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order <strong>to</strong> maintain computability with classful subnetting, by default, IOS does not<br />

permit <strong>the</strong> usage of <strong>network</strong> addresses that end on natural zero boundaries. This is<br />

done because <strong>the</strong>se addressees can be mistaken for <strong>network</strong> broadcast addresses<br />

because <strong>the</strong> netmask is not sent along in <strong>the</strong> packet's address information. To<br />

enable <strong>the</strong> use of "zero boundary" <strong>network</strong> addresses, which is particularly useful<br />

when working with CIDR's created out of Class C addresses, IOS provides <strong>the</strong> global<br />

configuration EXEC command . Here is an example of what<br />

happens when a zero boundary address is used without enabling :<br />

Router#config t<br />

Enter configuration commands, one per line. End with CNTL/Z.<br />

Router(config)#int e0<br />

Router(config-if)#ip address 192.168.0.1 255.255.255.252<br />

Bad mask /30 for address 192.168.0.1<br />

Router(config-if)#<br />

With <strong>the</strong> option enabled, <strong>the</strong> address is permitted:<br />

Router#config t<br />

Enter configuration commands, one per line. End with CNTL/Z<br />

Router(config)#ip subnet-zero<br />

Router(config)#int e0<br />

Router(config-if)#ip address 192.168.0.1 255.255.255.252<br />

Router(config-if)#^Z<br />

Router#<br />

Configuring IP Control Services<br />

IOS supports a number of options <strong>to</strong> adjust IP's behavior. Some of <strong>the</strong>se commands<br />

enhance <strong>the</strong> security of IP, and o<strong>the</strong>rs enhance IP's performance:<br />

• — This command allows IP UDP<br />

broadcasts <strong>to</strong> be forwarded between IP <strong>network</strong> segments that are<br />

connected <strong>to</strong> each o<strong>the</strong>r across <strong>the</strong> router. IP (Layer 3) broadcasts are not<br />

forwarded between <strong>network</strong>s by default. This option allows UDP broadcast<br />

service requests like DHCP and BOOTP <strong>to</strong> be relayed <strong>to</strong> servers on remote<br />

<strong>network</strong>s. This command must be used with , which is set on <strong>the</strong> router interface that is performing <strong>the</strong><br />

forwarding. The address specifies where <strong>the</strong> request should beforwarded. To<br />

configure DHCP/BOOTP forwarding:

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