A subnet mask is a 32 bit number like IP address and is also expressed in dotted decimal format. It defines which part of the IP address refer to network address and which to node address.
Each of the classes of IP address used for address assignment has a standard subnet mask associated with it. The default subnet masks are as follow,
Class A ----255.000.000.000
Class B ----255.255.000.000
Class C ----255.255.255.000
The default subnet masks are used when the network does not have any subnet.
N.B:-0 in the subnet mask represent the node ID and 1(255) represents network ID.
18.104.22.168-------> Class A IP
255.0.0.0 ------->Subnet mask
Here N/W ID is 92 and node ID is 134.56.26