This is getting a bit far afield from Unix, so my apologies to the list for that. But to avoid dumping it in the Internet-History list abruptly, let me answer here _briefly_ (believe it or not, the below _is_ brief). Not quite (see below). First, one needs to understand that there are two different timelines for changes to addressing: in the hosts, and in the routers (called 'gateways' originally). To start with, they were tied together, but as of RFC-1122, they were formally separated: hosts no longer fully understood the syntax/semantics of addresses, just (mostly) treated them as opaque 32-bit quantities. Originally, network numbers were 8 bits, and the 'rest' (local) part was 24. Mapping from IP addresses to physical network addresses was some with direct mapping - ARP did not exist - the actual local address (e.g. IMP/Port) was contained in the 'rest' field - each network had a document which specified the mapping. (Which is part of the interoperability issue with old implementations.) As some point early on, it was realized that 8 bits of network number were not enough, and the awful A/B/C kludge was added (it was dropped on the community, not discussed before-hand). Subnetting was indeed the next change. Then the host/router split happened. Classless routing (which effectively extended addesses, for path-computation purposes, to 32+N bits - since you couldn't look at a 32-bit IP address and immediately tell which was the 'network' part any more, you _had_ to have the mask as well, to tell you how many bits of any given address were the network number) was more of a process than a single change - the inter-AS routing (BGP) had to change, but so did IGP's (OSPF, IS-IS), etc, etc. There was this effort called ROAD which produced RFC-1338 and 1519, and IIRC there was an intermediate, involving blocks of network numbers (1338), and that slowly evolved into arbitrary blocks (1519). One should also note that the term "super-netting" comes from a proposal by Carl-Hubert ("Roki") Rokitansky which did not, alas, make it to RFC. (His purpose was different, but it used the same mechanism.) Alas, the authors of 1338/1519 failed to properly acknowledge his earlier work. Noel