RFC 1027 (rfc1027) - Page 2 of 8
Using ARP to implement transparent subnet gateways
Alternative Format: Original Text Document
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. Design 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 gateway can then respond for host B, saying that the network address for host B is that of the gateway itself. Host A will see this reply, cache it, and send future IP packets for host B to the gateway. The gateway will forward such packets to host B by the usual IP routing mechanisms. The gateway is acting as an agent for host B, which is why this technique is called "Proxy ARP"; we will refer to this as a transparent subnet gateway or ARP subnet gateway. When host B replies to traffic from host A, the same algorithm happens in reverse: the gateway connected to the network of host B answers the request for the network address of host A, and host B then sends IP packets for host A to gateway. The physical networks of host A and B need not be connected to the same gateway. All that is necessary is that the networks be reachable from the gateway. With this approach, all ARP subnet handling is done in the ARP subnet gateways. No changes to the normal ARP protocol or routing need to be made to the source and target hosts. From the host point of view, there are no subnets, and their physical networks are simply one big IP network. If a host has an implementation of subnets, its network masks must be set to cover only the IP network number, excluding the subnet bits, for the system to work properly. Carl-Mitchell & Quarterman
