RFC 950 August 1985
Internet Standard Subnetting Procedure
For example, the Internet address might be interpreted as: <network-number><subnet-number><host-number>
where the <network-number> field is as defined by IP [3], the<host-number> field is at least 1-bit wide, and the width of the<subnet-number> field is constant for a given network. No further structure is required for the <subnet-number> or <host-number> fields. If the width of the <subnet-number> field is zero, then the network is not subnetted (i.e., the interpretation of [3] is used).
For example, on a Class B network with a 6-bit wide subnet field, an address would be broken down like this:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0| NETWORK | SUBNET | Host Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Since the bits that identify the subnet are specified by a bitmask, they need not be adjacent in the address. However, we recommend that the subnet bits be contiguous and located as the most significant bits of the local address.
Special Addresses:
From the Assigned Numbers memo [9]:
"In certain contexts, it is useful to have fixed addresses with functional significance rather than as identifiers of specific hosts. When such usage is called for, the address zero is to be interpreted as meaning "this", as in "this network". The address of all ones are to be interpreted as meaning "all", as in "all hosts". For example, the address 128.9.255.255 could be interpreted as meaning all hosts on the network 128.9. Or, the address 0.0.0.37 could be interpreted as meaning host 37 on this network."
It is useful to preserve and extend the interpretation of these special addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets.
In the example above, the 6-bit wide subnet field may have any value except 0 and 63.