Class A

With Class A, part of the IP address (the first 8 bits) represents a network number, the rest of the address is used to number nodes. Any node in the same subnet can be reached directly from any other node in the same subnet.

Class A networks were assigned half the available address space of the entire Internet; if you see a network with a first octet <=127 then that is a class A network. There are 2^24 or 16,777,216 possible nodes. As nobody would ever need a single network with that many nodes, you can see how wasteful this approach is.

One special class A address is reserved for internal use only and does not route on the internet, that is 10.0.0.0 which was established by the standard RFC1918.

To identify which part of an address is the network and which part is the node, we use a subnet mask. This is a 32-bit number which has ‘1’ wherever we should treat the address as a network and zero wherever we should treat the address as a node. In the example above, our subnet mask would be;

Class A addresses are identified by the subnet mask 255.0.0.0 or another way we can describe this is to call is a /8, meaning that 8 bits are used for the network number.