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as m.shaker said, switches don't use subnet masks since these are use for ip addresses which are used in thee network layer which isn't available in switches...
subnet mask plays an important role in the routing process, ip adresses consists of two parts, the network ID and the host ID.. external routers store entries only for the network IDs. when a packet arrives with a certain ip, it will be anded with the subnet mask and compared with the stored network ID, if the result is equal to the stored network ID, the packet will be forwarded to the corresponding next hop.. the next figures explain that:
Subnet masks can able to divide the network.
For example:
When you look at your connection details you may see
192.168.0.5 and 255.255.255.0 like that this means your are the part of the 255.255.255.0 network.
And in your network there are 253 users with this subnet mask.And technically you can show this 192.168.0.5/24.Only the Routers understands the Subnet mask and also Layer 3 Switches.In both devices they have specific table ,When you connect a new pc ,it send own information to its network and both switches(L3) and routers writes this info to its table.
One more thing These devices keeps the info for their every ports.Ex: You have a 2 port router only for connecting the networks and Routers collects info and knows that in Port1 192.168.0.5 subnet mask is 255.255.255.0 ,Port2 172.16.10.3 255.255.240.0.
Ps:If you want more info I can explain more detailed.
for the importance of the subnet ,it is for managing and saving the available IPs
assume that we don't have subnetting ,then we have only the available network classes(A,B,C and D)
then we have limited the usage of each network like this example:class C has subent 255.255.255.0 it means that the max. no. of hosts is 2^8-2 =254 host while we have a network with 255 hosts or more then we have a unique solution which is to use another class while we don't need the rest IPs if we use Class B then we have available IPs=2^16-2=65534 which is a great waste of the available IPs ,so we may make subnetting by lending one bit or more of the network parts to the host parts to increase the no. of hosts
Subnetting Basics(from Cybex 5th edition)
What happens if you wanted to take one network address and create six networks from it? You would have to do something called subnetting, because that’s what allows you to take one larger network and break it into a bunch of smaller networks. There are loads of reasons in favor of subnetting. Some of the benefits include:
Reduced network traffic
Optimized network performance
Simplified management
you may find more details in any network books like Cybex
Regards
Mohamed Aziz
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