…Also Called “Address Masking”
The IP Address Mask is a configuration parameter used by a TCP/IP end-node and IP router to differentiate between that part of the IP address that represents the network and the part that represents the host.
A router uses the mask value to create a key value that is looked up in the router table to determine where to forward a frame. An end-node uses the mask value to create the same key value but the value is used to compare the destination address with the end-node address to determine whether the destination is directly reachable (on the same network) or remote (in which case the frame must be sent to a router and can not be sent directly to the destination).
The mask value can be assigned by default or it can be specified by the installer of the end-node or router software. The destination IP address and the mask value are combined with a Boolean AND operation to produce the resultant key value. Consider the following example:
An end-node is assigned the IP address 184.108.40.206 and a mask value of 255.255.0.0. This end-node wants to send a frame to 220.127.116.11. If 18.104.22.168 is on the same network as 22.214.171.124 then the end-node will broadcast an ARP (Address Resolution Protocol) frame to determine the data link address of the destination and it will then send the frame directly to the destination. If 126.96.36.199 is on a different network then the workstation must send the frame to a router for forwarding to the ultimate destination network.
All the dotted-decimal notation must be converted to the underlying 32-bit binary numbers to understand what is taking place.
End-Node 188.8.131.52 = 10001100 00000110 00001111 00000011 Mask 255.255.0.0 = 11111111 11111111 00000000 00000000 Destination 184.108.40.206 = 10001100 00000111 00001001 00000010
When the End-Node IP address is AND’ed with the mask the result is:
End-Node 220.127.116.11 = 10001100 00000110 00001111 00000011 Mask 255.255.0.0 = 11111111 11111111 00000000 00000000 ——————————————————————– RESULT OF “AND” = 10001100 00000110 00000000 00000000 (In dotted-decimal) = 140. 6. 0. 0
When the destination IP address is AND’ed with the mask the result is:
Destination 18.104.22.168 = 10001100 00000111 00001001 00000010 Mask 255.255.0.0 = 11111111 11111111 00000000 00000000 ——————————————————————– RESULT OF “AND” = 10001100 00000111 00000000 00000000 (In dotted-decimal) = 140. 7. 0. 0
Since the results (22.214.171.124 and 126.96.36.199) are not equal the end-node concludes that the destination must be on a different network and the frame is sent to a router. This is the way an end-node uses the mask value. A router, on the other hand, masks the destination address in an incoming frame and the result is used as a lookup key in the routing table.