This is a description of the ProxyARP functionality in the Netopia router.
The Netopia router is an excellent choice for connecting a local LAN to the Internet. In order to do so, an account must be established with an ISP or other service provider. The ISP will assign a block of addresses and a subnet so that each client on the local LAN will have a unique IP address from the block allocated by the ISP and share the same subnet mask.
The ProxyARP feature comes into play when another remote LAN or Terminal Adapter (TA) wants to connect to and share Internet access with the local LAN. The Netopia router is a full IP router. Thus, it requires that the remote LAN has its own block of addresses and a new subnet mask encompassing the number of hosts on the remote network.
Without the use of ProxyARP, the local LAN would have to subnet its addressing scheme and allocate roughly half the address space to the local LAN, and the other half to the remote LAN. With ProxyARP, this subnetting of the local LAN is no longer necessary, and a logical block of addresses and a subnet mask can be allocated to the remote LAN without subnetting the local LAN.
Below is a list of hardware and firmware loads that this Technical Note is based upon:
| Hardware | Firmware/Version | Installed Options |
| Any Netopia Router | 3.1.3 or later | None |
Below is a diagram of the network configuration that will be referenced in this Technical Note.
The Netopia router has a feature called ProxyARP. ProxyARP is an extremely powerful feature of the Netopia router, but before it can be fully understood, some basic principles have to be explained.
ARP stands for Address Resolution Protocol, which is a low-level protocol that maps a user-definable IP address (192.168.1.1) to a physical address (00-00-c5-60-04-f9) contained on the Ethernet hardware of an individual router or workstation. The physical address is what is used to communicate between two hosts on a given network.
This address mapping is first performed as an ARP broadcast, where host A (192.168.1.3) on a common network with host B (192.168.1.4) wants to talk to host B but does not know its physical address. Host A sends out an ARP broadcast to all hosts on the common network asking the host with the IP address of 192.168.1.4 to respond with its physical address. Once host B responds to host A with its physical address, end-to-end data transfer can begin.
The ProxyARP functionality of the Netopia router expands on the principles explained in the previous section. As you can see from the Network Configuration diagram, Customer Site A is allocated a Class C network address space. The network is 192.168.1.0 and the subnet mask is 255.255.255.0, which allows for 254 hosts. Customer Site A has two routers. One is a 'generic' router connecting to the ISP, and the other is a Netopia router performing the ProxyARP function as well as allowing a connection to Customer Site B.
Customer Site B on the other hand, has a subnetted network address space derived from Customer Site A. The network address for Customer Site B in this case is 192.168.1.128 with a subnet mask of 255.255.255.248. This allows for a total of six host addresses, ranging from 192.168.1.129 to 192.168.1.134.
The ProxyARP functionality comes into play on Netopia A. Internally in the IP routing table, the Netopia router realizes that the Customer Site B address space overlaps the Customer Site A address space. Even so, the Netopia router enters the route in its IP routing table and employs ProxyARP.
If a host on Customer Site A wants to reach a host on Customer Site B, it sends an ARP broadcast for the physical address of the host on Customer Site B as usual. Instead of the host on Customer Site B responding, the router Netopia A responds with its physical address, hence the term ProxyARP. The host on Customer Site A then uses the physical address of Netopia A to send data to the actual recipient on Customer Site B. When the router Netopia A receives data destined to the host on Customer Site B, Netopia A forwards the information downstream to Netopia B, and on to its destination.
Essentially, hosts on Customer Site A are unaware that hosts on Customer Site B are on a different network separated by a router. Hosts on Customer Site B think that they are on their own subnet off of Customer Site A. Using ProxyARP, both networks can communicate seamlessly.
Even though ProxyARP is easy to use, there are some things to be aware of. The main thing is to be sure that the address space and subnet allocated to the Customer Site B do not overlap any hosts on Customer Site A. For example, a host on Customer Site A cannot have an address within the range of addresses 192.168.1.128 to 192.168.1.135, that are allocated to Customer Site B.
Although ProxyARP is an easy, practical alternative for allowing remote LANs or TAs to connect and share Internet access with the local LAN, it should not be a direct replacement for fully subnetting the address space and allocating it to remote LANs or TAs.