X ranges from 1 to 127
Y ranges from 0 to 255
Below IDs must not be used because
X.0.0.0 is Network Id
X.255.255.255 is broadcast ID
127.0.0.1 is loopback address for every local system to check if TCP protocol is working

Class B
subnet mask:255.255.0.0
Format:     X1.X2.Y.Y
X (X1 and X2)is Network ID portion and Y is Host ID Portion
X1 ranges from 128 to 191
Y ranges from 0 to 255
Below IDs must not be used because
X1.X2.0.0 is Network Id
X1.X2.255.255 is broadcast ID

Class C
subnet mask:255.255.255.0
Format:     X1.X2.X3.Y
X (X1 X2 X3)is Network ID portion and Y is Host ID Portion
X1 ranges from 192 to 223
Y ranges from 0 to 255
Below IDs must not be used because
X1.X2.X3.0 is Network Id
X1.X2.X3.255 is broadcast ID

Class D and E are used for multicasting and Experimental purpose