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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter32 Configuring IP Unicast Routing
Configuring IP Addressing
Use of Subnet Zero
Subnetting with a subnet address of zero is strongly discouraged because of the pr oblems that can arise
if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as
255.255.255.0, subnet zero would be written as 131.108.0.0, which is th e sam e a s the netwo rk ad dre ss.
You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the
use of subnet zero if you need the entire subnet space for your IP address.
Beginning in privileged EXEC mode, follow these steps to enable subnet zero:
Use the no ip subnet-zero global configuration command to restore the default and disable the use of
subnet zero.
Classless Routing
By default, classless routing behavior is enabled on the switch when it is configured to route. With
classless routing, if a router receives packets for a subnet of a network with no default route, the router
forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C
address spaces used to simulate a single, larger address space and is designed to relieve the pressure on
the rapidly depleting Class B address space.
In Figure 32-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of
discarding the packet, the router forwards it to the best supernet rou t e. I f y ou disabl e c lassle ss r outing
and a router receives packets destined for a subnet of a network with no network defaul t route, the router
discards the packet.
Figure32-2 IP Classless Routing
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 ip subnet-zero Enable the use of subnet zero for interface addresses and routing updates.
Step3 end Return to privileged EXEC mode.
Step4 show running-config Verify your entry.
Step5 copy running-config startup-config (Optional) Save your entry in the configuration file.
Host
128.20.1.0
128.20.2.0
128.20.3.0
128.20.4.1
128.0.0.0/8
128.20.4.1
IP classless
45749
128.20.0.0