Using ARP to implement transparent subnet gateways (RFC1027)
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
S. Carl-Mitchell: AUTHOR [+1]
This RFC describes the use of the Address Resolution Protocol (ARP) by subnet gateways to permit hosts on the connected subnets to communicate without being aware of the existence of subnets, using the technique of "Proxy ARP".
Network Working Group Smoot Carl-Mitchell Request for Comments: 1027 Texas Internet Consulting John S. Quarterman Texas Internet Consulting October 1987
Using ARP to Implement Transparent Subnet Gateways
Status of this Memo
This RFC describes the use of the Ethernet Address Resolution Protocol (ARP) by subnet gateways to permit hosts on the connected subnets to communicate without being aware of the existence of subnets, using the technique of "Proxy ARP" . It is based on RFC-950 , RFC-922 , and RFC-826  and is a restricted subset of the mechanism of RFC-925 . Distribution of this memo is unlimited.
The work described in this memo was performed while the authors were employed by the Computer Sciences Department of the University of Texas at Austin.
The purpose of this memo is to describe in detail the implementation of transparent subnet ARP gateways using the technique of Proxy ARP. The intent is to document this widely used technique.
The Ethernet at the University of Texas at Austin is a large installation connecting over ten buildings. It currently has more than one hundred hosts connected to it . The size of the Ethernet and the amount of traffic it handles prohibit tying it together by use of repeaters. The use of subnets provided an attractive alternative for separating the network into smaller distinct units.
This is exactly the situation for which Internet subnets as described in RFC-950 are intended. Unfortunately, many vendors had not yet implemented subnets, and it was not practical to modify the more than half a dozen different operating systems running on hosts on the local networks.
Carl-Mitchell & Quarterman [Page 1]
RFC 1027 ARP and Transparent Subnet Gateways October 1987
Therefore a method for hiding the existence of subnets from hosts was highly desirable. Since all the local area networks supported ARP, an ARP-based method (commonly known as "Proxy ARP" or the "ARP hack") was chosen. In this memo, whenever the term "subnet" occurs the "RFC-950 subnet method" is assumed.
2.1 Basic method
On a network that supports ARP, when host A (the source) broadcasts an ARP request for the network address corresponding to the IP address of host B (the target), host B will recognize the IP address as its own and will send a point-to-point ARP reply. Host A keeps the IP-to-network-address mapping found in the reply in a local cache and uses it for later communication with host B.
If hosts A and B are on different physical networks, host B will not receive the ARP broadcast request from host A and cannot respond to it. However, if the physical network of host A is connected by a gateway to the physical network of host B, the gateway will see the ARP request from host A. Assuming that subnet numbers are made to correspond to physical networks, the gateway can also tell that the request is for a host that is on a different physical network from the requesting host. The ga...