Using ARP to implement transparent subnet gateways (RFC1027)
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2000-Sep-12
Internet Society Requests For Comment (RFCs)
S. Carl-Mitchell: AUTHOR [+2]
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.
Network Working Group Smoot Carl-Mitchell
Request for Comments: 1027 Texas Internet Consulting
John S. Quarterman
Texas Internet Consulting
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
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
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.
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 ga...