2.9 Using Helper Address
2.9.3 ip helper-address examples

The following examples show how helper addresses can be used when connecting to remote networks with one or more servers. 

Single Server-Remote Medium

In Figure , where a single server is on a single remote medium, a helper address allows the router to perform the desired function of forwarding a client request to a server. 

The basic ip helper-address statement must be placed on the router interface that receives the original client broadcast. It causes the router to convert the 255.255.255.255 (all-ones) broadcast to a unicast or a directed broadcast. A directed broadcast is a local broadcast within a particular subnet.

In Figure , the ip helper-address statement placed on interface Ethernet 0 causes the default eight UDP broadcasts sent by all hosts to be converted into unicasts with a destination address of the Boot Server --- 144.253.2.2. These unicasts then will be forwarded to the boot server.

The modification of the destination address from broadcast to unicast or directed broadcast lets the packet be routed, over several hops if necessary, to its final destination.

Single Server-Remote Medium

You may not want to forward all default UDP broadcasts to the server, but only those of a protocol type supported on that server. Use the ip forward-protocol command followed by the keyword udp and port number or protocol name for those UDP broadcasts that are not automatically forwarded. Turn off any automatically forwarded ports with the no ip forward-protocol udp port or port name command.

In Figure , in addition to the default UDP broadcasts, the forwarding of a custom application using UDP port 3000 has been enabled. Because the server does not support TFTP requests, the administrator has disabled the automatic forwarding of port 69. 

Multiple Servers-Remote Medium

To handle forwarding broadcasts to multiple servers on the same remote medium, you can use a directed broadcast into the subnet instead of using several unicast helpers. As shown in Figure , configuring a directed broadcast address, the broadcast address changes into a directed broadcast.

Multiple Servers-Remote Media

The most general case is when multiple servers are located on different remote media, as shown in the Figure . This case can be handled by a combination of multiple helper statements, some with a unicast and some with a directed-broadcast address. The helper statements can be used to address individual servers on different media, whereas the directed broadcast can be used to address sets of servers that reside together on a common medium.