A destination IP address is ANDed with its subnet mask to let the router understand which part of the address identifies the destination network and which part identifies the target host.
IP address 10.1.1.1 in binary is
Subnet mask is 255.255.0.0 which in binary is
If you AND in binary then
0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1
So AND ing the IP Address and Subnet Mask allows the 'network' part of the destination address to filter through while surpressing the part that identifies the destination host:
which identifies the destination network, 10.1.0.0
All the router then has to do is decide which interface it needs to forward the packet on to have it reach this destination network. Once on that network, the part of the address which identifies the host comes into play
Does that clear it up a bit?
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