I don't think there is an RFC that explicitly explains this, the reason it works is that if you take the address 127.1/24, the netmask specifies that the first 24 bits are the network address and the rest are the host address, the 127 falls within the network address part, setting the appropriate bits to one, and leaves all the other bits in the network part (even the unspecified ones) as zeros, since there is no third state that can represent the undefined value.
So, the way I understand it, it works something like this:
- 127.1 gets translated to binary and becomes 0111111100000001
- The 24 bit netmask tells us that the host part is the last 8 bits, or 00000001
- That leaves the other 24 bits for the network part, padded with zeros we get 011111110000000000000000
- Stick these back together and you get 01111111000000000000000000000001
- Split this long binary number into 4 bytes, and convert them back to decimal, and it leaves you with 127.0.0.1
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