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Subnet query
Hello all,
In my building for a large company the PCs (on a domain) all have an IP address starting with 10.x.x.x. Now this to me is a class A address and the subnet mask should be 255.0.0.0. however the subnet mask on all the Pcs in the office is
255.255.255.0 suggesting a class C address. It has always been this way and I wondered if anyone knew why this is so?
In my building for a large company the PCs (on a domain) all have an IP address starting with 10.x.x.x. Now this to me is a class A address and the subnet mask should be 255.0.0.0. however the subnet mask on all the Pcs in the office is
255.255.255.0 suggesting a class C address. It has always been this way and I wondered if anyone knew why this is so?
True classfull subneting would state that 10.0.0.0 255.0.0.0 would be a class A subnet but it is not as common to do that anymore. Classless addressing is used allowing more options for addressing schemes. Therefore 10.1.1.0 255.255.255.0 would allow a range of IP's from 10.1.1.1 to 10.1.1.254 with a network ID of 10.1.1.0 and a broadcast address of 10.1.1.255
As a result more diversified ranges can be chosen such as:
10.1.1.0 255.255.252.0 which results in 10.1.1.1 to 10.1.3.254 with a network ID of 10.1.1.0 and a broadcast address of 10.1.3.255
or
10.1.1.0 255.255.255.240 which results in 10.1.1.1 to 10.1.1.14 with a network ID of 10.1.1.0 and a broadcast address of 10.1.1.15
As a result more diversified ranges can be chosen such as:
10.1.1.0 255.255.252.0 which results in 10.1.1.1 to 10.1.3.254 with a network ID of 10.1.1.0 and a broadcast address of 10.1.3.255
or
10.1.1.0 255.255.255.240 which results in 10.1.1.1 to 10.1.1.14 with a network ID of 10.1.1.0 and a broadcast address of 10.1.1.15
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If you were going with the classful approach to subnetting, such would be true. But if you're using CIDR (Classless Internet Domain Routing, which is used by most), then the subnet mask you gave would not necessarily apply.
Why 255.255.255.0 for the subnet mask? Because they figured that 256 IP addresses would suit the needs of the particular area or office to which those addresses are assigned. By not taking a classful approach to subnetting, it's a heck of a lot easier to give an office about as many IP addresses as needed without worrying about overkill. Plus you reduce the amount of machines affected by broadcasts. Imagine if you had an office needing 1000 IP addresses (assume growth is built into this number), and you were given a class B network. Whereas if you were given a chunk of IPs where you had a subnet mask of 22 bits, you'd have 1024 addresses. The other approach would be obviously to assign 4 class C networks, but why bother when you can just have one network?
Hope this helps.
Why 255.255.255.0 for the subnet mask? Because they figured that 256 IP addresses would suit the needs of the particular area or office to which those addresses are assigned. By not taking a classful approach to subnetting, it's a heck of a lot easier to give an office about as many IP addresses as needed without worrying about overkill. Plus you reduce the amount of machines affected by broadcasts. Imagine if you had an office needing 1000 IP addresses (assume growth is built into this number), and you were given a class B network. Whereas if you were given a chunk of IPs where you had a subnet mask of 22 bits, you'd have 1024 addresses. The other approach would be obviously to assign 4 class C networks, but why bother when you can just have one network?
Hope this helps.
https://www.experts-exchange.com/questions/21782082/Classless-vs-Classful.html
With subnetting, it really doesn't matter what you do. Like your example above, you could use a 172.16.x.x with a 255.255.255.0 mask as well.