Posted by arlene

Sometimes, different network segments both use the same network ID and are connected by a router or another device. Because ARP uses broadcast packets to resolve IP addresses to hardware addresses, it would appear that computers on different network segments that use the same network ID would never be able to communicate.

Proxy ARP allows for just such a situation. The router or other device that connects the physical network segments is configured to provide the proxy ARP service. When a host broadcasts an ARP packet to learn the hardware address of a device that is on a different physical segment, the ARP proxy device recognizes this situation and acts as a go-between. The proxy device responds to the ARP broadcast and sends the originating computer a datagram that contains the proxy device’s IP address instead of the actual target computer’s IP address. From that point on, the host that originated the ARP request will communicate with the host on the other segment by sending packets to the proxy device, which will know to forward them to the computer on the other subnet.

Another use of proxy ARP comes into play for remote access servers. For example, when users dial into a computer that is acting as a remote access server, they are communicating with software on the remote access server and are not actually physically connected to the subnet. The remote access server recognizes this and will intercept any ARP broadcast packets that are trying to resolve the dial-in computer’s IP address. Communications then take place between the host on the local subnet and theremote computer through the remote access server. The host on the local subnet sends unicast packets to the remote access server, which forwards them to the remote client.

Yet another use for proxy ARP is to support older systems that use a TCP/IP stack that doesn’t understand subnetting or those that use the older method for broadcast packets—a host address of all zeros instead of the current standard of all ones. Although this is not really much of a problem today, you might still find older legacy systems that cannot be abandoned, yet they cannot properly interactwith newer systems when you subnet your network. The solution for this is to place the older systems on a separate network segment and let the proxy ARP device take care of resolving protocol addresses.

RARP—The Reverse Address Resolution Protocol

The Reverse Address Resolution Protocol (RARP) does just what it sounds like it would do. It performs the opposite function of ARP. It is most commonly used by diskless workstations that need to discover what their IP address is when they boot. Because the diskless workstation already knows its hardware address (because the address is burned into the network card), the workstation uses RARP to send a broadcast packet requesting that a server respond to its request by sending it an ARP frame containing an IP address it can use.

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