Networking – Difference Between Subnet Masks 255.255.255.0 and 255.0.0.0

We need a subnet mask for IPv4 addresses because the address doesn't give any information on the network size. Class sizes are not the network sizes. In practical networks all IPv4 networks are broken up into subnets that are smaller than the class size.

For example you could break up the class C network 200.200.200.0/255.255.255.0 into two smaller networks (potentially at separate locations) 200.200.200.0/255.255.255.128 and 200.200.200.128/255.255.255.128 assuming neither needed more than 126 hosts. In reality most companies only get enough IPv4 addresses for the servers that need to be on the public Internet. I've personally seen set ups with 32,16, & 8 address networks (that would be masks of 255.255.255.224, 255.255.255.240, & 255.255.255.248 respectively)

Having IP networks only in class size blocks was too restrictive in limited the number of networks that could be allowed - the 127 class A networks taking half of the space. Not to mention that having a 24 billion node network is completely unmanageable Instead in 1993 Classless Inter-Domain Routing (CIDR) was introduced to allow the networks to be split up.

Also to be clear the purpose of the subnet mask is to determine which hosts are on the local network and which are outside of the network. Hosts can talk directly to hosts on the same network, but they need to communicate with a router to talk to hosts on external networks.

There are certain classes of IPs that are considered private and "not routable", including all of 10.0.0.0 - 10.255.255.255, 192.168.0.0 - 192.168.255.255, and 172.16.0.0 - 172.31.255.255.

Often the subnet masks for these ranges will line up with old class boundaries, but they don't have to. Consumer equipment that you use in your home will often use a /24 (255.255.255.0) by default, even if they use an address in the 10.x.x.x range. Part of the reason for this is protection in case the device is brought into a larger network, with a larger subnet, where the device IP may conflict with something else. This give a kind of protection for that network.

Larger networks also often are divided up into smaller logical groups called vlans. These vlans may need to occupy the same large address space, but still have distinct network and broadcast addresses. So I might set up a vlan using 10.1.0.0 for the network address and set a broadcast address of 10.1.255.255 by using 255.255.0.0 for the subnet mask. Then I can have a different vlan starting 10.2.0.0 that will be separated from the first network. You can get specific about how much of an address is used for the network and how much is used for the host.

Aside from all that, with IPv4 space filling up, most of the large blocks have now been broken up, and we don't really talk about class A,B,C,D any more anyway.