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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.
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