Dell POWEREDGE M1000E - Manual (Page 900)

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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
OL-13270-03
Chapter 38 Configuring IP Unicast Routing
Configuring IP Addressing
Use of Subnet Zero
We strongly discourage subnetting with a subnet address of zero because of the problems 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 the same as the network address.
You can use the all ones subnet (131.108.255.0) and, even though we discourage this practice, you can
enable the 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 to disable the use of
subnet zero.
Classless Routing
By default, classless routing is enabled when the switch 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 is contiguous blocks of Class C address spaces simulating a single,
larger address space and is designed to relieve the pressure on the rapidly depleting Class B address
space.
In Figure 38-2 , 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 route. If you disable classless routing
and a router receives packets destined for a subnet of a network with no network default route, the router
discards the packet.
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 ip subnet-zero Enable the use of subnet zero for interface addresses and routing updates.
Step 3 end Return to privileged EXEC mode.
Step 4 show running-config Verify your entry.
Step 5 copy running-config startup-config (Optional) Save your entry in the configuration file.