Variable-Length Subnetting in the RFCs
Ordinarily, I would point out an IETF source document that is the basis for an Internet technology and then expound on that technology. In the case of Variable-Length Subnet Masking (VLSM), there is no clear-cut genesis document. Searching the Internet or the RFC Editor's database turns up a variety of references, mostly in documents dedicated to other topics. The more salient of these tangential reference documents are RFC 1009 and RFC 1878. They provide you with the context for the development of variable-length subnets and supporting mathematics and helps you appreciate a more thorough examination of VLSM. The following sections discuss each of these RFCs.
The first “official” acknowledgment ...