RFC 925 (rfc925) - Page 1 of 15
Multi-LAN address resolution
Alternative Format: Original Text Document
Network Working Group J. Postel
Request for Comments: 925 ISI
October 1984
Multi-LAN Address Resolution
STATUS OF THIS MEMO
This memo is prompted by RFC-917 by Jeffery Mogul on "Internet
Subnets". In that memo, Mogul makes a case for the use of "explicit
subnets" in a multi-LAN environment. In this memo, I attempt to make
a case for "transparent subnets". This RFC suggests a proposed
protocol for the ARPA-Internet community, and requests discussion and
suggestions for improvements. Distribution of this memo is
unlimited.
INTRODUCTION
The problem of treating a set of local area networks (LANs) as one
Internet network has generated some interest and concern. It is
inappropriate to give each LAN within an site a distinct Internet
network number. It is desirable to hide the details of the
interconnections between the LANs within an site from people,
gateways, and hosts outside the site. The question arises on how to
best do this, and even how to do it at all. One proposal is to use
"explicit subnets" [1]. The explicit subnet scheme is a call to
recursively apply the mechanisms the Internet uses to manage networks
to the problem of managing LANs within one network. In this note I
urge another approach: the use of "transparent subnets" supported by
a multi-LAN extension of the Address Resolution Protocol [2].
OVERVIEW
To quickly review the Address Resolution Protocol (ARP). Each host
on a broadcast LAN knows both its own physical hardware address (HA)
on the LAN and its own Internet Address (IA). When Host-A is given
the IA of Host-B and told to send a datagram to it, Host-A must find
the HA that corresponds to Host-B's IA. To do this Host-A forms an
ARP packet that contains its own HA and IA and the IA of the
destination host (Host-B). Host-A broadcasts this ARP packet. The
hosts that receive this ARP packet check to see if they are
destination sought. If so, they (it should be only Host-B) send a
reply specifically addressed to the originator of the query (Host-A)
and supplying the HA that was needed. The Host-A now has both the HA
and the IA of the destination (Host-B). The Host-A adds this
information to a local cache for future use.
Note: The ARP is actually more general purpose than this brief
sketch indicates.
Postel
