Summary

  • RFC 796 described how local host, terminal, process and logical addresses on several networks fit inside the local portion of a 32-bit Internet address, often with a zero octet or a reserved field.
  • The mappings preserved real differences: packet-radio hosts could move without changing their 16-bit local identity, satellite networks could assign several logical addresses to one machine, and ARPANET's logical-host field remained zero because its IMPs did not yet support it.

One Internet address looks tidy on paper: 32 bits, a network number, a local address. The machines beneath it were less obliging. A packet-radio host might move, a satellite host might answer to more than one logical address, and an ARPANET leader had a field its IMPs could not yet use. Jon Postel's September 1981 RFC 796 was the tailor's pattern book for fitting those different local identities into an IP address. Universal envelope, custom lining.

Postel began with the classful shapes then in use. A Class A address set aside seven network-number bits and 24 local bits after its leading class bit. Class B divided the remaining space into 14 network and 16 local bits; Class C into 21 and eight. The point of his memo was what happened after the network number identified a particular network. Those local bits had to mean something to the equipment on the other side. The answer varied across AUTODIN II, ARPANET, distributed-computing networks, MIT's LCSNET, packet radio and two satellite networks.

AUTODIN II used 16-bit subscriber addresses for a host or terminal. They could be assigned independently of location. In its Class A IP representation, Postel placed the subscriber value in the low 16 local bits and filled the preceding local octet with zero. PRNET used a similar shape for its 16-bit host addresses. Its hosts could be mobile, so location independence was practical rather than philosophical. Postel listed six packet-radio network numbers, each surrounding the same kind of 16-bit local host value with its own network prefix and a zero octet. The envelope said which network; the smaller card still identified the host.

ARPANET's 96-bit leader supplied another puzzle. Its 24-bit address was arranged as host, logical host and IMP fields, carried directly in IP's 24-bit Class A local part. The logical-host slot was drawn in the format, but the IMPs did not yet support logical addressing. The field went out as zero in the leader. EDN, an experimental data network, used the same kind of arrangement and the same zero in the unsupported slot. A bit field can reserve a future possibility; it cannot make deployed equipment understand it on schedule.

Other networks put different things in the local portion. COMSAT's and UCL's distributed-computing networks had an eight-bit host identifier plus an eight-bit process identifier, placed below a zero octet. MIT's LCSNET used several 32-bit address forms; the common one contributed its low 24 bits—subnet, reserved and host—to IP. Postel's COMSAT/UCL paragraph names network numbers 29 and 30, while its adjacent drawing prints 18 in the network octet. That small mismatch sits in the original document. It is a useful reminder to read the prose and the diagram together before turning a number into a routing assertion.

The satellite cases pulled apart machine and address. On SATNET, a 16-bit address could be assigned independently of ground-station location, and one physical host could hold several such logical addresses. Its IP form put a zero octet ahead of the 16-bit local value. WBCNET also allowed several logical addresses for one physical host. There the 16 bits split into HAP number and local address, with a zero octet between those pieces in the IP layout. Neither drawing makes the satellite dish itself the identity. The bits describe how a host is found in the network's own scheme.

RFC 796 did more than copy smaller numbers into a larger box. It recorded which zeros were padding, which zeros marked a capability not yet in the IMPs, and which address fields referred to a process, a logical host or a movable radio station. The Internet address supplied a common outer format. Postel's seven pages show the seams, where an engineer had to know the local network before interpreting the supposedly universal last three octets.